MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1d Fatty Acid Protein Metabolism
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1d Fatty Acid Protein MetabolismQuestion 1 of 20

A patient with exercise intolerance is found to carry a homozygous missense variant in the gene encoding medium-chain acyl-CoA dehydrogenase (MCAD), a mitochondrial enzyme used during beta-oxidation of medium-chain fatty acids. During a supervised 18-hour fast, plasma measurements were obtained.

Based on the experimental setup, what is the most likely effect of the mutation on metabolism?

Decreased ketone body production due to reduced acetyl-CoA generation from beta-oxidation, with increased reliance on glucose
Increased ketone body production due to diversion of fatty acids toward cytosolic beta-oxidation
Decreased blood glucose because beta-oxidation directly consumes glucose as a co-substrate in the mitochondrial matrix
Increased urea production because MCAD normally catalyzes oxidative deamination of amino acids during fasting
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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 1d Fatty Acid Protein Metabolism

Practice 1d Fatty Acid Protein Metabolism in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on 1d Fatty Acid Protein Metabolism, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A patient with exercise intolerance is found to carry a homozygous missense variant in the gene encoding medium-chain acyl-CoA dehydrogenase (MCAD), a mitochondrial enzyme used during beta-oxidation of medium-chain fatty acids. During a supervised 18-hour fast, plasma measurements were obtained.

Based on the experimental setup, what is the most likely effect of the mutation on metabolism?

  1. Decreased ketone body production due to reduced acetyl-CoA generation from beta-oxidation, with increased reliance on glucose (correct answer)
  2. Increased ketone body production due to diversion of fatty acids toward cytosolic beta-oxidation
  3. Decreased blood glucose because beta-oxidation directly consumes glucose as a co-substrate in the mitochondrial matrix
  4. Increased urea production because MCAD normally catalyzes oxidative deamination of amino acids during fasting
Explanation: This question tests understanding of beta-oxidation defects and their metabolic consequences during fasting. MCAD catalyzes the first dehydrogenation step in beta-oxidation of medium-chain fatty acids, and its deficiency impairs the breakdown of these fatty acids into acetyl-CoA. During fasting, reduced acetyl-CoA production from beta-oxidation leads to decreased ketone body synthesis in the liver, forcing increased reliance on glucose for energy. The correct answer A accurately describes this metabolic shift with reduced ketogenesis and increased glucose utilization. Choice B incorrectly suggests increased ketone production, which is impossible when beta-oxidation is impaired since acetyl-CoA is the substrate for ketogenesis. When evaluating metabolic defects, trace the pathway from substrate to product to predict downstream effects.

Question 2

To probe insulin+cs effect on lipid handling in muscle, researchers measure respiratory quotient (RQ) and plasma FFAs before and after insulin infusion during euglycemic clamp.

Observations:

  • RQ increases from 0.80 to 0.92
  • Plasma FFAs decrease by 50%

What conclusion can be drawn about the role of insulin in fat storage?

  1. Insulin reduces carbohydrate oxidation, and the rise in RQ reflects preferential oxidation of long-chain fatty acids
  2. Insulin increases fatty acid oxidation by activating CPT1, which raises RQ toward 1.0
  3. Insulin increases lipolysis, and the rise in RQ reflects increased ketone oxidation
  4. Insulin shifts whole-body fuel utilization toward carbohydrate oxidation and away from fatty acid oxidation, consistent with reduced circulating FFAs (correct answer)
Explanation: This question tests understanding of fatty acid metabolism, focusing on insulin's shift in fuel utilization. Insulin promotes glucose oxidation and inhibits lipolysis, reducing fatty acid oxidation; this raises respiratory quotient (RQ) toward 1.0 and lowers plasma FFAs. Observations of increased RQ and decreased FFAs confirm insulin's preference for carbohydrate over fat oxidation. Choice D is correct as it describes the shift toward carbohydrate oxidation and reduced FFAs. Choice B is incorrect because insulin inhibits, not activates, CPT1, decreasing fatty acid oxidation. In clamp studies, use RQ to infer fuel mix and correlate with metabolites like FFAs. Recall RQ values: ~0.7 for fat, ~1.0 for glucose.

