Unit 16 Question Bank
Format note: the TMU Biochemistry paper gives five suggested
answers (A–E), not four — these MCQs match that. Items tagged TMU 2019 or
TMU 2020/21 come from the real papers. Answers are verified against Harper's
Illustrated Biochemistry; the "marking schemes" in the source folder are other students'
answer sheets, not official, so they are never used as the authority.
0 / 25 answered
1Fatty acid synthesis occurs in the ( ), and fatty acid oxidation in the ( ).
Answer: B
The separation is deliberate. A pathway and its opposite must be independently regulable or they run as a futile cycle — and here the cell achieves it by compartmentation: different rooms, different coenzymes (NADPH vs NAD⁺/FAD), different acyl carriers (ACP vs CoA).Harper's ch.23, p.232
2The initial and controlling step of fatty acid synthesis is ( ).
Answer: C
Production of malonyl-CoA is the initial and controlling step in fatty acid synthesis, and acetyl-CoA carboxylase has a major role in the regulation of the pathway. It requires bicarbonate as the source of CO₂, ATP, and the B vitamin biotin.Harper's ch.23, p.233
3Acetyl-CoA carboxylase requires which vitamin?
Answer: D
Biotin is the carboxylation vitamin — the same requirement as pyruvate carboxylase in gluconeogenesis (Unit 13). The enzyme is a multienzyme protein containing biotin, biotin carboxylase, biotin carboxyl carrier protein and a carboxyl transferase, plus a regulatory allosteric site.Harper's ch.23, p.233
4The reaction catalysed by acetyl-CoA carboxylase takes place in two steps, which are ( ).
Answer: E
This is the standard biotin mechanism: the vitamin is carboxylated first, at the expense of ATP, and then hands the carboxyl group on. Note the design logic — the cell spends an ATP to make a leaving group, because the decarboxylation at each subsequent condensation is what drives chain elongation forward.Harper's ch.23, p.233
5Acetyl-CoA is made in the mitochondrial matrix but fatty acid synthesis is cytosolic. Acetyl-CoA reaches the cytosol ( ).
Answer: A
Acetyl-CoA does not diffuse readily across the mitochondrial membranes, so transport requires a special mechanism involving citrate. This is Unit 11 §7 seen from the other side — citrate as the vehicle that smuggles two carbons out. The carnitine shuttle carries fatty acids IN, not acetyl units out.Harper's ch.23, p.234 · Harper's ch.16, p.166
6The fatty acid synthase complex is ( ).
Answer: B
X-ray crystallography shows the two subunits arranged in an X shape. The positions of the ACP and thioesterase domains cannot yet be resolved, possibly because they are too flexible, but they are thought to lie close to the 3-ketoacylreductase.Harper's ch.23, p.233
7Acyl carrier protein contains which vitamin, and in what form?
Answer: C
ACP has a function similar to CoA in the β-oxidation pathway — and that is not a coincidence, since coenzyme A also derives from pantothenic acid. The same vitamin equips both the synthetic and the degradative carrier.Harper's ch.23, p.233
8Harper's gives two advantages of using one multienzyme complex for fatty acid synthesis. One is coordinated synthesis from a single gene. The other is that it ( ).
Answer: D
The intermediates never leave the complex, so they cannot diffuse away or be diverted by competing pathways. This is the same principle as channelling at aconitase in Unit 11 — keep the intermediate on the protein and it cannot go astray.Harper's ch.23, p.233
9In palmitate synthesised de novo, carbons 15 and 16 come from ( ).
Answer: E
The acetyl-CoA used as a primer forms carbons 15 and 16 of palmitate; the addition of all subsequent C₂ units is via malonyl-CoA. Note the corollary: propionyl-CoA acts as primer for long-chain fatty acids having an ODD number of carbons, found particularly in ruminant fat and milk.Harper's ch.23, p.234
10Free palmitate is released from the synthase complex by ( ).
