BIOC final exam

What should the 6-mark extended response likely be about?

Signal transduction — e.g. 'describe glucagon signalling in liver cells.' Model high-mark answer

structure: 1) hormone binds 7TM/GPCR receptor 2) G-protein activated (GDP->GTP swap on alpha

subunit) 3) activates adenylate cyclase 4) generates 2nd messenger cAMP 5) activates Protein Kinase A

(PKA) 6) PKA phosphorylates glycogen phosphorylase -> glycogen breakdown -> glucose released to

blood. Name every molecule/enzyme, don't just describe vaguely.

Where does dietary protein digestion begin, and with what?

Stomach — acidic environment (HCl) denatures protein; pepsin (from pepsinogen, a zymogen) begins

digestion.

What happens to dietary protein in the small intestine?

Pancreatic proteolytic enzymes (e.g. trypsin, chymotrypsin) continue breakdown; membrane-bound

aminopeptidases on intestinal cells and intracellular peptidases finish the job into amino

acids/di/tripeptides

What is a zymogen and why is it needed?

An inactive precursor protein (proenzyme) that requires proteolytic cleavage to activate. Needed so

digestive enzymes don't destroy the tissue that makes them — activation happens only once it's in the right

place (gut lumen) with the right substrate.

What activates pepsinogen -> pepsin?

HCl in the stomach.

What activates trypsinogen -> trypsin?

Enteropeptidase, secreted by duodenal cells. Trypsin then activates more trypsinogen and other

zymogens (autocatalytic amplification).

What determines substrate specificity of chymotrypsin vs trypsin vs elastase?

A few amino acids at the substrate binding site. Chymotrypsin cleaves after hydrophobic side chains;

Trypsin after positively-charged side chains; Elastase after small side chains.

What is Pancreatic Trypsin Inhibitor (PTI) and how does it work?

A polypeptide that binds tightly to trypsin's active site but is cleaved very slowly (a poor substrate) —

this is essentially competitive inhibition, protecting the pancreas from premature trypsin activity.

What is the correct order of dietary protein degradation?

Pepsin (stomach) -> Secreted pancreatic proteolytic enzymes (e.g. trypsin, in intestine) ->

Membrane-bound (aminopeptidases) -> Cellular (intracellular) peptidases -> amino acids into blood.

What two processes does cellular protein turnover involve?

Regulated synthesis (gene expression) AND regulated degradation of existing proteins — both control

protein function/levels.

What is ubiquitin and what does it do?

A small (8.5 kDa), highly conserved protein (differs by only 3/76 residues between yeast and human)

that gets covalently attached (via isopeptide bonds to Lys residues) to proteins destined for degradation —

'tags' them.

How many ubiquitins are needed to signal degradation?

A single Ub is a weak signal; a chain of 4+ Ub molecules is required to strongly signal degradation.

What enzymes attach ubiquitin, and what's special about E3?

E1, E2, E3 (requires ATP). E3 is the 'reader' — it identifies/recognises the specific protein target for Ub

attachment

What degrades the ubiquitin-tagged protein, and what happens to the Ub?

The proteasome. Its cap recognises/unfolds tagged proteins; Ub is released and reused; the core

(catalytic β-subunits) breaks the protein into peptides using ATP hydrolysis for energy; peptides are further

broken into amino acids by other peptidases

What can determine whether Ub is attached to a protein?

The protein's N-terminal residue, PEST boxes, or cyclin destruction boxes.

What diseases are linked to ubiquitin/proteasome dysfunction?

Early-onset Parkinson's disease; cervical cancer caused by HPV infection

What happens to amino acids released from protein turnover?

Used to synthesise new proteins, OR the amino group is removed (urea cycle) and the carbon skeleton

used as fuel (enters TCA cycle/glycolysis etc).

What two reactions remove the alpha-amino group from an amino acid?

1) Transamination, then 2) Oxidative deamination.

What happens in transamination? What enzyme, what cofactor?

The alpha-amino group is transferred from an amino acid to an alpha-ketoacid acceptor (usually

alpha-ketoglutarate), converting the acceptor into a new amino acid (usually glutamate) and the donor into

its alpha-ketoacid. Catalysed by aminotransferases (transaminases), which require PLP

(pyridoxal-5'-phosphate, a vitamin B6 derivative) as coenzyme.

