[MCQs] Integrated Metabolism—Vitamins, Minerals, and Hormones - Part 2 (18 test - end)

Quizzes about Integrated Metabolism—Vitamins, Minerals, and Hormones - Part 2 (18 test - end)

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Question 1

A 14-year-old African American adolescent has shown weight loss and increased thirst with fatigue and decreased athletic performance. His physician does a urinalysis and the reagent strip is strongly positive for glucose and ketones. A likely diagnosis of diabetes mellitus is made and the patient started on long-acting insulin at night with short-acting insulin at breakfast and lunch. Which of the following statements correctly describes insulin?

A	It is an anabolic signal to cells that glucose is scarce.
B	It is converted from proinsulin to insulin primarily following secretion from β-cells.
C	It does not have a prohormone form.
D	It is a small polypeptide composed of a single chain bridged by disulfide groups.
E	Its action is antagonistic to that of glucagon.

Question 1 Explanation:

Early signs of diabetes mellitus reflect the inability of glucose to enter peripheral tissues, accompanied by decreased energy metabolism (fatigue), excess glucagon action, hyperglycemia, increased glucose excretion (polyuria), and dehydration (thirst). The anabolic hormone called insulin is synthesized in pancreatic β-cells in response to signals of glucose availability and is processed to its active form (incorrect answers a, c). The action of insulin is antagonistic to that of glucagon, a catabolic hormone secreted by pancreatic α-cells. Insulin is synthesized as a nascent polypeptide chain called preproinsulin, then cleaved immediately following synthesis to form proinsulin (incorrect answer b). Proinsulin is composed of one continuous polypeptide that contains in sequence an A chain of 21 residues, a connecting peptide (C peptide) of about 30 residues, and a B chain of 30 residues. The molecule is folded so that two disulfide bridges span the A and B chains (incorrect answer d). The proinsulin molecule is transported from the lumen of the endoplasmic reticulum to the Golgi apparatus, where it is packaged into storage granules. In the Golgi apparatus and in the storage granules, proteolysis of the C peptide occurs. Exocytosis of the granules releases insulin as well as C peptides into the bloodstream. Neither proinsulin nor the C peptide is biologically active.

Question 2

Some individuals with diabetes mellitus are susceptible to rapid drops in blood sugar (hypoglycemia) with lethargy and potential seizures or coma. Such diabetics are called “brittle” and require careful monitoring of glucose levels as proper insulin doses are titrated. Of the many actions of insulin, decrease in which of the following cellular activities best accounts for “brittleness” of certain diabetics?

A	Plasma membrane transfer of glucose
B	Glucose oxidation
C	Gluconeogenesis
D	Lipogenesis
E	Formation of ATP, DNA, and RNA

Question 2 Explanation:

Gluconeogenesis is a catabolic process for the synthesis of glucose, mainly from the amino acids of degraded proteins. Gluconeogenesis is the adaptive response of the organism to low blood levels of glucose and is, therefore, diminished by insulin. Insulin regulates the disposition and utilization of glucose, particularly exogenous glucose. Glucose can enter liver and pancreatic β-cells freely, and high blood glucose signals the latter cells to secrete insulin. This hormone acts through various protein kinase cascades (sometimes called group II hormones) to stimulate entry of glucose and amino acids into certain extra-hepatic tissues that include muscle and fat cells. The presence of glucose stimulates synthesis of glucose oxidation, lipogenesis, and macromolecular precursors such as nucleotides. Chronic insulin therapy as with type I (juvenile onset, insulin-dependent) diabetes may stimulate extrahepatic glucose entry under conditions of inhibited gluconeogenesis, potentiating hypoglycemia while suppressing its correction.

Question 3

The absorption of glucose from the gut into intestinal mucosal cells is coupled to Na+, K+-ATPase. In contrast, the movement of glucose from the intestinal epithelial cells into the submucosal bloodstream occurs through passive transport. Given these facts, which of the following statements is most accurate?

A	Cytosolic levels of glucose in intestinal mucosal cells are regulated by levels of glucose in skeletal muscle cells.
B	Free glucose levels in the lumen of the intestine can never be higher than levels in intestinal cells.
C	Plasma glucose levels are much higher than intestinal cell cytosolic levels of glucose.
D	Levels of glucose in the intestinal lumen are always higher than those in the cytosol of intestinal epithelial cells.
E	Levels of plasma glucose are approximately equal to levels in the cytosol of intestinal epithelial cells.

Question 3 Explanation:

Glucose travels from intestinal cells into plasma by passive diffusion, ensuring that plasma glucose levels will be slightly less or equal to those of the intestinal cytosol (incorrect answers b-d). Glucose levels in other tissues of the body (eg, muscle) will not affect those in intestinal cells (incorrect answer a). Absorption of glucose from the intestinal lumen into intestinal epithelial cells uses active transport, coupling glucose import to a sodium gradient glucose. Sodium ions entering the cell in the company of glucose are pumped out again by Na+, K+-ATPase. Once in the cytosol of the intestinal cell, the glucose moves across the cell and diffuses out of the cell into the interstitial fluid of the submucosa and then into the plasma of the capillaries underlying the intestinal epithelium.