Answer: A
The free palmitate must be activated to acyl-CoA before it can proceed via any other metabolic pathway. Its possible fates are esterification into acylglycerols, chain elongation or desaturation, or esterification into cholesteryl ester. In the mammary gland a separate thioesterase specific for C8, C10 or C12 residues produces the shorter fatty acids of milk lipids.Harper's ch.23, pp.233–234
11How many NADPH are required for the synthesis of one molecule of palmitate?
Answer: B
The overall equation is acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H⁺ → palmitate + 7 CO₂ + 6 H₂O + 8 CoA-SH + 14 NADP⁺. Seven cycles, two reductions each — NADPH reduces both the 3-ketoacyl and the 2,3-unsaturated acyl derivatives.Harper's ch.23, p.234
12The chief source of NADPH for lipogenesis is ( ).
Answer: C
Two facts make it ideal, and Harper's states both. Distribution: the tissues specialising in active lipogenesis — liver, adipose tissue and lactating mammary gland — also possess an active pentose phosphate pathway (compare Unit 14's tissue list). Location: both pathways are cytosolic, so there are no permeability barriers to the transfer of NADPH.Harper's ch.23, p.234
13Besides the pentose phosphate pathway, NADPH for lipogenesis is supplied by ( ).
Answer: D
“Malic enzyme” is NADP malate dehydrogenase. A third source, the extramitochondrial isocitrate dehydrogenase reaction, is probably not a substantial source except in ruminants — note the qualification.Harper's ch.23, p.234
14Acetyl-CoA carboxylase is allosterically ACTIVATED by ( ).
Answer: E
Citrate increases in concentration in the well-fed state and is an indicator of a plentiful supply of acetyl-CoA. Note that citrate is doing two jobs at once: it is the carrier that delivers the substrate to the cytosol and the activator of the enzyme that consumes it. The raw material announces its own arrival.Harper's ch.23, p.237
15Citrate activates acetyl-CoA carboxylase by ( ).
Answer: A
The active polymer has a molecular mass of several million. This is allosteric regulation altering the state of aggregation — an unusual mechanism worth being able to state precisely, since “activates it” alone will not earn the mark.Harper's ch.23, p.237
16Long-chain acyl-CoA inhibits lipogenesis at how many separate points?
Answer: B
1 · Direct feedback on acetyl-CoA carboxylase. 2 · It inhibits the mitochondrial tricarboxylate transporter, so citrate cannot leave to activate the enzyme. 3 · It inhibits pyruvate dehydrogenase, by blocking the ATP-ADP exchange transporter and raising the intramitochondrial ATP/ADP ratio. The cell stops making what it already has too much of.Harper's ch.23, p.237
17Inhibition of acetyl-CoA carboxylase by long-chain acyl-CoA is an example of ( ).
Answer: C
Harper's gives the physiological rationale directly: if acyl-CoA accumulates because it is not esterified quickly enough, or because of increased lipolysis or an influx of free fatty acids into the tissue, it will automatically reduce the synthesis of new fatty acid.Harper's ch.23, p.237
18Acyl-CoA inhibits pyruvate dehydrogenase by ( ).
Answer: D
A raised ATP/ADP ratio converts active to inactive pyruvate dehydrogenase — exactly the PDH kinase regulation of Unit 10 §8. Furthermore, oxidation of acyl-CoA may raise the (acetyl-CoA)/(CoA) and (NADH)/(NAD⁺) ratios, which also inhibit PDH. Arsenite is what reacts with lipoic acid — a different mechanism.Harper's ch.23, p.237
19Insulin stimulates lipogenesis by all of the following EXCEPT ( ).
Answer: E
Inactivation of acetyl-CoA carboxylase is promoted by PHOSPHORYLATION — so phosphorylating it would switch it off, not on. Insulin acts in the opposite direction. Note the tissue detail in option D: insulin activates PDH in adipose tissue, but not in liver.Harper's ch.23, p.237
20Which hormones regulate acetyl-CoA carboxylase through changes in its phosphorylation state?