Question 3

Investigators measure initial velocity of carnitine acetyltransferase using acetyl-CoA as the variable substrate and excess carnitine. CoA release is monitored.

Data (acetyl-CoA, ;cM ;2 v0, ;cM/min): 2 ;2 1.0 5 ;2 2.2 10 ;2 3.6 20 ;2 4.6 50 ;2 5.0

Which outcome is most consistent with the data on enzyme activity?

  1. The data indicate the reaction is zero-order at all substrate concentrations because transport is rate-limiting
  2. The enzyme rate will decrease at higher acetyl-CoA because acetyl-CoA is a product of the reaction
  3. The enzyme is only active in the cytosol, so mitochondrial acetyl-CoA cannot be used as a substrate
  4. The enzyme exhibits saturable kinetics with respect to acetyl-CoA, approaching a maximal rate at higher concentrations (correct answer)
Explanation: This question tests understanding of fatty acid metabolism, focusing on carnitine acetyltransferase kinetics. The enzyme transfers acetyl groups from acetyl-CoA to carnitine, exhibiting saturable Michaelis-Menten kinetics. Data show v0 increasing to a plateau at higher acetyl-CoA, indicating saturation. Choice D is correct as it describes saturable kinetics approaching Vmax. Choice B is incorrect because acetyl-CoA is the substrate, driving the forward reaction. In shuttle enzyme assays, ensure excess cosubstrate and monitor appropriate products. Recall the enzyme's role in acetyl group transport across membranes.

Question 4

Purified human pancreatic lipase is assayed with increasing concentrations of a triglyceride emulsion substrate. Fatty acid release is measured over the first minute to approximate initial velocity.

Data (substrate, mg/mL ;2 v0, ;cmol FA/min): 0.1 ;2 1.2 0.2 ;2 2.1 0.5 ;2 3.8 1.0 ;2 4.6 2.0 ;2 4.9

Which outcome is most consistent with the data on enzyme activity?

  1. The plateau indicates the reaction has reversed and is now synthesizing triglycerides from fatty acids
  2. The lipase rate will double with every doubling of substrate because lipolysis is first-order at all concentrations
  3. The lipase must be mitochondrial because fatty acid release plateaus, indicating respiratory control
  4. The lipase approaches a maximal rate at higher substrate, consistent with saturation of catalytic sites (correct answer)
Explanation: This question tests understanding of fatty acid metabolism, specifically kinetics of pancreatic lipase in triglyceride hydrolysis. Lipase catalyzes fatty acid release from triglycerides, following saturable kinetics at increasing substrate concentrations. Data show v0 rising but approaching a plateau, indicating active-site saturation. Choice D is correct as it describes maximal rate achievement consistent with saturation. Choice B is incorrect because it assumes perpetual first-order kinetics, ignoring Vmax limitations. For lipolytic enzyme assays, use initial velocity to avoid product inhibition and plot data for hyperbolic fit. Recall lipases' interfacial activation but note saturation still applies.

Question 5

A patient has a mutation that reduces activity of medium-chain acyl-CoA dehydrogenase (MCAD). During an overnight fast, plasma acylcarnitine profiling shows elevated medium-chain acylcarnitines, while urinary ketones are low despite hypoglycemia.

Based on the experimental setup, what is the most likely effect of the mutation on metabolism?

  1. Impaired mitochondrial b2-oxidation reduces acetyl-CoA supply for ketogenesis, lowering ketone production during fasting (correct answer)
  2. Enhanced peroxisomal oxidation increases acetyl-CoA export to cytosol, increasing ketone production
  3. Blocked fatty acid synthesis decreases malonyl-CoA, directly causing hypoglycemia via reduced glycogen breakdown
  4. Increased gluconeogenesis from fatty acids compensates, so ketones decrease because glucose rises
Explanation: This question tests understanding of fatty acid metabolism, specifically medium-chain acyl-CoA dehydrogenase (MCAD) deficiency's effect on β-oxidation and ketogenesis. MCAD is crucial for mitochondrial β-oxidation of medium-chain fatty acids, producing acetyl-CoA for ketogenesis during fasting. The mutation leads to elevated acylcarnitines, low urinary ketones, and hypoglycemia, indicating blocked oxidation and reduced ketone production. Choice A is correct because impaired β-oxidation limits acetyl-CoA for ketogenesis, causing hypoketotic hypoglycemia. Choice B is incorrect as it suggests enhanced peroxisomal oxidation increasing ketones, but MCAD is mitochondrial and deficiency reduces overall oxidation. In similar inborn error cases, link enzyme defect to substrate accumulation and downstream fuel shortages. Check for compensatory pathways like peroxisomal oxidation but note their limitations.