Question 4

Several weight-loss diets are based on decreased carbohydrate intake to prevent stimulation of insulin secretion; insulin drives the feeling of hunger and also inhibits lipolysis. The suggested way of monitoring the success of low carbohydrate intake during fasting reflects the greatest increase in plasma concentration of which of the following?

A	Palmitate, oleate (free fatty acids)
B	Glucose, galactose
C	Lactate, pyruvate
D	Acetoacetate, β-hydroxybutyrate
E	Triacylglycerols

Question 4 Explanation:

Urine ketone measurements by dipstick were recommended for monitoring of success in limiting carbohydrate intake (incorrect answers a-c, e). Under fed conditions, glucose is the preferred fuel for most tissues. However, under starvation conditions, glucose must be reserved for use by the central nervous system. There is a small initial decrease in plasma glucose upon starvation but the concentration levels off after a time because of conversion of glycogen in the liver. Under starvation conditions, ketone bodies are made in the liver and provide a major metabolic fuel source for skeletal and heart muscle and can serve to meet some of the brain’s energy needs. Another effect of starvation and carbohydrate limitation is accelerated lipogenesis and elevated serum cholesterol/triglycerides, which was exaggerated by substituting fat (meat, eggs) in one famous low-carbohydrate diet. These effects may have contributed to the sudden collapse and death of the diet’s founder; low-fat, heart-friendly low carbohydrate diets have since been promoted.

Question 5

Defects in the ability to oxidize fatty acids can be asymptomatic in the presence of frequent feeding; a therapy applied to all but the most severe long chain fatty acid oxidation disorders that are frequently lethal. In asymptomatic children with medium- or short-chain fatty acid oxidation defects, unintentional fasting due to infections can produce lethargy and even coma after 4 to 6 hours without food. A deficit in which of the following compounds in serum most likely causes these neurologic symptoms?

A	Pyruvate and lactate
B	Free fatty acids
C	Propionate and methylmalonate
D	Glucose and ketone bodies
E	Triacylglycerols

Question 5 Explanation:

The brain uses glucose as its major energy source but can also use ketone bodies to provide up to 20% of its energy needs. Other metabolites are not efficient energy sources (incorrect answers a-c, e). Glucose is the preferred metabolic fuel for most tissues. However, extrahepatic tissues such as heart prefer ketone bodies and fatty acids as energy sources over glucose. Children with medium- or short-chain fatty acid oxidation defects can breakdown glycogen over the first 1 to 3 hours of fasting to maintain glucose, but have deficient fat breakdown when glycogen is depleted. They have normal lipases and can mobilize triacylglycerols, but cannot metabolize the fatty acid side chains. Some abnormal ketones are produced as expected from sequential breakdown of fatty acyl-CoAs in two-carbon steps—dehydrogenase to make β-double bond, enolase to make β-hydroxyl, dehydrogenase to make β-ketone, and thiolase to cleave off acetyl-CoA and an acyl-CoA two carbons shorter. A block like that in medium chain-CoA dehydrogenase (MCAD) deficiency (MIM*201450) will cause accumulation of 8 to 12 carbon β-hydroxyl and ketone β-compounds, but these will have much less serum concentrations and metabolic efficiency than the normal 4-carbon acetoacetate, β-hydroxybutyrate, and 2-carbon acetone. Even greater deficiency of ketone bodies with long chain oxidation defects may explain their early and lethal cardiac failure. Lactate but not pyruvate will be deficient in mitochondrial pyruvate dehydrogenase deficiency (eg, MIM*300502), while propionate and methylmalonate will accumulate in disorders such as methylmalonic aciduria (MIM*251000).

Question 6

A 25-year-old African American male seeks evaluation for increasing fatigue and muscle aches during exercise. He recalls avoiding athletics when young because he could not keep up with his peers and recently has embarked on an exercise program to help lose weight. Stress testing reveals normal cardiac function, but serial blood tests show increased serum lactic acid and ammonia levels when he is at rest. His physician postulates a mitochondrial DNA depletion disorder that affects mitochondrial function due to defects in mitochondrial DNA polymerase or tRNAs. Which of the following options: (1) list mitochondrial pathways that would explain the man’s blood findings, and (2) name a compound linking these pathways?