Answer: A
The same fed/fasting hormonal axis that controls glycogen (Unit 12) and glycolysis/gluconeogenesis (Unit 13). Insulin favours synthesis; glucagon and epinephrine oppose it — one hormonal signal coordinating every anabolic pathway in the liver at once.Harper's ch.23, p.237
21Which molecule links the regulation of fatty acid synthesis to that of fatty acid oxidation?
Answer: B
Malonyl-CoA is the first committed intermediate of synthesis AND the inhibitor of carnitine palmitoyltransferase-I, the gateway to oxidation. The act of starting to make fat simultaneously blocks the pathway that burns it — one molecule enforcing reciprocal regulation across two compartments, exactly as fructose 2,6-bisphosphate does in Unit 13.Harper's ch.23 · Harper's ch.22, p.229
22In fatty acid SYNTHESIS the acyl carrier is ( ); in OXIDATION it is ( ).
Answer: C
Learn the contrast as a set: cytosol vs matrix · NADPH vs NAD⁺/FAD · ACP vs CoA · malonyl-CoA added vs acetyl-CoA removed · one multienzyme complex vs separate enzymes · insulin vs glucagon. Almost every examinable point about either pathway is one of those rows.Harper's ch.23, p.233
23Propionyl-CoA acts as the primer for the synthesis of ( ).
Answer: D
These are found particularly in ruminant fat and milk. Note the symmetry with Unit 17: propionyl-CoA is also the product left over when an odd-chain fatty acid is β-oxidised, and it is glucogenic via methylmalonyl-CoA to succinyl-CoA.Harper's ch.23, p.234
24After release from the synthase, palmitate must first be ( ) before entering any other pathway.
Answer: E
The same activation step that begins β-oxidation (Unit 17 §2), catalysed by acyl-CoA synthetase at a cost of two ATP equivalents. Its possible fates thereafter are esterification into acylglycerols, chain elongation or desaturation, or esterification into cholesteryl ester.Harper's ch.23, p.234
25Which tissue list is correct for ACTIVE lipogenesis?
Answer: A
And that list is identical to Unit 14's list of tissues with an active pentose phosphate pathway — because lipogenesis consumes 14 NADPH per palmitate and the pentose phosphate pathway is its chief supplier. Option E is Unit 14's list of tissues where the pathway is low.Harper's ch.23, p.234 · TMU “Chapter 20” Slide 15
1 Lipogenesis — 3′+
The de novo synthesis of long-chain fatty acids from acetyl-CoA, occurring in the cytosol of liver, adipose tissue and the lactating mammary gland.
Acetyl-CoA is the immediate substrate and free palmitate is the end product. Reducing equivalents are supplied as NADPH, chiefly from the pentose phosphate pathway, and the acyl carrier is acyl carrier protein (ACP).
Overall: acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H⁺ → palmitate + 7 CO₂ + 6 H₂O + 8 CoA-SH + 14 NADP⁺.Harper's ch.23, pp.232–234
Acetyl-CoA is the immediate substrate and free palmitate is the end product. Reducing equivalents are supplied as NADPH, chiefly from the pentose phosphate pathway, and the acyl carrier is acyl carrier protein (ACP).
Overall: acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H⁺ → palmitate + 7 CO₂ + 6 H₂O + 8 CoA-SH + 14 NADP⁺.Harper's ch.23, pp.232–234
2 Acetyl-CoA carboxylase — 3′+
The enzyme catalysing the carboxylation of acetyl-CoA to malonyl-CoA — the initial and controlling step in fatty acid synthesis and the most important enzyme in the regulation of lipogenesis.
It requires the B vitamin biotin, and is a multienzyme protein containing biotin, biotin carboxylase, biotin carboxyl carrier protein and a carboxyl transferase, plus a regulatory allosteric site. The reaction occurs in two steps: carboxylation of biotin involving ATP, then transfer of the carboxyl group to acetyl-CoA.
