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

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.

Question 1

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

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 3

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 $v_0$ 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 $v_0$ at fixed b1-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 4

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 5

1. Increasing glutamate will linearly increase $v_0$ 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 $v_0$ (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 6

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 7

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 8

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 9

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 10

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 11

1. Malonyl-CoA increases $V_{max}$ by improving CPT1 catalytic turnover while leaving $K_m$ 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 12

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 13

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 14

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.

Question 15

1. Impaired transport of long-chain acyl groups into mitochondria, reducing beta-oxidation and ketone production during fasting (correct answer)
2. Impaired transport of fatty acids into the cytosol, reducing fatty acid synthesis and thereby decreasing ketones
3. Increased beta-oxidation because carnitine normally inhibits CPT2 in the mitochondrial matrix
4. Increased ketone production because carnitine deficiency forces amino acids to be converted into fatty acids for oxidation

Explanation: This question tests understanding of carnitine's essential role in fatty acid oxidation. Carnitine is required for transporting long-chain fatty acyl groups across the inner mitochondrial membrane via the carnitine shuttle (CPT1 and CPT2 system). Without adequate carnitine, long-chain fatty acids cannot enter mitochondria for beta-oxidation, preventing acetyl-CoA generation and subsequent ketone body production during fasting, despite elevated free fatty acids in plasma. The correct answer A accurately describes this transport defect and its consequences. Choice B incorrectly places the defect in cytosolic transport, when the actual problem is mitochondrial entry for beta-oxidation. When evaluating metabolic defects, identify the specific transport or enzymatic step affected and trace the consequences.

Question 16

1. Leucine depletion increases translation initiation, raising protein synthesis and diverting acetyl-CoA away from lipid storage
2. Leucine depletion decreases global protein synthesis via stress signaling, which can reduce apolipoprotein production and favor intracellular triglyceride accumulation (correct answer)
3. Leucine depletion directly activates hormone-sensitive lipase, lowering triglyceride content by increasing lipolysis in hepatocytes
4. Leucine depletion increases mitochondrial b2-oxidation by raising malonyl-CoA, thereby preventing triglyceride accumulation

Explanation: This question tests understanding of amino acid sensing, protein synthesis regulation, and its effects on lipid metabolism. Leucine is an essential amino acid that serves as a nutrient signal for protein synthesis through the mTOR pathway. Leucine depletion activates stress responses including phosphorylation of eIF2α, which globally decreases translation initiation and protein synthesis, as shown by reduced methionine incorporation. When protein synthesis decreases, hepatocytes produce fewer apolipoproteins needed for VLDL assembly and secretion, causing triglycerides to accumulate intracellularly rather than being exported. Choice A is incorrect because leucine depletion decreases, not increases, translation initiation through stress signaling pathways. When analyzing metabolic stress responses, consider how changes in one pathway (protein synthesis) can indirectly affect another (lipid export).

Question 17

1. Insulin potentiates epinephrine signaling to increase HSL activation and accelerate triglyceride breakdown
2. Insulin antagonizes catecholamine-stimulated lipolysis, decreasing HSL activation and reducing release of glycerol and free fatty acids (correct answer)
3. Insulin lowers glycerol release by inhibiting fatty acid uptake into adipocytes, preventing triglyceride synthesis
4. Insulin decreases glycerol release by increasing mitochondrial b2-oxidation in adipocytes, consuming stored triglyceride

Explanation: This question tests understanding of insulin's anti-lipolytic effects and the regulation of hormone-sensitive lipase (HSL). HSL catalyzes the rate-limiting step of lipolysis in adipocytes, and its activity is increased by PKA-mediated phosphorylation in response to catecholamines like epinephrine. Insulin antagonizes this process by activating phosphodiesterases that degrade cAMP and by activating protein phosphatases that dephosphorylate HSL, thereby reducing its activity. The experimental data shows that insulin pretreatment reduces PKA-mediated HSL phosphorylation and glycerol release (a marker of lipolysis), demonstrating insulin's role in promoting fat storage by inhibiting triglyceride breakdown. Choice A is incorrect because insulin opposes, rather than potentiates, catecholamine-stimulated lipolysis. When analyzing hormone interactions, consider how insulin's anabolic effects oppose catabolic signals.