Question 6

In a human study, insulin is infused while maintaining euglycemia. Liver biopsies (obtained for clinical reasons) show increased acetyl-CoA carboxylase (ACC) activity and increased malonyl-CoA concentration compared with pre-infusion. Plasma ketone bodies decrease.

What conclusion can be drawn about the role of insulin in fat storage?

  1. Insulin suppresses lipogenesis by inhibiting ACC, lowering malonyl-CoA and increasing TAG breakdown
  2. Insulin decreases malonyl-CoA, which activates CPT1 and increases ketone body formation
  3. Insulin increases ketone bodies by activating HSL in adipose, increasing FFA delivery to liver
  4. Insulin increases malonyl-CoA, which tends to reduce mitochondrial fatty acid entry and oxidation, favoring lipid storage over ketogenesis (correct answer)
Explanation: This question tests understanding of fatty acid metabolism, particularly insulin's regulation of lipogenesis and ketogenesis via malonyl-CoA. Insulin activates acetyl-CoA carboxylase (ACC), increasing malonyl-CoA, which inhibits CPT1 and mitochondrial fatty acid oxidation, reducing ketogenesis. Biopsies show increased ACC and malonyl-CoA with decreased plasma ketones, aligning with insulin's suppression of oxidation. Choice D is correct as elevated malonyl-CoA reduces fatty acid entry and ketogenesis, favoring storage. Choice B is incorrect because insulin increases, not decreases, malonyl-CoA, inhibiting CPT1. For regulatory studies, trace hormone effects on intermediates like malonyl-CoA and correlate with outcomes like ketone levels. Distinguish between acute and chronic insulin effects on liver metabolism.

Question 7

An enzyme kinetics experiment measures initial velocity of glutamate dehydrogenase (GDH) in isolated human liver mitochondria while varying glutamate concentration; NAD+ is saturating. Ammonia production is quantified.

Data (glutamate, mM ;2 v0, nmol NH3/min): 0.5 ;2 12 1.0 ;2 20 2.0 ;2 30 4.0 ;2 36 8.0 ;2 39

Which outcome is most consistent with the data on enzyme activity?

  1. Increasing glutamate will linearly increase v0v_0 indefinitely because deamination is diffusion-limited
  2. GDH rate should decrease at high glutamate because glutamate is a product, not a substrate, of GDH
  3. GDH is cytosolic, so mitochondrial measurements cannot show saturation behavior
  4. At high glutamate, GDH approaches a maximum rate, so additional glutamate produces diminishing increases in v0v_0 (correct answer)
Explanation: This question tests understanding of protein metabolism, focusing on kinetics of glutamate dehydrogenase (GDH) in amino acid catabolism. GDH catalyzes oxidative deamination of glutamate to α-ketoglutarate and ammonia, exhibiting saturable kinetics with respect to glutamate. The data show v0 increasing but plateauing at higher glutamate, consistent with Michaelis-Menten behavior. Choice D is correct as GDH approaches maximum rate at high substrate, yielding diminishing v0 increases. Choice B is incorrect because glutamate is the substrate, not product, so high levels drive forward reaction. For mitochondrial enzyme kinetics, ensure cofactors like NAD+ are saturating and monitor appropriate products. Recall GDH's role linking amino acid and carbohydrate metabolism.

Question 8

A lab deprives cells of phenylalanine for 6 hours and measures secretion of a phenylalanine-rich plasma protein from hepatocytes. Intracellular mRNA levels for the protein are unchanged, but secreted protein decreases to 55% of control.