What happens in oxidative deamination? What enzyme, where?

Glutamate's amino group is converted to free ammonium ion (NH4+). Catalysed by glutamate

dehydrogenase (GDH), located in mitochondria. Reaction: glutamate + NAD(P)+ -> alpha-ketoglutarate +

NH4+ + NAD(P)H

Which two amino acids skip the 2-step process and go straight to ammonium?

Serine and threonine — converted directly to ammonium by dehydratases.

During prolonged exercise/fasting, which amino acids does muscle use as fuel?

Branched-chain amino acids (BCAAs): Leucine, Isoleucine, Valine (muscle uses these because muscle

lacks urea cycle enzymes and can't dispose of nitrogen itself)

How does muscle transport nitrogen to the liver (2 ways)?

1) Alanine cycle: glutamate + pyruvate -> alanine (+ alpha-ketoacid); alanine travels to liver and is also

a glucose precursor there. 2) Glutamine: glutamate + ATP + NH4+ -> glutamine (via glutamine

synthetase); glutamine carries N to liver where it feeds the urea cycle.

How do different organisms excrete excess nitrogen?

Humans/mammals: urea (also excrete uric acid from purine breakdown). Birds: uric acid (saves water).

Fish: ammonia (via gills)

Why are humans prone to gout?

We lack urate oxidase, the enzyme that would convert uric acid to the more soluble allantoin.

List the 5 urea cycle enzymes IN ORDER with their reactions.

1) Carbamoyl phosphate synthetase: NH4+ + HCO3- + 2ATP -> carbamoyl phosphate (mitochondria).

2) Ornithine transcarbamoylase: carbamoyl phosphate + ornithine -> citrulline (mitochondrial matrix;

citrulline exported to cytoplasm). 3) Argininosuccinate synthetase: citrulline + aspartate + ATP ->

argininosuccinate (aspartate donates 2nd N; ATP -> AMP + PPi). 4) Argininosuccinase (argininosuccinate

lyase): argininosuccinate -> arginine + fumarate. 5) Arginase: arginine + H2O -> urea + ornithine (ornithine

recycled back to mitochondria).

Where do the two nitrogen atoms in urea come from?

One from NH4+ (via carbamoyl phosphate, step 1). One from aspartate (step 3).

What's the ATP cost of making one urea?

Equivalent of 4 ATP (2 ATP used directly in step 1; ATP->AMP+PPi in step 3 costs 2 more

ATP-equivalents to regenerate ATP from AMP, since PPi is hydrolysed irreversibly).

What is the stoichiometry of the urea cycle?

CO2 + NH4+ + aspartate + 3ATP + 2H2O -> urea + 2ADP + 2Pi + AMP + PPi + fumarate.

What is a urea cycle disorder and give an example?

A mutation causing deficiency in one of the 6 urea cycle enzymes -> ammonia can't be cleared ->

hyperammonemia. Example: carbamoyl phosphate synthetase (CPS1) deficiency.

What is a glucogenic vs ketogenic amino acid?

Glucogenic: carbon skeleton can be converted to glucose via gluconeogenesis. Ketogenic: carbon

skeleton forms acetyl-CoA, used for oxidation or ketone body production.

Which amino acids are purely ketogenic (can't make oxaloacetate)?

Leucine and Lysine — their breakdown yields only acetyl-CoA

What is phenylketonuria (PKU)?

A disorder in amino acid catabolism: mutation impairs the enzyme (requires a cofactor) that converts

phenylalanine to tyrosine (important in neurotransmitter synthesis) — many different mutations give

differing enzyme function/severity

Why store glucose as glycogen rather than rely only on fat?

A constant glucose supply is essential for certain tissues (RBCs — no mitochondria, fully

glucose-dependent; brain — needs rapid consistent energy). Fat takes longer to break down and needs

O2

Where is glycogen stored and in what amounts?

Skeletal muscle: 1-2% by weight, used locally only (for that muscle's contraction). Liver: up to 10% by

weight, shares glucose with ALL tissues. Stored in the cytoplasm

Describe glycogen's structure.

Glucose backbone linked by α(1->4) glycosidic bonds; branches attach via α(1->6) bonds roughly every

10th residue; highly branched -> many non-reducing ends (free OH on C4) for fast synthesis/breakdown,

one reducing end (free OH on C1).