A	Glycogenolysis, glycolysis, and glucose 1-P
B	Glycogen synthesis, gluconeogenesis, and pyruvate
C	Citric acid cycle, electron transport, and isocitrate
D	Citric acid and urea cycles, and arginine
E	Citric acid and urea cycles, and fumarate

Question 6 Explanation:

The citric acid and urea cycle are located primarily in the mitochondria, while pathways for glycogen and glucose metabolism are primarily in the cytosol (incorrect answers a, b). Isocitrate and fumarate are intermediates of the citric acid cycle, but only fumarate is an intermediate of both the citric acid and urea cycles (incorrect answer c). Fumarate and arginine are produced from argininosuccinate in the urea cycle, after which arginine is converted to ornithine to yield urea and fumarate is converted to malate and then oxaloacetate in the citric acid cycle. Depending upon the organism’s needs, oxaloacetate can either enter gluconeogenesis or react with acetyl-CoA to form citrate and traverse the citric acid cycle with generation of NADH and then energy through oxidative phosphorylation. Starvation will simultaneously increase urea cycle activity to remove ammonia from protein degradation and increase fumarate for oxaloacetate production; the oxaloacetate is then available for gluconeogenesis to maintain glucose levels that are essential for brain metabolism. Depletion of mitochondrial pathway functions will mainly impact electron transport and cause more dependence on glycolysis and lactate production as exercise depletes blood oxygen saturation. A vicious cycle of exercise intolerance leading to muscle inactivity with fewer mitochondria exacerbates myopathy (muscle weakness and incoordination) in mitochondrial disorders.

Question 7

Which of the following can be converted to an intermediate of either the citric acid cycle or the urea cycle?

A	Tyrosine
B	Lysine
C	Leucine
D	Tryptophan
E	Aspartate

Question 7 Explanation:

Aspartate is a glucogenic amino acid that is also used to carry NH4+ into the urea cycle. Aspartate aminotransferase catalyzes the direct transamination of aspartate to oxaloacetate: aspartate + α-ketoglutarate → oxaloacetate + glutamate Oxaloacetate may either be utilized in the citric acid cycle or undergo gluconeogenesis. Argininosuccinate synthetase catalyzes the condensation of citrulline and aspartate to form argininosuccinate: citrulline + aspartate + ATP → argininosuccinate + AMP + PPi In this manner, one of the two nitrogens of urea is introduced into the urea cycle.

Question 8

An 18-year-old Caucasian female is admitted to the hospital for evaluation of extreme weight loss over the past 6 months. She has decreased subcutaneous tissue, sores on the back of her fingers, severe dental decay, and sparse hair. Her parents report that she jogs each morning despite her weight loss, yet is preoccupied with food. A diagnosis of anorexia nervosa with bulimia is made and the female is started on a counseling program with intravenous feeding. Which of the following pathways would be least affected by this female’s catabolic state?

A	Lipolysis
B	Glycolysis
C	β-Oxidation of fatty acids
D	Citric acid cycle
E	Gluconeogenesis

Question 8 Explanation:

The citric acid cycle generates energy (through production of NADH/FADH2) and precursors of anabolic pathways (such as succinic acid); it also catabolizes compounds such as acetyl-CoA to carbon dioxide and water. For these reasons, the citric acid cycle is often called an amphibolic pathway. Catabolic pathways degrade protein, lipid, and carbohydrate to acetyl-CoA to produce energy and conserve essential molecules such as glucose when there are caloric deficits as in starvation or eating disorders (incorrect answers a-c, e). Anabolic pathways store energy through conversion of foodstuffs to glycogen or fat during caloric excess (fed states). In general, the catabolic and anabolic pathways may share metabolites but involve different enzyme steps and cofactors that yield or consume energy (glycolysis vs gluconeogenesis, lipolysis and β-oxidation of fatty acids vs fatty acid synthesis, glycogenolysis vs glycogenesis).

Question 9

A 46-year-old Caucasian female has had spells of irritability, headaches, and trembling when she does not eat for several hours. She is hospitalized overnight for metabolic studies that demonstrate a low blood sugar in the presence of adequate gluconeogenesis. Which of the following would be an indicator of active gluconeogenesis?

A	Increased urea production
B	Increased conversion of alanine to pyruvate
C	Increased conversion of acetyl-CoA to malonyl-CoA
D	Increased conversion of oxaloacetate to citrate
E	Increased conversion of glucose 6-phosphate to glucose 1-phosphate

Question 9 Explanation:

Transamination of alanine to pyruvate, like that of other amino acids, provides citric acid cycle intermediates leading to oxaloacetate and gluconeogenesis. Urea is excreted (incorrect answer a) although fumarate from the urea cycle is a precursor to oxaloacetate,conversion of acetyl-CoA to malonyl-CoA initiates fatty acid synthesis (incorrect answer c), and diversion of oxaloacetate to citrate rather than pyruvate or glucose 6- P to glucose 1-P rather than glucose would decrease gluconeogenesis (incorrect answers d, e). Many amino acids are degraded and transaminated to yield citric acid cycle intermediates, including arginine, histidine, proline, and glutamine to glutamate; and then α-ketoglutarate, tyrosine, and phenylalanine to fumarate, asparagine to aspartate, and then to oxaloacetate. Others such as cysteine, glycine, serine, and threonine can be converted to alanine and transaminated to pyruvate that directly contributes to gluconeogenesis (through pyruvate carboxylase and phosphoenolpyruvate carboxykinase). During the early phases of starvation, the catabolism of proteins is at its highest level. Anabolic enzymes, which are not utilized during starvation, are targeted for degradation (with ubiquitin) and their synthesis repressed. The transamination of amino acids is a first step in amino acid degradation and also yields ketoacids for gluconeogenesis. The protein and amino acid degradation with ketogenesis results in a negative nitrogen balance, increasing ammonia and urea levels in the urine (from the urea cycle). The glucose formed from gluconeogenic amino acids becomes the major source of blood glucose following depletion of liver glycogen stores. Complete oxidation of this glucose, as well as the ketone bodies formed from ketogenic amino acids, leads to a relative increase in the CO2 and H2O formed from amino acid carbon skeletons.

Question 10

A 60-year-old Egyptian male has a hemoglobin of 12 g/dL (13.5 lower limit of normal) during his annual checkup and was taking Bactrim (antibiotic with trimethoprim folate inhibitor and a sulfonamide) for a skin infection. The medical student who evaluated him had suggested a diagnosis of glucose-6-phosphate dehydrogenase (G6PD) deficiency (MIM*305900), recognizing the patient’s Mediterranean origin and exposure to sulfa antibiotics that can precipitate hemolysis in these individuals. The attending suggested G6PD deficiency would give much more severe symptoms of rapid hemolysis, severe anemia, and heart Given that PHI converts glucose 6-phosphate to fructose 6-phosphate and G6PD initiates the reverse conversion (with several subsequent enzymes), which of the following statements do these different clinical presentations support?

A	Chemical conversions in metabolic pathways often use different enzymes and intermediates than reactions proceeding in the reverse direction.
B	The reaction steps for degradative (catabolic) pathways are the exact reverse of those for biosynthetic sequence.
C	Enzymes found in an anabolic pathway are never found in the corresponding catabolic pathway.
D	The first steps in a metabolic pathway are rarely rate limiting or subject to regulation.
E	Steps in both anabolic and catabolic pathways are usually reversible.

Question 10 Explanation:

Chemical conversions in metabolic pathways usually employ different enzymes and intermediates than those occurring in the reverse direction (eg, phosphohexose isomerase—PHI enzyme) (MIM*172400) converting glucose 6-phosphate to fructose 6-phosphate but several enzymes including G6PD (MIM*305900) to accomplish the reverse conversion (incorrect answers b, e). The first steps of metabolic pathways are often rate limiting and subject to regulation, allowing shifts to the appropriate pathway during times of energy excess/depletion or synthetic product excess/deficiency (incorrect answer d). Thus, oxaloacetate or pyruvate can be diverted toward the citric acid cycle to generate energy or toward gluconeogenesis to generate glucose, using different enzymes and intermediates in each case. Some enzymes such as PHI occur in different pathways but are not rate limiting (incorrect answer d)

Question 11

Which of the following statements is true in the well-fed state?

A	NADPH production by the hexose monophosphate shunt is decreased.
B	Acetoacetate is the major fuel for muscle.
C	Glucose transport into adipose tissue is decreased.
D	The major fuel used by the brain is glucose.
E	Amino acids are utilized for glucose production.

Question 11 Explanation:

Glucose is the major fuel for the brain in the well-fed state. The brain requires a continuous supply of glucose at all times. In fact, if glucose drops to a low level, convulsions may follow. However, during starvation or fasting, the brain is capable of obtaining approximately 75% of its energy from circulating ketone bodies. During the absorptive phase, ketone bodies such as acetoacetate and 3-hydroxybutyrate are low (with glucose rather than these ketones used by muscle—incorrect answer b). Circulating amino acids are utilized for protein synthesis, not glucose production (incorrect answer e). Liver production of NADPH is at a high level because it is needed for fatty acid synthesis (incorrect answer a). Glucose is actively transported into all cells, including adipocytes, which require it to form glycerol-3-phosphate for esterifying fatty acids into triacylglyceride (incorrect answer c).

Question 12

The jinga bean, found in the jungles of Brazil, is unique in that it is composed almost exclusively of protein. Studies have shown that, immediately following a meal composed exclusively of jinga beans, which of the following occurs?