Regulation: activated allosterically by citrate, which converts the inactive dimer to an active polymeric form; inhibited by phosphorylation and by long-chain acyl-CoA — negative feedback by a product.Harper's ch.23, pp.233, 237
It requires the B vitamin biotin, and is a multienzyme protein containing biotin, biotin carboxylase, biotin carboxyl carrier protein and a carboxyl transferase, plus a regulatory allosteric site. The reaction occurs in two steps: carboxylation of biotin involving ATP, then transfer of the carboxyl group to acetyl-CoA.
Regulation: activated allosterically by citrate, which converts the inactive dimer to an active polymeric form; inhibited by phosphorylation and by long-chain acyl-CoA — negative feedback by a product.Harper's ch.23, pp.233, 237
3 Fatty acid synthase — 3′+
The multienzyme polypeptide complex in which the individual enzymes of fatty acid synthesis are linked, incorporating the acyl carrier protein (ACP).
It is a homodimer of two identical subunits, each containing six enzymes and an ACP, arranged in an X shape. ACP contains the vitamin pantothenic acid as 4′-phosphopantetheine and has a function similar to CoA in β-oxidation.
Two advantages of the multienzyme arrangement: it achieves compartmentalisation of the process without the erection of permeability barriers, and synthesis of all the enzymes is coordinated, since it is encoded by a single gene.Harper's ch.23, pp.233–234
It is a homodimer of two identical subunits, each containing six enzymes and an ACP, arranged in an X shape. ACP contains the vitamin pantothenic acid as 4′-phosphopantetheine and has a function similar to CoA in β-oxidation.
Two advantages of the multienzyme arrangement: it achieves compartmentalisation of the process without the erection of permeability barriers, and synthesis of all the enzymes is coordinated, since it is encoded by a single gene.Harper's ch.23, pp.233–234
4 The citrate shuttle — 2′+
The mechanism by which acetyl-CoA formed in the mitochondrial matrix reaches the cytosol, where fatty acid synthesis occurs.
Acetyl-CoA does not diffuse readily across the mitochondrial membranes. It therefore condenses with oxaloacetate to form citrate, which is transported into the cytosol and cleaved by ATP-citrate lyase, regenerating acetyl-CoA and oxaloacetate.
Note that citrate is simultaneously the carrier of the substrate and the allosteric activator of acetyl-CoA carboxylase.Harper's ch.23, p.234
Acetyl-CoA does not diffuse readily across the mitochondrial membranes. It therefore condenses with oxaloacetate to form citrate, which is transported into the cytosol and cleaved by ATP-citrate lyase, regenerating acetyl-CoA and oxaloacetate.
Note that citrate is simultaneously the carrier of the substrate and the allosteric activator of acetyl-CoA carboxylase.Harper's ch.23, p.234
1
Outline briefly the biosynthesis of fatty acids and its regulation.
5′ — 'outline briefly'
Location and materials
Fatty acid synthesis occurs in the cytosol of liver, adipose tissue and the lactating mammary gland. Acetyl-CoA is the immediate substrate and free palmitate the end product; the reducing equivalents are supplied as NADPH.
Supply of acetyl-CoA — the citrate shuttle
Acetyl-CoA is formed from glucose by oxidation of pyruvate in the mitochondrial matrix, but does not diffuse readily across the mitochondrial membranes. It therefore condenses with oxaloacetate to form citrate, which is transported into the cytosol and cleaved by ATP-citrate lyase.
The committed step
Production of malonyl-CoA is the initial and controlling step. Acetyl-CoA carboxylase — requiring biotin, ATP and bicarbonate — carboxylates acetyl-CoA in two steps: carboxylation of biotin involving ATP, then transfer of the carboxyl group to acetyl-CoA.
Chain elongation
Fatty acid synthase is a homodimer of two identical subunits, each containing six enzymes and an acyl carrier protein, arranged in an X shape. ACP contains pantothenic acid as 4′-phosphopantetheine and functions like CoA. A priming acetyl-CoA forms carbons 15 and 16 of palmitate; all subsequent two-carbon units come from malonyl-CoA. After seven cycles, thioesterase releases free palmitate.