Which interpretation best explains the data?

  1. Amino acid limitation reduces translation efficiency for proteins requiring that amino acid, lowering output despite unchanged mRNA (correct answer)
  2. Phenylalanine deprivation increases secretion because fewer aromatic residues reduce folding time
  3. Secretion decreases because phenylalanine is required for transcription initiation, so mRNA must have fallen
  4. Secretion decreases because phenylalanine is produced from tyrosine in humans, causing toxic tyrosine depletion
Explanation: This question tests understanding of protein metabolism, focusing on phenylalanine deprivation's impact on secretion of phenylalanine-rich proteins. Deprivation limits translation of proteins requiring phenylalanine, reducing secretion without affecting mRNA levels. Decreased secreted protein despite stable mRNA indicates translational bottleneck. Choice A is correct because amino acid limitation reduces translation efficiency for dependent proteins. Choice C is incorrect as phenylalanine isn't required for transcription; effect is post-transcriptional. For secretion studies, compare mRNA and protein levels to localize effects. Note essential amino acids' roles in limiting synthesis of specific proteins.

Question 9

A patient has recurrent fasting intolerance. Whole-exome sequencing identifies a variant that increases malonyl-CoA levels in liver by constitutively activating acetyl-CoA carboxylase (ACC). During fasting, plasma ketones are low despite elevated FFAs.

Based on the experimental setup, what is the most likely effect of this alteration on metabolism?

  1. Elevated malonyl-CoA tends to inhibit mitochondrial fatty acid entry, reducing hepatic b2-oxidation and ketone production during fasting (correct answer)
  2. Elevated malonyl-CoA activates CPT1, increasing hepatic fatty acid oxidation and ketogenesis
  3. Elevated malonyl-CoA directly stimulates hormone-sensitive lipase, increasing adipose lipolysis and ketones
  4. Elevated malonyl-CoA blocks glycolysis, forcing acetyl-CoA into ketogenesis and raising ketones
Explanation: This question tests understanding of fatty acid metabolism, particularly elevated malonyl-CoA's effect on ketogenesis. Malonyl-CoA inhibits CPT1, reducing mitochondrial fatty acid entry and β-oxidation, leading to low ketones despite high FFAs in fasting. The variant constitutively activates ACC, raising malonyl-CoA and causing fasting intolerance with hypoketosis. Choice A is correct as elevated malonyl-CoA inhibits entry, reducing oxidation and ketogenesis. Choice B is incorrect because malonyl-CoA inhibits, not activates, CPT1. For regulatory variants, predict effects on downstream pathways like oxidation. Correlate with clinical symptoms like fasting intolerance to confirm.

Question 10

A patient with episodic rhabdomyolysis is found to have a mutation reducing muscle carnitine uptake (systemic carnitine deficiency). During prolonged exercise, plasma shows elevated long-chain acylcarnitines are low, while long-chain acyl-CoA accumulates in muscle biopsy.

Based on the experimental setup, what is the most likely effect of the mutation on metabolism?

  1. Fatty acid oxidation shifts to the cytosol, preserving ATP yield while lowering acyl-CoA accumulation
  2. Increased mitochondrial import of long-chain fatty acids raises acetyl-CoA, causing excessive ketone production in muscle
  3. Reduced carnitine availability limits transport of long-chain fatty acids into mitochondria, decreasing b2-oxidation and ATP generation during exercise (correct answer)
  4. Carnitine deficiency primarily blocks amino acid transamination, reducing urea formation and causing hyperammonemia during exercise
Explanation: This question tests understanding of fatty acid metabolism, specifically carnitine deficiency's impact on muscle β-oxidation during exercise. Carnitine is required for long-chain fatty acid transport into mitochondria via CPT systems; deficiency limits this, reducing ATP from oxidation and causing acyl-CoA accumulation. Elevated plasma acylcarnitines and muscle acyl-CoA during exercise indicate blocked transport and impaired energy production. Choice C is correct because reduced carnitine limits mitochondrial entry, decreasing β-oxidation and ATP. Choice B is incorrect as it suggests increased import and ketones, opposite to deficiency effects. In transport defect cases, look for substrate accumulation patterns to confirm blockade. Consider tissue-specific effects, like muscle fatigue in exercise.