A	A decreased release of epinephrine
B	A complete absence of liver glycogen
C	Hypoglycemia
D	An increased release of insulin
E	Ketosis caused by the metabolism of ketogenic amino acids

Question 12 Explanation:

High blood levels of amino acids, in addition to glucose, promote the release of insulin through their action on receptors at the surface of the β-cells of the pancreas. Although insulin alone could lead to a hypoglycemic effect, hypoglycemia should not be observed because glucagon is also released in response to the elevated levels of circulating amino acids (incorrect answer c). The balance of glucagon and glucose tends to keep blood levels of glucose within normal ranges while amino acid transport into cells is promoted. Because of the normal insulin levels in the fed state, ketosis and depletion of liver glycogen are not observed (incorrect answers b, e). Both of these events occur during fasting and starvation due to the abundance of glucagon and epinephrine (incorrect answer a) in the blood as opposed to the low levels of insulin.

Question 13

Approximately 3 hours following a well-balanced meal, blood levels of which of the following are elevated?

A	Fatty acids
B	Glucagon
C	Glycerol
D	Epinephrine
E	Chylomicrons

Question 13 Explanation:

Following digestion, the products of digestion enter the bloodstream. These include glucose, amino acids, triacylglycerides packaged into chylomicrons from the intestine, and very low density lipoproteins from the liver. The hormone of anabolism, insulin, is also elevated because of the signaling of the glucose and amino acids in the blood, which allows release of insulin from the β-cells of the pancreas. Insulin aids the movement of glucose and amino acids into cells. In contrast, all the hormones and energy sources associated with catabolism are decreased in the blood during this time. Long-chain fatty acids and glycerol released by lipolysis from adipocytes are not elevated. Glucagon and epinephrine are not released. The only time glucose levels rise significantly above approximately 80 mM is following a well-balanced meal when glucose is obtained from the diet. The concentration of glucose reaches a peak 30 to 45 minutes after a meal and returns to normal within 2 hours after eating. This response of blood glucose after eating (mimicked by giving 50 g of oral glucose) is the basis for the glucose tolerance test. In the event of insulin deficiency (diabetes mellitus), the peak glucose concentration is abnormally high and its return to normal is delayed.

Question 14

If a homogenate of liver cells is centrifuged to remove all cell membranes and organelles, which of the following enzyme activities will remain in the supernatant?

A	Glucose-6-phosphate dehydrogenase
B	Glycogen synthetase
C	Aconitase
D	Acyl-CoA hydratase
E	Hydroxybutyrate dehydrogenase

Question 14 Explanation:

Centrifugation of a cellular homogenate at a force of 100,000 × g will pellet all cellular organelles and membranes. Only soluble cellular molecules found in the cytosol will remain in the supernatant. Thus, the enzymes of glycolysis and most of those of gluconeogenesis, fatty acid synthesis, and the pentose phosphate pathway will be in the supernatant. Glucose-6-phosphate dehydrogenase, which results in the formation of 6-phosphoglucono-δ-lactone from glucose 6-phosphate, is the committed step in the pentose phosphate pathway. In the pellet will be the enzymes within mitochondria, including those of the citric acid cycle (aconitase), fatty acid β-oxidation (acyl-CoA hydratase), and ketogenesis (hydroxybutyrate dehydrogenase). Enzymes of glycogen degradation and synthesis (glycogen synthetase) will also be in the pellet associated with glycogen particles.

Question 15

Which of the following enzymes is active in adipocytes following a heavy meal?

A	Glycogen phosphorylase
B	Glycerol kinase
C	Hormone-sensitive triacylglyceride lipase
D	Glucose-6-phosphatase
E	Phosphatidate phosphatase

Question 15 Explanation:

The enzyme phosphatidate phosphatase converts phosphatidic acid to diacylglycerol during synthesis of triacylglycerides. The function of adipose tissue is the storage of fatty acids as triacylglycerols in times of plenty and the release of fatty acids during times of fasting or starvation. Fatty acids taken in by adipocytes are stored by esterification to glycerol-3-phosphate. Glycerol-3-phosphate is derived almost entirely from the glycolytic intermediate dihydroxyacetone phosphate through the action of glycerol-3-phosphate dehydrogenase. Glycolytic enzymes are active in adipocytes during triglyceride synthesis, but those of glycogen degradation (low levels in adipocytes) and gluconeogenesis (ie, glucose-6-phosphatase) are not. Glycerol kinase is not present to any great extent in adipocytes, so that glycerol freed during lipolysis is not used to reesterify the fatty acids being released. The enzyme triacylglyceride lipase is turned on by phosphorylation by a cyclic AMP-dependent protein kinase following epinephrine stimulation.

Question 16

Which of the following statements correctly apply to energy metabolism?

A	Fatty acids can be precursors of glucose.
B	High energy levels turn on glycolysis.
C	Pyruvate is committed to the citric acid cycle by acetylation through pyruvate dehydrogenase.
D	Phosphorylation activates enzymes that store fat and glycogen.
E	Guanosine triphosphate (GTP) is the major donor for enzyme phosphorylation.