Overall: acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 14 H⁺ → palmitate + 7 CO₂ + 6 H₂O + 8 CoA-SH + 14 NADP⁺.
Source of NADPH
Chiefly the oxidative reactions of the pentose phosphate pathway. Significantly, the tissues specialising in active lipogenesis also possess an active pentose phosphate pathway, and both are cytosolic, so there are no permeability barriers to NADPH transfer. Other sources are malic enzyme and, marginally, extramitochondrial isocitrate dehydrogenase.
Regulation
- Citrate ACTIVATES acetyl-CoA carboxylase, converting the inactive dimer to the active polymer; it rises in the well-fed state and is an indicator of a plentiful supply of acetyl-CoA.
- Long-chain acyl-CoA INHIBITS — negative feedback by a product — and also inhibits the tricarboxylate transporter, preventing the egress of citrate, and inhibits pyruvate dehydrogenase.
- Phosphorylation inactivates; the enzyme is regulated by glucagon, epinephrine and insulin. Insulin also increases glucose transport into the cell and activates pyruvate dehydrogenase in adipose tissue.
Marking guide: cytosolic location and NADPH requirement 0.5 · citrate shuttle with ATP-citrate lyase 1 · acetyl-CoA carboxylase as the committed step, with biotin 1.5 · fatty acid synthase structure 1 · pentose phosphate pathway as NADPH source 0.5 · citrate activation and acyl-CoA inhibition 0.5.
2
Compare and contrast the synthesis and the oxidation of fatty acids.
8′
The organising principle
Synthesis and oxidation are opposite pathways that must be independently regulable, or they would run simultaneously as a futile cycle. The cell achieves this by compartmentation — different locations, different coenzymes, different acyl carriers, different enzyme organisation and opposite hormonal control.
| Synthesis | Oxidation | |
|---|---|---|
| Location | Cytosol | Mitochondrial matrix |
| Acyl carrier | ACP (4′-phosphopantetheine) | CoA |
| Coenzyme | NADPH — 14 per palmitate | NAD⁺ and FAD |
| Two-carbon unit | Added as malonyl-CoA | Removed as acetyl-CoA |
| Key regulated enzyme | Acetyl-CoA carboxylase | Carnitine palmitoyltransferase-I |
| Rate-limiting step | Malonyl-CoA formation | Entry into the mitochondrion |
| Enzyme organisation | One multienzyme complex | Separate enzymes |
| Stimulated by | Insulin, citrate, the fed state | Glucagon, epinephrine, fasting |
| Transport problem solved by | The citrate shuttle (acetyl units OUT) | The carnitine shuttle (acyl groups IN) |
The chemistry is a mirror image
β-oxidation runs oxidation → hydration → oxidation → thiolysis, using NAD⁺ and FAD. Synthesis runs the reverse sequence — condensation → reduction → dehydration → reduction — using NADPH. The intermediates correspond, but the enzymes, coenzymes and compartments are entirely distinct.
The link between them
Malonyl-CoA is the point of contact. It is the first committed intermediate of synthesis and simultaneously the inhibitor of carnitine palmitoyltransferase-I, the gateway to oxidation. The act of beginning to synthesise fat therefore blocks the pathway that degrades it — reciprocal regulation enforced across two compartments by a single molecule, in the same way that fructose 2,6-bisphosphate coordinates glycolysis and gluconeogenesis.
A second link operates through acyl-CoA: long-chain acyl-CoA inhibits acetyl-CoA carboxylase, the tricarboxylate transporter and pyruvate dehydrogenase, so an accumulation of the product of oxidation automatically suppresses synthesis.
Yields
Synthesis of palmitate consumes 8 acetyl-CoA and 14 NADPH; its complete oxidation yields 106 ATP net, against 30–32 for a mole of glucose.
Marking guide: statement of why the pathways must be separated 1 · at least six correct contrasts from the table 3 · the two shuttles correctly assigned 1 · the mirror-image chemistry 1 · malonyl-CoA as the regulatory link 1.5 · a numerical yield 0.5.