Name the 3 enzymes of glycogen BREAKDOWN (glycogenolysis) and their roles.

1) Glycogen phosphorylase — rate-controlling step; removes glucose units from non-reducing ends as

G1P (phosphorolysis) until ~4-5 residues from a branch point; needs PLP cofactor; regulated allosterically

AND by phosphorylation. 2) Glycogen debranching enzyme — transfers a 3-glucose block to another chain

(new (1->4) link) then hydrolyses the remaining (1->6) branch bond to release free glucose. 3)

Phosphoglucomutase — converts G1P to G6P.

What happens to G6P after glycogen breakdown?

Can enter glycolysis or the pentose phosphate pathway (not examined). In the LIVER, G6P is

hydrolysed by glucose-6-phosphatase (G6Pase) to free glucose + Pi, which exits via GLUT2 to the blood.

Muscle LACKS G6Pase so it keeps its G6P for its own use — can't share glycogen with the body

What is McArdle's Disease?

Muscle glycogen phosphorylase deficiency. Muscle can't break down glycogen -> painful cramps on

exertion (liver phosphorylase is a different isoenzyme, unaffected). 'Second wind' effect occurs once the

body shifts to using blood-borne fuels (fatty acids, glucose) instead.

Name the 3 enzymes of glycogen SYNTHESIS and their roles

1) UDP-glucose pyrophosphorylase — G1P + UTP -> UDP-glucose (activated glucose donor). 2)

Glycogen synthase — transfers glucose from UDP-glucose to the non-reducing end, forms (1->4) links

only. 3) Glycogen branching enzyme — transfers a 7-residue segment to make a new (1->6) branch.

Why can't glycogen synthesis and breakdown just reverse each other?

Synthesis from G1P is thermodynamically unfavourable without energy input (needs UTP) — so

separate, oppositely-regulated pathways are required to avoid wasteful simultaneous

synthesis+breakdown (futile cycle).

How is glycogen metabolism controlled allosterically?

High ATP/G6P levels inhibit glycogen phosphorylase and activate glycogen synthase (cell has enough

energy, so store glucose). High AMP activates glycogen phosphorylase (low energy, break down

glycogen).

How does covalent modification (phosphorylation) control glycogen metabolism?

Phosphorylation: ACTIVATES glycogen phosphorylase, DEACTIVATES glycogen synthase -> net

glycogen breakdown. Dephosphorylation: DEACTIVATES phosphorylase, ACTIVATES synthase -> net

glycogen synthesis.

What are glycogen storage diseases?

Inherited disorders producing abnormal glycogen quantity/quality. Liver forms -> hepatomegaly +

hypoglycemia. Muscle forms -> cramps/weakness. Both can cause cardiovascular/renal issues. ~10 types

characterised.

What is gluconeogenesis?

Synthesis of glucose from non-carbohydrate precursors — occurs when dietary glucose is unavailable

and liver glycogen is exhausted. A universal pathway (plants, animals, fungi, microbes).

Where does gluconeogenesis occur?

Primarily the LIVER, to a lesser extent the KIDNEYS.

What are the main precursors for gluconeogenesis?

Lactate, glycerol (from fat), pyruvate/alanine, TCA cycle intermediates, and carbon skeletons of MOST

amino acids. ALL must first be converted to oxaloacetate (4C).

Which molecules CANNOT be gluconeogenic precursors, and why

Leucine and Lysine (only yield acetyl-CoA) and fatty acids in general (mostly degraded fully to

acetyl-CoA) — acetyl-CoA cannot feed into gluconeogenesis because pyruvate dehydrogenase is

irreversible.

Is gluconeogenesis simply glycolysis reversed?

Mostly, but NOT entirely — 3 of the 10 glycolysis reactions are thermodynamically IRREVERSIBLE and

must be bypassed by different enzymes; the other 7 run in reverse.

Describe Bypass I (Pyruvate -> PEP).

Pyruvate carboxylase (needs biotin cofactor, carries 'activated CO2') converts pyruvate -> oxaloacetate

in the mitochondrial matrix. OAA can't cross the mito membrane, so it's reduced to malate (malate

dehydrogenase), exported to cytoplasm, re-oxidised to OAA, then decarboxylated to PEP by PEP

carboxykinase (PEPCK).