Question 16 Explanation:

The acetylation of pyruvate to citric acid by pyruvate dehydrogenase rather than its reduction by lactate dehydrogenase is a key regulatory step between high energy yields of citric acid cycle/oxidative phosphorylation or lower energy yields of glycolysis (incorrect answers a, b, and d). GTP can be a phosphate donor (eg, in the phosphoenolpyruvate carboxykinase reaction of gluconeogenesis), but ATP is much more common (incorrect answer e). There are certain properties of metabolism that are considered truisms:1. Futile cycles involving useless synthesis and degradation of a fuel do not occur simultaneously. 2. Acetyl-CoA or substances that produce it, such as fatty acids or ketogenic amino acids, cannot be precursors of glucose. 3. ATP is a major phosphate donor and energy source; it must be present in cells at all times in order for them to function. 4. Protein phosphorylation inactivates enzymes that store glycogen and fat and activates enzymes that increase blood glucose and fatty acids. 5. Low blood glucose stimulates gluconeogenesis and glycogenolysis. 6. Low energy levels stimulate glycolysis and lipolysis. 7. High energy levels inhibit glycolysis and β-oxidation of fatty acids.

Question 17

Which of the following diseases reflects the loss of ability to move specific molecules between membrane-separated cellular compartments?

A	McArdle phosphorylase disease
B	Carnitine deficiency
C	Methanol poisoning
D	Ethylene glycol poisoning
E	Diphtheria

Question 17 Explanation:

A deficiency in carnitine, carnitine acyltransferase I, carnitine acyltransferase II, or acylcarnitine translocase can lead to an inability to oxidize long chain fatty acids. This occurs because all of these components are needed to translocate activated long chain (>10 carbons long) fatty acyl-CoA across mitochondrial inner membrane into the matrix where β-oxidation takes place. Once long chain fatty acids are coupled to the sulfur atom of CoA on the outer mitochondrial membrane, they can be transferred to carnitine by the enzyme carnitine acyltransferase I, which is located on the cytosolic side of the inner mitochondrial membrane. Acylcarnitines are transferred across the inner membrane to the matrix surface by translocase. At this point, the acyl group is reattached to a CoA sulfhydryl by the carnitine acyltransferase II located on the matrix face of the inner mitochondrial membrane. McArdle disease (deficiency of muscle glycogen phosphorylase) is one of several glycogen storage diseases. Muscle cramping and fatigue after exercise are characteristics of muscle glycogen storage diseases (types V and VII), whereas hypoglycemia, hyperuricemia, and liver disease are characteristics of liver glycogen storage diseases (types I, III, IV, VI, and VIII)—see Table 3 of the High-Yield Facts. Wood alcohol (methanol) is a cause of death or serious illness (including blindness) among patients who ignorantly substitute it for ethanol or mistakenly ingest it. Ingestion of automotive antifreeze (ethylene glycol) can also result in death if not treated. In both cases, death or serious injury can be averted by quickly administering an intoxicating dose of ethanol. The success of this treatment is based on the fact that methanol and ethylene glycol are not poisons as such. First, they must be converted by the action of the enzyme alcohol dehydrogenase to precursors of potentially toxic substances. Administration of large doses of ethanol inhibits oxidation of both methanol and ethylene glycol by effectively competing as a preferred substrate for the active sites of alcohol dehydrogenase. Over time, methanol and ethylene glycol are excreted. One of the primary killers of children prior to immunization was upper respiratory tract infections by Corynebacterium diphtheriae. Toxin produced by a lysogenic phage that is carried by some strains of this bacteria causes the lethal effects. It is lethal in small amounts because it blocks protein synthesis. The viral toxin is composed of two parts. The B portion binds a cell’s surface and injects the A portion into the cytosol of cells. The A portion ADP ribosylates a unique histidinederived amino acid of the elongation factor 2 (EF-2) known as diphthamide. This action completely blocks the ability of EF-2 to translocate the growing polypeptide chain.

Question 18

Which of the following sets of blood values most closely correlates with a patient who has conducted a hunger strike for 1 month? The blood levels listed by option (a) represent the immediate postprandial state—choose option a if blood levels after the hunger strike will remain the same, or other options if they will change as specified.