Question 11

A team examines the effect of leucine deprivation on protein synthesis in human skeletal muscle cells. Cells are incubated for 4 hours in complete media or leucine-free media. ATP levels remain stable, but phosphorylation of eIF2;1 (a translation initiation regulator) increases in leucine-free conditions.

Measured outcome: global protein synthesis (normalized)

  • Complete: 1.0
  • Leucine-free: 0.60

Which interpretation best explains the reduction in protein synthesis?

  1. Amino acid limitation triggers signaling that suppresses translation initiation/elongation, reducing global protein synthesis (correct answer)
  2. Leucine deprivation increases protein synthesis by diverting acetyl-CoA away from fatty acid oxidation
  3. Leucine deprivation has no effect because leucine is nonessential and can be synthesized from pyruvate
  4. Protein synthesis decreases because peptide bond formation requires NADH, which is depleted only when leucine is absent
Explanation: This question tests understanding of protein metabolism, particularly how leucine deprivation affects translation regulation. Leucine, an essential amino acid, signals through mTOR to promote translation initiation; its absence increases eIF2α phosphorylation, suppressing global protein synthesis. In leucine-free media, synthesis decreases with elevated eIF2α phosphorylation and stable ATP, indicating regulatory inhibition. Choice A is correct because amino acid limitation triggers signaling that suppresses translation, reducing synthesis. Choice C is incorrect as leucine is essential and cannot be synthesized de novo in humans. To evaluate similar deprivations, check signaling markers like eIF2α and energy levels to pinpoint mechanisms. Differentiate essential from nonessential amino acids in human metabolism.

Question 12

An enzyme activity experiment measures initial velocity of aspartate aminotransferase (AST) while varying aspartate concentration at fixed, saturating αα-ketoglutarate. Oxaloacetate formation is monitored via a coupled assay.

Data (aspartate, mM ;2 v0, units): 0.1 ;2 5 0.2 ;2 9 0.5 ;2 18 1.0 ;2 26 2.0 ;2 30

Which outcome is most consistent with the data on enzyme activity?

  1. The plateau indicates aspartate is being converted into acetyl-CoA, which inhibits the coupled assay
  2. AST rate must be proportional to aspartate at all concentrations because transamination is diffusion-limited
  3. AST catalyzes fatty acid activation, so higher aspartate should reduce oxaloacetate formation
  4. AST shows saturable kinetics with respect to aspartate, so increasing aspartate beyond ~2 mM yields little additional rate increase (correct answer)
Explanation: This question tests understanding of protein metabolism, specifically kinetics of aspartate aminotransferase (AST) in transamination. AST transfers amino groups from aspartate to α-ketoglutarate, producing oxaloacetate, with saturable kinetics. Data show v0 plateauing at higher aspartate, indicating saturation. Choice D is correct as AST saturates beyond ~2 mM, limiting further rate increases. Choice B is incorrect because transamination isn't diffusion-limited but enzyme-bound. In coupled assays, ensure variable substrate is identified and check for hyperbolic patterns. Recall AST's role in malate-aspartate shuttle and amino acid catabolism.

Question 13

A newborn has cardiomyopathy and hypoketotic hypoglycemia. Sequencing identifies a defect in the mitochondrial trifunctional protein (long-chain ββ-oxidation). In a fibroblast assay with palmitate, labeled acetyl-CoA production is reduced, while labeled medium-chain products accumulate.

Based on the experimental setup, what is the most likely effect of the mutation on metabolism?