Describe Bypass II.

Fructose-1,6-bisphosphate + H2O -> Fructose-6-phosphate + Pi, catalysed by

fructose-1,6-bisphosphatase (hydrolysis, bypasses PFK's irreversible step).

Describe Bypass III.

Glucose-6-phosphate + H2O -> Glucose + Pi, catalysed by glucose-6-phosphatase (bypasses

hexokinase/glucokinase's irreversible step).

Which enzyme deficiency (G6Pase vs FBPase-1) is more devastating, and why?

Glucose-6-phosphatase deficiency — because G6Pase is the SHARED final step for BOTH

gluconeogenesis AND glycogenolysis (glycogen breakdown also produces G6P which needs G6Pase to

release free glucose). Losing it cripples both pathways for supplying blood glucose. FBPase-1 deficiency

only affects gluconeogenesis.

What is the Cori Cycle?

Cooperation between skeletal muscle and liver under low O2: muscle does glycolysis + lactate

fermentation from glucose; lactate is transported to liver; liver uses lactate for gluconeogenesis; the new

glucose is sent back to muscle via blood.

What's the net ATP cost of gluconeogenesis, and why must it be tightly regulated against

glycolysis?

2 pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 6H2O -> glucose + 2NAD+ + 4ADP + 2GDP + 6Pi. Costs

6 ATP-equivalents to make 1 glucose from 2 pyruvate. If glycolysis + gluconeogenesis ran simultaneously,

net 4 ATP would be wasted per cycle — so liver reciprocally regulates them.

What is the master allosteric regulator switching glycolysis vs gluconeogenesis, and how

does it work

Fructose-2,6-bisphosphate. It ACTIVATES phosphofructokinase (PFK) -> promotes glycolysis, and

INHIBITS fructose-1,6-bisphosphatase -> inhibits gluconeogenesis. High F2,6BP = glycolysis on; Low

F2,6BP = gluconeogenesis on.

How does alcohol (ethanol) inhibit gluconeogenesis?

Ethanol catabolism raises the cytoplasmic NADH/NAD+ ratio in liver cells. Since GNG needs

cytoplasmic NAD+ for the malate->OAA and lactate->pyruvate steps, high NADH inhibits these — pyruvate

gets diverted to lactate instead of GNG. This is why alcohol + fasting/low food + insulin can cause

dangerous hypoglycemia.

Why is fat used for energy storage over carbs?

Fat: 37 kJ/g (energy-dense, anhydrous storage, no water needed) vs carbohydrate: 16 kJ/g. Fat also

protects organs, insulates, is a major membrane component.

Compare triglyceride vs phospholipid structure.

Triglyceride (triacylglycerol): glycerol + 3 fatty acids via ester linkages — neutral, storage form.

Phospholipid: glycerol + 2 fatty acids + phosphate-head group (e.g. phosphatidylcholine/lecithin) — polar,

membrane form.

Compare cholesterol vs cholesteryl ester.

Cholesterol: amphipathic (polar OH head + hydrophobic body) — membrane component. Cholesteryl

ester: cholesterol + fatty acid (esterified) — neutral, storage form (found in lipoprotein cores).

What are saturated, monounsaturated, and polyunsaturated fatty acids? Give examples.

Saturated = no double bonds (e.g. palmitic acid 16:0, stearic acid 18:0). Monounsaturated = 1 double

bond (e.g. oleic acid 18:1(9), main fat in olive oil). Polyunsaturated = 2+ double bonds — includes

ESSENTIAL fatty acids we can't synthesise (n-6: linoleic acid 18:2, arachidonic acid; n-3/omega-3:

linolenic acid 18:3, DHA 22:6 — rich in brain/retina/testes).

Cis vs trans double bonds — what's the difference and significance?

Cis = natural, produces a 'kink' in the chain (fluid). Trans = produced by hydrogenation/processing (e.g.

margarine 'hardening') — behaves like a saturated fat, straighter chain.

Which fatty acid dominates the brain, and what's the dietary link?

Docosahexaenoic acid (DHA, 22:6 n-3) — ~60% of the brain is lipid; DHA mainly comes from fish. 'A

fish-rich diet was essential for the evolution of the human brain.

Name the 4 fat-soluble vitamins and their deficiency diseases.