CONCENTRATION OF BLOOD FUELS (mM)

A	a
B	b
C	c
D	d
E	e

Question 18 Explanation:

In a normal postabsorptive patient, blood fuel values are 4.5 mM glucose, 0.5 mM free fatty acids, 0.02 mM ketone bodies, and 4.5 mM amino acids as shown beside answer a; fasting will elevate levels of ketone bodies, making answer a incorrect. Following several days of starvation, a catabolic homeostasis has set in, such that free fatty acids ketone bodies have increased concentrations with glucose remaining the same (incorrect answers b-d). At this point, glycogen stores have been depleted. Blood glucose is maintained at about 4.5 mM during starvation by gluconeogenesis of amino acids from protein breakdown. Most of the brain’s fuel supply still derives from glucose at this time—a considerable demand because the brain accounts for at least 20% of the body’s total consumption of fuel. Following prolonged starvation, utilization of glucose and hence catabolism of protein is spared by the induction of brain enzymes that utilize ketone bodies. In prolonged starvation, the blood concentration of amino acids decreases, whereas that of free fatty acids and ketone bodies increases with preservation of blood glucose (correct answer e). A diabetic could have blood values of free fatty acids, ketone bodies, and amino acids but not high levels of blood glucose shown in answer c (answer c incorrect). The lack of insulin does not allow glucose to enter cells (aside from liver), stimulating lipolysis, glycogenolysis, gluconeogenesis, and ketogenesis.

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Part 1 (20 test) | Part 2 (18 test – end)

1. A 14-year-old African American adolescent has shown weight loss and increased thirst with fatigue and decreased athletic performance. His physician does a urinalysis and the reagent strip is strongly positive for glucose and ketones. A likely diagnosis of diabetes mellitus is made and the patient started on long-acting insulin at night with short-acting insulin at breakfast and lunch. Which of the following statements correctly describes insulin?

It is an anabolic signal to cells that glucose is scarce.
It is converted from proinsulin to insulin primarily following secretion from β-cells.
It does not have a prohormone form.
It is a small polypeptide composed of a single chain bridged by disulfide groups.
Its action is antagonistic to that of glucagon.

2. Some individuals with diabetes mellitus are susceptible to rapid drops in blood sugar (hypoglycemia) with lethargy and potential seizures or Such diabetics are called “brittle” and require careful monitoring of glucose levels as proper insulin doses are titrated. Of the many actions of insulin, decrease in which of the following cellular activities best accounts for “brittleness” of certain diabetics?

Plasma membrane transfer of glucose
Glucose oxidation
Gluconeogenesis
Lipogenesis
Formation of ATP, DNA, and RNA

3. The absorption of glucose from the gut into intestinal mucosal cells is coupled to Na+, K+-ATPase. In contrast, the movement of glucose from the intestinal epithelial cells into the submucosal bloodstream occurs through passive Given these facts, which of the following statements is most accurate?

Cytosolic levels of glucose in intestinal mucosal cells are regulated by levels of glucose in skeletal muscle cells.
Free glucose levels in the lumen of the intestine can never be higher than levels in intestinal cells.
Plasma glucose levels are much higher than intestinal cell cytosolic levels of glucose.
Levels of glucose in the intestinal lumen are always higher than those in the cytosol of intestinal epithelial cells.
Levels of plasma glucose are approximately equal to levels in the cytosol of intestinal epithelial cells.

4. Several weight-loss diets are based on decreased carbohydrate intake to prevent stimulation of insulin secretion; insulin drives the feeling of hunger and also inhibits The suggested way of monitoring the success of low carbohydrate intake during fasting reflects the greatest increase in plasma concentration of which of the following?

Palmitate, oleate (free fatty acids)
Glucose, galactose
Lactate, pyruvate
Acetoacetate, β-hydroxybutyrate
Triacylglycerols

5. Defects in the ability to oxidize fatty acids can be asymptomatic in the presence of frequent feeding; a therapy applied to all but the most severe long chain fatty acid oxidation disorders that are frequently lethal. In asymptomatic children with medium- or short-chain fatty acid oxidation defects, unintentional fasting due to infections can produce lethargy and even coma after 4 to 6 hours without food. A deficit in which of the following compounds in serum most likely causes these neurologic symptoms?

Pyruvate and lactate
Free fatty acids
Propionate and methylmalonate
Glucose and ketone bodies
Triacylglycerols

6. A 25-year-old African American male seeks evaluation for increasing fatigue and muscle aches during He recalls avoiding athletics when young because he could not keep up with his peers and recently has embarked on an exercise program to help lose weight. Stress testing reveals normal cardiac function, but serial blood tests show increased serum lactic acid and ammonia levels when he is at rest. His physician postulates a mitochondrial DNA depletion disorder that affects mitochondrial function due to defects in mitochondrial DNA polymerase or tRNAs. Which of the following options: (1) list mitochondrial pathways that would explain the man’s blood findings, and (2) name a compound linking these pathways?

