Question 1

Shuttling Cytosolic NADH—Malate–Aspartate Shuttle Sites In which tissues is the malate–aspartate shuttle especially active for transferring cytosolic reducing equivalents into mitochondria?

Accepted Answer

Introduction / Context: The inner mitochondrial membrane is impermeable to NADH. Cells therefore use shuttles to transfer the reducing power of cytosolic NADH into the mitochondrial matrix for oxidative phosphorylation. Two main systems exist: the malate–aspartate shuttle and the glycerol 3-phosphate shuttle. Tissue distribution differs and has metabolic consequences. Given Data / Assumptions: Malate–aspartate shuttle is highly efficient (yields more ATP per cytosolic NADH). Heart and liver have high oxidative capacity and rely on efficient NADH transfer. Some skeletal muscles preferentially use the glycerol 3-phosphate shuttle. Concept / Approach: In the malate–aspartate shuttle, cytosolic oxaloacetate is reduced to malate using NADH; malate enters mitochondria via carriers, is reoxidized to oxaloacetate producing NADH in the matrix, and transamination with glutamate regenerates aspartate to complete the cycle. Heart and liver prominently use this shuttle to maximize ATP yield from cytosolic NADH. Step-by-Step Solution: Identify tissues needing efficient ATP production from cytosolic NADH: heart and liver.Recall pathway steps: OAA ⇄ malate (cytosol) → malate entry → OAA (matrix) → aspartate transamination and export.Choose heart and liver as the correct pair. Verification / Alternative check: Biochemical measurements show higher malate–aspartate shuttle enzyme activities in heart and liver compared with tissues favoring the glycerol 3-phosphate shuttle. Why Other Options Are Wrong: Lungs/pancreas/skin: not classic high-activity sites for this shuttle. “None of these” is incorrect because a specific pair is well established. Common Pitfalls: Assuming a single universal shuttle; in fact, shuttle choice varies by tissue and metabolic demand. Final Answer: Heart and liver

Question 2

Why the TCA Cycle Is “Central”—Metabolic Rationale Why is the tricarboxylic acid (TCA) cycle considered the central pathway of cellular metabolism?

Accepted Answer

Introduction / Context: The TCA cycle sits at a crossroads where carbohydrates, fats, and proteins converge. It not only oxidizes acetyl units but also supplies intermediates for biosynthesis, making it a hub for both energy production and building-block generation. Given Data / Assumptions: Intermediates include citrate, α-ketoglutarate, succinyl-CoA, malate, oxaloacetate. Anaplerosis replenishes TCA intermediates; cataplerosis removes them for biosynthesis. Cells integrate multiple pathways simultaneously. Concept / Approach: Centrality refers to connectivity, not physical location. TCA intermediates feed amino acid synthesis (e.g., glutamate from α-ketoglutarate), heme synthesis (from succinyl-CoA), gluconeogenesis (via oxaloacetate/malate), and lipid synthesis (via citrate export for acetyl-CoA). This shared use underpins the description of the TCA as a central pathway. Step-by-Step Solution: List biosynthetic branches: amino acids, porphyrins, glucose (via OAA), fatty acids (via citrate).Explain replenishment: pyruvate carboxylase forms oxaloacetate; transamination interconversions balance pools.Conclude that widespread intermediate usage defines the TCA cycle’s centrality. Verification / Alternative check: Metabolic flux analyses show heavy traffic through TCA nodes and multiple entry/exit points during varied nutrient states. Why Other Options Are Wrong: Geometric “center” is irrelevant to metabolic centrality. “All pathways depend solely upon it” is overstated; some microbes under certain conditions can bypass or modify TCA usage. Other statements do not capture the integrative role of intermediates. Common Pitfalls: Equating “central” with “largest ATP producer”; oxidative phosphorylation, not TCA alone, yields most ATP. Final Answer: Its intermediates are commonly used by other metabolic reactions (anaplerosis and cataplerosis)

Question 3

TCA Cycle—Identify the First Isolated Intermediate What is the first stable intermediate formed when acetyl-CoA enters the tricarboxylic acid (TCA) cycle?