A (retinol) — night-blindness; part of rhodopsin. D (cholecalciferol) — rickets; made via UV on skin

('sunshine vitamin'), important for calcium/bone. E (tocopherol) — reduced RBC survival; antioxidant. K

(menaquinones/phylloquinone) — reduced clotting; ~half normally from gut bacteria.

Where does fat digestion begin and what enzymes are involved?

Mouth (lingual lipase) -> stomach (gastric lipase, both prefer medium-chain triglycerides <12C,

important for infants) -> small intestine, where bile acids emulsify fat and pancreatic lipase splits

triglycerides into free fatty acids + monoglycerides.

What are bile acids, where made, what's their role?

Synthesised in the LIVER from cholesterol, stored in gall bladder, secreted into small intestine. More

polar than cholesterol -> water-soluble/detergent properties -> emulsify (solubilise) fats for digestion

What hormone stimulates bile/lipase secretion, and what does absorption involve?

Cholecystokinin (CCK) stimulates bile secretion (emulsification) and pancreatic lipase secretion

(lipolysis). Free fatty acids + monoglycerides form micelles -> absorbed across mucosal membrane into

enterocytes -> micelle formation -> enterocyte uptake.

What happens to short/medium chain (C4-C12) vs long chain (>C12) fatty acids after

absorption?

Short/medium (C4-C12): absorbed directly into portal blood (no chylomicrons needed). Long (>C12):

re-esterified to triglyceride in the ER of enterocytes -> packaged into chylomicrons -> lymphatic transport

-> blood.

What 2 drugs impair fat absorption and how?

Orlistat/Xenical — inhibits pancreatic lipase. Olestra — a non-absorbable fat substitute.

What's the general structure of a lipoprotein?

Core: neutral cargo — triglyceride, cholesteryl esters. Surface: polar — apoproteins, phospholipids

(emulsifiers), free cholesterol. Apoproteins: help transport out of gut/liver, provide receptor recognition

sites, act as enzyme cofactors.

List the lipoprotein classes and their major lipid cargo

Chylomicron — dietary (exogenous) triglyceride, highest TG. Chylomicron remnant — dietary

cholesterol (after LPL removes most TG). VLDL — endogenous triglyceride (made by liver). IDL —

endogenous cholesterol (transient, from VLDL after LPL). LDL — endogenous cholesterol (from IDL after

hepatic lipase). HDL — cholesterol; 'reverse transport' from tissues back to liver.

Which lipoprotein carries newly-ingested (exogenous) fat, and which is made by the liver

(endogenous)?

Chylomicrons = exogenous (dietary). VLDL = endogenous, synthesised in the liver.

What is the conversion pathway VLDL -> IDL -> LDL, and what enzymes drive it?

Lipoprotein lipase (LPL, on capillary endothelium in muscle/adipose/heart) strips triglyceride from VLDL

-> forms IDL (~50% taken up by liver). Hepatic lipase (HL) then converts remaining IDL -> LDL by further

removing triglyceride.

What's the role of LDL vs HDL?

LDL: delivers cholesterol (endogenous + exogenous) TO tissues. HDL: 'reverse cholesterol transport' —

carries cholesterol FROM cells BACK to the liver.

How does lipoprotein density relate to lipid content?

MORE lipid = LESS dense (lipid is less dense than protein/water). So density order (increasing):

chylomicron < VLDL < IDL < LDL < HDL (as lipid content drops and protein content rises).

What activates fat mobilisation in adipocytes, and what enzyme carries it out?

A FALL in blood glucose (fasting/diabetes) activates Hormone-Sensitive Lipase (HSL), which

hydrolyses stored triglyceride -> glycerol + free fatty acids. FFAs travel in blood bound to serum albumin.

How are fatty acids activated before beta-oxidation? Where, and what's the cost?

On the OUTER mitochondrial membrane: Fatty acid + ATP + CoA-SH --(Acyl-CoA synthetase)--> Fatty

acyl-CoA + AMP + PPi (irreversible; PPi rapidly hydrolysed by pyrophosphatase, so effectively 2

ATP-equivalents used).

What is the rate-limiting/key transport step of beta-oxidation, and what carries it out?