  1. Impaired long-chain ββ-oxidation causes incomplete fatty acid breakdown, reducing acetyl-CoA generation and ketone production during fasting (correct answer)
  2. Increased long-chain fatty acid import into mitochondria increases acetyl-CoA, causing hyperketonemia
  3. The defect primarily blocks fatty acid synthesis, so palmitate cannot be formed and accumulates as glucose
  4. The defect shifts fatty acid oxidation to the cytosol, maintaining acetyl-CoA output while increasing lactate
Explanation: This question tests understanding of fatty acid metabolism, specifically defects in mitochondrial trifunctional protein affecting long-chain β-oxidation. The protein catalyzes later steps of β-oxidation; deficiency causes incomplete breakdown, reducing acetyl-CoA and ketones, leading to hypoketotic hypoglycemia. Fibroblast assay shows reduced acetyl-CoA and accumulated medium-chain products, confirming blocked oxidation. Choice A is correct because impaired oxidation reduces acetyl-CoA for ketogenesis. Choice B is incorrect as deficiency reduces, not increases, fatty acid import effects. For β-oxidation defects, analyze chain-length specific accumulations and downstream fuel deficits. Consider clinical presentations like cardiomyopathy for long-chain issues.

Question 14

Investigators measure initial velocity of purified human alanine aminotransferase (ALT) at varying alanine concentrations with αα-ketoglutarate held constant. NADH-coupled detection is used to quantify pyruvate formation. Initial rates plateau at high alanine.

Data (alanine, mM ;2 v0, ;cM/min): 0.2 ;2 8 0.5 ;2 18 1.0 ;2 30 2.0 ;2 40 5.0 ;2 46

Which outcome is most consistent with the data on enzyme activity?

  1. ALT catalyzes alanine oxidation in the cytosol, so maximal rate requires mitochondrial alanine transport
  2. ALT activity will continue to rise linearly with alanine concentration because transamination is not saturable
  3. Increasing alanine will decrease v0v_0 by shifting equilibrium toward alanine, reducing pyruvate formation
  4. ALT is saturated with alanine near 5 mM, so further increases in alanine would minimally change v0v_0 at fixed αα-ketoglutarate (correct answer)
Explanation: This question tests understanding of protein metabolism, focusing on enzyme kinetics in amino acid transamination by alanine aminotransferase (ALT). ALT catalyzes the reversible transfer of an amino group from alanine to α-ketoglutarate, producing pyruvate and glutamate, following Michaelis-Menten kinetics with saturation at high substrate levels. The data show initial velocity plateauing at higher alanine concentrations with fixed α-ketoglutarate, indicating enzyme saturation. Choice D is correct because ALT becomes saturated near 5 mM alanine, so further increases yield minimal changes in v0, consistent with saturable kinetics. Choice B is incorrect as it assumes non-saturable, linear kinetics, ignoring the enzyme's finite active sites and typical hyperbolic behavior. For similar kinetics problems, plot or visualize the data to check for hyperbolic saturation and recall that enzymes approach Vmax at high substrate. Also, distinguish between substrates to identify which is variable and potentially limiting.

Question 15

A lab investigates whether a missense variant in branched-chain αα-ketoacid dehydrogenase (BCKDH) alters amino acid catabolism. After a protein-rich meal, plasma shows elevated leucine, isoleucine, and valine, and urine organic acids include increased corresponding αα-ketoacids.

Based on the experimental setup, what is the most likely effect of the mutation on metabolism?

  1. Impaired oxidative decarboxylation of branched-chain αα-ketoacids decreases their entry into downstream catabolism, causing accumulation of BCAAs and their ketoacids (correct answer)
  2. Increased transamination of BCAAs directly increases urea cycle flux, lowering plasma BCAAs after meals
  3. Blocked cytosolic ββ-oxidation prevents breakdown of BCAAs, which normally occurs in peroxisomes
  4. Enhanced ketogenesis consumes BCAA-derived acetyl-CoA, preventing ketoacid accumulation in urine
Explanation: This question tests understanding of protein metabolism, focusing on branched-chain amino acid (BCAA) catabolism defects. Branched-chain α-ketoacid dehydrogenase (BCKDH) catalyzes oxidative decarboxylation of BCAA-derived ketoacids, enabling their breakdown; deficiency causes accumulation. Post-meal elevated BCAAs and ketoacids indicate impaired catabolism at the BCKDH step. Choice A is correct because blocked decarboxylation prevents downstream catabolism, leading to BCAA and ketoacid buildup. Choice B is incorrect as increased transamination wouldn't elevate BCAAs; deficiency affects post-transamination steps. In amino acid disorder analyses, identify the blocked step and predict accumulating precursors. Use meal challenges to reveal catabolic deficiencies.