Accepted Answer

Introduction / Context: The TCA cycle begins when acetyl-CoA condenses with oxaloacetate. Knowing the sequence of intermediates is essential for tracing carbon flow and understanding regulation points in central metabolism. Given Data / Assumptions: Entry substrate: acetyl-CoA (2 carbons). Acceptor: oxaloacetate (4 carbons). Enzyme: citrate synthase. Concept / Approach: Citrate synthase catalyzes condensation of acetyl-CoA with oxaloacetate to form citrate (6 carbons) via an intermediate citryl-CoA. Citrate is the first stable and isolable TCA intermediate after entry, preceding isomerization to isocitrate by aconitase. Step-by-Step Solution: Combine acetyl-CoA (2C) + oxaloacetate (4C) → citrate (6C).Recognize citrate as the first product after the entry condensation step.Note subsequent steps: citrate → isocitrate via aconitase (not required for this answer). Verification / Alternative check: Enzyme assays for citrate synthase activity and accumulation of citrate in conditions of aconitase inhibition corroborate this sequence. Why Other Options Are Wrong: Succinate, fumarate, malate: later TCA intermediates. Oxaloacetate: the acceptor that reacts with acetyl-CoA; not the first product formed after entry. Common Pitfalls: Confusing oxaloacetate (the acceptor) with citrate (the first product). Final Answer: Citrate

Question 4

Cholesterol Biosynthesis—Primary Site of de novo Synthesis In humans, which organ is the principal site for de novo cholesterol synthesis under normal physiological conditions?

Accepted Answer

Introduction / Context: Cholesterol is a vital lipid serving as a membrane component and precursor for bile acids, steroid hormones, and vitamin D. While many tissues can synthesize cholesterol, one organ plays the dominant role in overall production and distribution via lipoproteins. Given Data / Assumptions: Pathway: acetyl-CoA → HMG-CoA → mevalonate → isoprenoids → squalene → cholesterol. Rate-limiting enzyme: HMG-CoA reductase (target of statins). Need for systemic export via very-low-density lipoproteins (VLDL). Concept / Approach: The liver is the central metabolic hub for lipid synthesis and trafficking. It expresses high levels of HMG-CoA reductase and packages cholesterol into lipoproteins for delivery to peripheral tissues. Although intestine and other tissues contribute, hepatic synthesis is predominant in whole-body balance. Step-by-Step Solution: Identify the organ with robust lipoprotein secretion (VLDL) and bile acid production: liver.Relate control point: hepatic HMG-CoA reductase regulated by SREBP pathway, insulin/glucagon, and intracellular cholesterol.Select liver as the principal site of de novo synthesis. Verification / Alternative check: Clinical pharmacology: statins primarily affect hepatic cholesterol synthesis, lowering LDL-cholesterol via upregulated LDL receptors—evidence of the liver’s central role. Why Other Options Are Wrong: Pancreas/kidney: can synthesize some cholesterol but are not the main systemic source. Intestine: synthesizes and absorbs cholesterol but is not the principal producer in most contexts. Cell membrane: a structure, not an organ capable of de novo synthesis. Common Pitfalls: Equating “site of cholesterol” (membranes) with “site of synthesis” (hepatocytes and other cells). Final Answer: Liver

Question 5

Bioenergetics focus: During one full turn of the citric acid cycle (tricarboxylic acid cycle) per acetyl-CoA oxidized, how many ATP equivalents are produced using the conventional textbook P/O ratios?