Question 18

1. Increased mitochondrial oxidation of octanoate due to compensatory upregulation of long-chain acyl-CoA dehydrogenase activity
2. Increased cytosolic b2-oxidation of medium-chain fatty acids, increasing ketone body production during fasting
3. Reduced conversion of acetyl-CoA into malonyl-CoA, directly blocking ketone body synthesis from fatty acids
4. Reduced mitochondrial b2-oxidation of medium-chain fatty acyl-CoA, lowering hepatic acetyl-CoA supply for ketogenesis during fasting (correct answer)

Explanation: This question tests understanding of fatty acid metabolism and the role of medium-chain acyl-CoA dehydrogenase (MCAD) in β-oxidation. MCAD specifically catalyzes the first oxidation step of medium-chain fatty acids (6-12 carbons) in mitochondrial β-oxidation, while long-chain acyl-CoA dehydrogenase (LCAD) handles longer fatty acids like palmitate (C16). The experimental data shows that octanoate (C8, a medium-chain fatty acid) metabolism is impaired in mutant cells (lower oxygen consumption and acetoacetate production), while palmitate metabolism remains normal. Since MCAD deficiency prevents efficient β-oxidation of medium-chain fatty acids, less acetyl-CoA is generated from octanoate, resulting in reduced substrate for ketogenesis during fasting. Choice B is incorrect because β-oxidation occurs in mitochondria, not the cytosol, and MCAD deficiency would decrease, not increase, oxidation of medium-chain fatty acids. To approach similar problems, identify which specific enzyme is affected and match the substrate specificity to the experimental observations.

Question 19

1. Impaired peroxisomal shortening of VLCFAs, reducing generation of shorter-chain substrates that can be further oxidized in mitochondria (correct answer)
2. Impaired mitochondrial import of long-chain fatty acids due to defective carnitine shuttle, causing VLCFA accumulation
3. Increased cytosolic fatty acid synthesis from acetyl-CoA, producing excess VLCFAs that overwhelm mitochondria
4. Direct block of ketone body synthesis from acetyl-CoA due to loss of peroxisomal oxidation enzymes

Explanation: This question tests understanding of peroxisomal fatty acid oxidation and its role in very-long-chain fatty acid (VLCFA) metabolism. Peroxisomes are specialized for oxidizing VLCFAs (>C20) through a process that shortens them to medium-chain fatty acids, which can then be completely oxidized in mitochondria. ACOX1 catalyzes the first step of peroxisomal β-oxidation, and its deficiency prevents the breakdown of VLCFAs like lignoceric acid (C24) into shorter chains. The observation that palmitate (C16) oxidation remains normal confirms that mitochondrial β-oxidation is intact, but the inability to shorten VLCFAs in peroxisomes prevents their subsequent mitochondrial oxidation. Choice B is incorrect because the defect is in peroxisomal, not mitochondrial, fatty acid processing, and the carnitine shuttle handles long-chain, not very-long-chain, fatty acids. To understand organelle-specific metabolism, recognize that peroxisomes and mitochondria have complementary roles in complete fatty acid oxidation.

Question 20

1. Defective ketone body utilization in muscle due to reduced CPT1 activity, causing ketone accumulation in cytosol
2. Enhanced entry of long-chain fatty acids into mitochondria, increasing ATP production from b2-oxidation in low glucose
3. Block of cytosolic fatty acid synthesis due to insufficient carnitine, decreasing malonyl-CoA formation
4. Impaired transport of long-chain fatty acids into mitochondria, reducing mitochondrial b2-oxidation during low glucose availability (correct answer)

Explanation: This question tests understanding of the carnitine shuttle system and its role in fatty acid metabolism. CPT1 catalyzes the rate-limiting step of fatty acid β-oxidation by converting long-chain acyl-CoA to acyl-carnitine for transport into mitochondria. The mutant CPT1 has reduced affinity for carnitine, meaning it requires higher carnitine concentrations to function normally. In low glucose conditions when cells rely on fatty acid oxidation, the mutant's impaired ability to bind carnitine prevents efficient transport of long-chain fatty acids into mitochondria, causing cytosolic accumulation of long-chain acyl-CoA and reduced oxygen consumption from decreased β-oxidation. Choice B is incorrect because the mutation impairs, not enhances, fatty acid entry into mitochondria. When analyzing metabolic mutations, consider how changes in substrate affinity affect enzyme function under physiological substrate concentrations.