Question 16

To probe how amino acid availability constrains protein synthesis, researchers cultured primary human myotubes for 6 hours in complete medium or medium lacking leucine. They then added puromycin for 10 minutes and quantified puromycin incorporation into nascent polypeptides (a proxy for translation rate). Total cellular mRNA levels for a housekeeping gene were unchanged between conditions.

Which outcome is most consistent with the data on enzyme activity (translation machinery function) under leucine deprivation?

  1. Increased puromycin incorporation because leucine depletion stimulates global translation to compensate for missing amino acids
  2. Decreased puromycin incorporation because limited charged Leu-tRNA reduces elongation despite unchanged mRNA abundance (correct answer)
  3. No change in puromycin incorporation because ribosomes can substitute isoleucine for leucine during elongation
  4. Increased puromycin incorporation because leucine deprivation directly increases aminoacyl-tRNA synthetase catalytic rate
Explanation: This question tests understanding of protein metabolism regulation, specifically how amino acid availability controls translation through charged tRNA levels. Leucine is an essential amino acid that must be attached to its cognate tRNA by leucyl-tRNA synthetase for incorporation during translation elongation. When leucine is depleted, the pool of charged Leu-tRNA decreases, causing ribosomes to stall at leucine codons during elongation, thereby reducing overall translation rate despite unchanged mRNA levels. Puromycin incorporation serves as a readout for active translation because it mimics aminoacyl-tRNA and terminates growing peptide chains. Choice A is incorrect because amino acid deprivation typically suppresses, not stimulates, global translation through mechanisms like GCN2 kinase activation and eIF2α phosphorylation. To assess translation regulation, distinguish between changes in mRNA abundance versus ribosome activity, and remember that amino acid availability affects elongation through charged tRNA pools.

Question 17

In a study of human skeletal muscle homogenates, investigators measured carnitine palmitoyltransferase I (CPT1) activity by tracking formation of palmitoylcarnitine from palmitoyl-CoA. Reactions were run at fixed enzyme concentration with increasing palmitoyl-CoA, either without malonyl-CoA or with 10 µM malonyl-CoA (a physiologic inhibitor during the fed state). Initial rates were recorded over the first 60 seconds.

Which outcome is most consistent with the data on enzyme activity?

  1. Malonyl-CoA increases VmaxV_{max} by improving CPT1 catalytic turnover while leaving KmK_m unchanged
  2. Malonyl-CoA decreases the apparent affinity of CPT1 for palmitoyl-CoA, shifting the substrate–rate curve rightward without requiring a change in enzyme amount (correct answer)
  3. Malonyl-CoA increases mitochondrial matrix beta-oxidation by promoting acyl-CoA entry into the cytosol for oxidation
  4. Malonyl-CoA increases the measured rate by serving as an alternative substrate that is converted into palmitoylcarnitine
Explanation: This question tests understanding of enzyme kinetics and regulation of fatty acid metabolism, specifically how malonyl-CoA inhibits CPT1. CPT1 catalyzes the rate-limiting step of fatty acid oxidation by converting fatty acyl-CoA to acylcarnitine for mitochondrial entry. Malonyl-CoA acts as a competitive inhibitor of CPT1, competing with palmitoyl-CoA for the active site, which increases the apparent Km (decreases affinity) without changing Vmax. The correct answer B accurately describes this competitive inhibition pattern where the substrate-rate curve shifts rightward. Choice A incorrectly suggests malonyl-CoA increases Vmax, which would indicate activation rather than inhibition. To identify competitive inhibition, look for increased Km with unchanged Vmax when inhibitor is present.

Question 18

Researchers cultured human hepatocytes in media containing either all essential amino acids (control) or media lacking only leucine. Cells were then exposed to a fixed insulin concentration and pulsed with labeled methionine for 20 minutes to estimate global protein synthesis. ATP levels and cell viability were unchanged between conditions.