Glycogenolysis, glycolysis, and glucose 1-P
Glycogen synthesis, gluconeogenesis, and pyruvate
Citric acid cycle, electron transport, and isocitrate
Citric acid and urea cycles, and arginine
Citric acid and urea cycles, and fumarate

7. Which of the following can be converted to an intermediate of either the citric acid cycle or the urea cycle?

Tyrosine
Lysine
Leucine
Tryptophan
Aspartate

8. An 18-year-old Caucasian female is admitted to the hospital for evaluation of extreme weight loss over the past 6 months. She has decreased subcutaneous tissue, sores on the back of her fingers, severe dental decay, and sparse Her parents report that she jogs each morning despite her weight loss, yet is preoccupied with food. A diagnosis of anorexia nervosa with bulimia is made and the female is started on a counseling program with intravenous feeding. Which of the following pathways would be least affected by this female’s catabolic state?

Lipolysis
Glycolysis
β-Oxidation of fatty acids
Citric acid cycle
Gluconeogenesis

9. A 46-year-old Caucasian female has had spells of irritability, headaches, and trembling when she does not eat for several hours. She is hospitalized overnight for metabolic studies that demonstrate a low blood sugar in the presence of adequate Which of the following would be an indicator of active gluconeogenesis?

Increased urea production
Increased conversion of alanine to pyruvate
Increased conversion of acetyl-CoA to malonyl-CoA
Increased conversion of oxaloacetate to citrate
Increased conversion of glucose 6-phosphate to glucose 1-phosphate

10. A 60-year-old Egyptian male has a hemoglobin of 12 g/dL (13.5 lower limit of normal) during his annual checkup and was taking Bactrim (antibiotic with trimethoprim folate inhibitor and a sulfonamide) for a skin infection. The medical student who evaluated him had suggested a diagnosis of glucose-6-phosphate dehydrogenase (G6PD) deficiency (MIM*305900), recognizing the patient’s Mediterranean origin and exposure to sulfa antibiotics that can precipitate hemolysis in these individuals. The attending suggested G6PD deficiency would give much more severe symptoms of rapid hemolysis, severe anemia, and heart Given that PHI converts glucose 6-phosphate to fructose 6-phosphate and G6PD initiates the reverse conversion (with several subsequent enzymes), which of the following statements do these different clinical presentations support?

Chemical conversions in metabolic pathways often use different enzymes and intermediates than reactions proceeding in the reverse direction.
The reaction steps for degradative (catabolic) pathways are the exact reverse of those for biosynthetic sequence.
Enzymes found in an anabolic pathway are never found in the corresponding catabolic pathway.
The first steps in a metabolic pathway are rarely rate limiting or subject to regulation.
Steps in both anabolic and catabolic pathways are usually reversible.

11. Which of the following statements is true in the well-fed state?

NADPH production by the hexose monophosphate shunt is decreased.
Acetoacetate is the major fuel for muscle.
Glucose transport into adipose tissue is decreased.
The major fuel used by the brain is glucose.
Amino acids are utilized for glucose production.

12. The jinga bean, found in the jungles of Brazil, is unique in that it is composed almost exclusively of protein. Studies have shown that, immediately following a meal composed exclusively of jinga beans, which of the following occurs?

A decreased release of epinephrine
A complete absence of liver glycogen
Hypoglycemia
An increased release of insulin
Ketosis caused by the metabolism of ketogenic amino acids

13. Approximately 3 hours following a well-balanced meal, blood levels of which of the following are elevated?

Fatty acids
Glucagon
Glycerol
Epinephrine
Chylomicrons

14. If a homogenate of liver cells is centrifuged to remove all cell membranes and organelles, which of the following enzyme activities will remain in the supernatant?

Glucose-6-phosphate dehydrogenase
Glycogen synthetase
Aconitase
Acyl-CoA hydratase
Hydroxybutyrate dehydrogenase

15. Which of the following enzymes is active in adipocytes following a heavy meal?

Glycogen phosphorylase
Glycerol kinase
Hormone-sensitive triacylglyceride lipase
Glucose-6-phosphatase
Phosphatidate phosphatase

16. Which of the following statements correctly apply to energy metabolism?

Fatty acids can be precursors of glucose.
High energy levels turn on glycolysis.
Pyruvate is committed to the citric acid cycle by acetylation through pyruvate dehydrogenase.
Phosphorylation activates enzymes that store fat and glycogen.
Guanosine triphosphate (GTP) is the major donor for enzyme phosphorylation.

17. Which of the following diseases reflects the loss of ability to move specific molecules between membrane-separated cellular compartments?

McArdle phosphorylase disease
Carnitine deficiency
Methanol poisoning
Ethylene glycol poisoning
Diphtheria

18. Which of the following sets of blood values most closely correlates with a patient who has conducted a hunger strike for 1 month? The blood levels listed by option (a) represent the immediate postprandial state—choose option a if blood levels after the hunger strike will remain the same, or other options if they will change as

CONCENTRATION OF BLOOD FUELS (mM)

[MCQs] Integrated Metabolism—Vitamins, Minerals, and Hormones – Part 2 (18 test – end)

Quizzes about Integrated Metabolism—Vitamins, Minerals, and Hormones - Part 2 (18 test - end)

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