Accepted Answer

Introduction / Context: The citric acid cycle (also called the tricarboxylic acid cycle or Krebs cycle) harvests high-energy electrons in the form of reduced cofactors. A common exam convention asks for ATP equivalents produced per acetyl-CoA using the classic P/O ratios (3 ATP per NADH and 2 ATP per FADH2). This question uses that traditional convention to compute the total ATP yield per cycle turn. Given Data / Assumptions: Accounting is per acetyl-CoA oxidized through one turn of the cycle. Products per turn: 3 NADH, 1 FADH2, and 1 GTP (substrate-level phosphorylation). Classic P/O ratios assumed: 1 NADH → 3 ATP; 1 FADH2 → 2 ATP; 1 GTP ≈ 1 ATP equivalent. Concept / Approach: The cycle itself produces reduced cofactors; ATP is realized when these donate electrons to the respiratory chain. Under the specified classic P/O values, we convert cofactors into ATP equivalents, then sum with the substrate-level GTP to obtain the total ATP yield for one cycle turn. Step-by-Step Solution: List cycle outputs: 3 NADH, 1 FADH2, 1 GTP per acetyl-CoA.Convert NADH to ATP equivalents: 3 NADH * 3 ATP/NADH = 9 ATP equivalents.Convert FADH2 to ATP equivalents: 1 FADH2 * 2 ATP/FADH2 = 2 ATP equivalents.Add substrate-level phosphorylation: 1 GTP ≈ 1 ATP equivalent.Total ATP equivalents = 9 + 2 + 1 = 12. Verification / Alternative check: Some modern texts use refined values (approximately 2.5 ATP per NADH and 1.5 ATP per FADH2). Under those values, the same cycle turn yields roughly 10 ATP equivalents (3 * 2.5 + 1 * 1.5 + 1 ≈ 10). However, the conventional exam answer under classic P/O ratios is 12, which matches the options provided here and the requested assumption set. Why Other Options Are Wrong: 10: Reflects modern P/O ratios, not the classic convention explicitly used by this question. 13 or 8 or 9: Do not match either classic or modern accepted accounting per acetyl-CoA. Common Pitfalls: Mixing modern P/O ratios with classic exam conventions, or accidentally counting ATP yield per glucose (two acetyl-CoA) rather than per acetyl-CoA. Always read the assumption set and the unit of accounting. Final Answer: 12

Question 6

TCA cycle membership check: Which of the following enzymes does not catalyze a step within the citric acid (tricarboxylic acid) cycle itself?

Accepted Answer

Introduction / Context: The citric acid cycle is a defined series of mitochondrial matrix reactions. Some closely linked enzymes feed substrates into the cycle but are not considered part of the cycle itself. This question probes whether you can distinguish a true cycle enzyme from a pre-cycle linking enzyme. Given Data / Assumptions: Citrate synthase, aconitase, isocitrate dehydrogenase, and malate dehydrogenase are canonical cycle enzymes. Pyruvate dehydrogenase converts pyruvate to acetyl-CoA in a separate multi-enzyme complex. The cycle proper begins with acetyl-CoA condensing with oxaloacetate to form citrate. Concept / Approach: Identify whether each enzyme catalyzes a reaction that occurs after acetyl-CoA has entered the cycle. The pyruvate dehydrogenase complex operates upstream, generating acetyl-CoA and NADH but not participating within the cycle's ring of reactions. Step-by-Step Solution: Recall the cycle sequence: citrate synthase → aconitase → isocitrate dehydrogenase → α-ketoglutarate dehydrogenase → succinyl-CoA synthetase → succinate dehydrogenase → fumarase → malate dehydrogenase.Locate pyruvate dehydrogenase: upstream conversion of pyruvate to acetyl-CoA, before citrate synthase.Therefore, pyruvate dehydrogenase is not part of the TCA cycle. Verification / Alternative check: Maps of central metabolism consistently depict the pyruvate dehydrogenase complex as the link between glycolysis and the TCA cycle, physically adjacent in the matrix but not counted within the eight-step cycle. Why Other Options Are Wrong: Citrate synthase, isocitrate dehydrogenase, malate dehydrogenase, and aconitase all catalyze established steps of the cycle. Common Pitfalls: Confusing physical proximity in the mitochondrial matrix with cycle membership; proximity does not equal inclusion in the defined cycle steps. Final Answer: Pyruvate dehydrogenase

TCA Cycle Questions