Which outcome is most consistent with the data on protein synthesis under leucine deficiency?

  1. Increased protein synthesis because leucine deficiency upregulates translation initiation to compensate for missing amino acids
  2. Unchanged protein synthesis because leucine can be synthesized from acetyl-CoA during the pulse period
  3. Decreased protein synthesis because translation elongation stalls at leucine codons despite adequate energy status (correct answer)
  4. Decreased protein synthesis because insulin blocks amino acid uptake and thereby prevents charging of tRNAs
Explanation: This question tests understanding of essential amino acid requirements for protein synthesis. Leucine is an essential amino acid that cannot be synthesized by human cells and must be obtained from the diet. During translation, the absence of leucine causes ribosomes to stall at leucine codons because leucyl-tRNA cannot be formed, halting elongation despite adequate ATP and other resources. The correct answer C accurately describes this translation block at leucine codons. Choice B incorrectly claims leucine can be synthesized from acetyl-CoA, which is impossible in humans as we lack the necessary biosynthetic enzymes for branched-chain amino acids. Remember that essential amino acids create absolute requirements for protein synthesis that cannot be bypassed.

Question 19

To probe protein catabolism during prolonged fasting, investigators measured urea nitrogen production in healthy adults before and after administration of a drug that inhibits hepatic transaminases (but does not affect renal clearance). Participants were kept on the same caloric restriction protocol throughout.

Which outcome is most consistent with inhibiting transamination during fasting?

  1. Decreased ketone bodies because transamination is required to transport long-chain fatty acids into mitochondria
  2. Increased urea production because transamination normally consumes free ammonia and prevents urea formation
  3. Unchanged urea production because urea is generated directly from fatty acid beta-oxidation in mitochondria
  4. Decreased urea production because limiting amino group transfer reduces substrate flow into deamination and the urea cycle (correct answer)
Explanation: This question tests understanding of amino acid catabolism and urea cycle regulation during fasting. Transaminases catalyze the transfer of amino groups from amino acids to α-ketoglutarate, producing glutamate which can then be deaminated to release ammonia for urea synthesis. Inhibiting transaminases blocks this initial step of amino acid catabolism, reducing the flow of nitrogen into the urea cycle and decreasing urea production during protein breakdown in fasting. The correct answer D accurately describes this reduction in urea synthesis. Choice B incorrectly suggests transaminases consume ammonia, when they actually facilitate its eventual release through the glutamate dehydrogenase reaction. To trace nitrogen flow, follow amino groups from proteins through transamination to deamination to urea.

Question 20

In an ex vivo assay, human adipose tissue slices were incubated with either epinephrine alone or epinephrine plus insulin. Glycerol release into the medium over 30 minutes was used as a proxy for triglyceride breakdown. Tissue mass and perfusion were equivalent across conditions.

What conclusion can be drawn about the role of insulin in fat storage?

  1. Insulin enhances epinephrine-stimulated lipolysis, increasing glycerol release due to activation of protein kinase A
  2. Insulin attenuates epinephrine-stimulated lipolysis, decreasing glycerol release consistent with promoting triglyceride storage (correct answer)
  3. Insulin increases glycerol release by directly converting triglycerides into glycerol-3-phosphate for gluconeogenesis
  4. Insulin has no effect because glycerol release reflects beta-oxidation rate in mitochondria rather than lipolysis
Explanation: This question tests understanding of hormonal cross-talk in adipose tissue lipolysis regulation. Epinephrine stimulates lipolysis through β-adrenergic receptors, activating PKA to phosphorylate and activate hormone-sensitive lipase, leading to triglyceride breakdown and glycerol release. Insulin opposes this effect by activating phosphodiesterase and phosphatases, reducing PKA activity and HSL phosphorylation, thereby attenuating epinephrine-stimulated lipolysis. The correct answer B accurately describes insulin's anti-lipolytic effect even in the presence of epinephrine. Choice A incorrectly suggests insulin enhances lipolysis, which contradicts insulin's fundamental role in promoting storage and opposing catabolism. Remember that insulin generally opposes catabolic signals to maintain energy storage.