Transport of the fatty acyl group into the mitochondrial matrix via CARNITINE. CAT I (carnitine

acyltransferase I, outer membrane/intermembrane space) transfers acyl-CoA's acyl group to carnitine ->

acylcarnitine crosses inner membrane via translocase -> CAT II (matrix side) transfers it back to

mitochondrial CoA-SH, regenerating fatty acyl-CoA + free carnitine

What inhibits CAT I (linking fatty acid synthesis and breakdown)?

Malonyl-CoA (an intermediate of fatty acid SYNTHESIS) inhibits CAT I — prevents fat synthesis and

breakdown happening simultaneously (futile cycle).

Where does carnitine come from?

Synthesised in the body from lysine + S-adenosylmethionine (requires vitamin C); also from diet (meat).

Extra supplementation gives no benefit beyond normal levels.

List the 4 steps of one beta-oxidation round, enzymes, and cofactors.

1. Oxidation: Acyl-CoA dehydrogenase; FAD -> FADH2 (removes H from alpha/beta carbons, forms

trans-2-enoyl-CoA). 2. Hydration: Enoyl-CoA hydratase; adds H2O across double bond ->

L-beta-hydroxyacyl-CoA. 3. Oxidation: beta-hydroxyacyl-CoA dehydrogenase; NAD+ -> NADH (oxidises

OH to ketone) -> beta-ketoacyl-CoA. 4. Cleavage (thiolysis): beta-ketothiolase + CoASH releases

Acetyl-CoA, leaving fatty acyl-CoA shortened by 2 carbons, ready for another round.

Why is it called 'BETA-oxidation'?

Because oxidation/modification happens at the beta-carbon, which is cleaved off (as part of the

2-carbon acetyl-CoA unit released each round).

How many rounds of beta-oxidation for a 16-carbon fatty acid (palmitate), and what's

produced?

7 rounds -> 8 x acetyl-CoA total (16C / 2C = 8 units; but only 7 oxidation cycles are needed since the

last cleavage yields 2 acetyl-CoA directly).

What's the total ATP yield from one palmitate (16:0), and how is it calculated?

106 ATP net. 8 acetyl-CoA through TCA: 24 NADH=60 ATP, 8 FADH2=12 ATP, 8 GTP=8 ATP. Plus

beta-oxidation itself: 7 NADH=17.5 ATP, 7 FADH2=10.5 ATP. Total=108, MINUS 2 ATP used for activation

= 106 ATP. (1 NADH=2.5 ATP, 1 FADH2=1.5 ATP by oxidative phosphorylation)

Why do fats yield more ATP per gram than glucose?

Fatty acids are more REDUCED (more C-H bonds) than glucose. Fat ~0.41 ATP/g vs glucose ~0.18

ATP/g (roughly matches 37 kJ/g fat vs 16 kJ/g carb).

What happens to odd-chain fatty acids in beta-oxidation? (not examinable in detail but

know the concept)

Last round produces a 3-carbon propionyl-CoA (instead of acetyl-CoA), which converts to succinyl-CoA

-> CAN enter gluconeogenesis in the liver (unlike acetyl-CoA, which can't, because pyruvate

dehydrogenase is irreversible).

What special enzymes handle unsaturated and very-long-chain fatty acids

Unsaturated: accessory enzymes convert cis double bonds to trans to allow beta-oxidation to continue.

Very-long-chain (20-26C): oxidised in peroxisomes; first step makes H2O2 instead of directly generating

ATP.

What are ketone bodies and when are they made?

Acetoacetate, 3(beta)-hydroxybutyrate, and acetone. Made in LIVER MITOCHONDRIA when

beta-oxidation outstrips carbohydrate breakdown (starvation, uncontrolled diabetes, low-carb diets).

Excess acetyl-CoA (that can't enter TCA due to low OAA) is converted to KBs

Why does the liver make ketone bodies during fasting (mechanism)?

Liver is doing gluconeogenesis -> depletes mitochondrial oxaloacetate -> TCA cycle activity depressed

-> acetyl-CoA (from FA beta-oxidation) builds up -> acetoacetyl-CoA backs up -> stimulates ketone body

production.

Why are ketone bodies useful for transporting energy?

They are water/blood soluble (unlike fatty acids which need albumin), so they easily carry fat-derived

energy from liver to muscle/brain, where they're converted BACK to acetyl-CoA and enter the TCA cycle.

Can the liver use the ketone bodies it makes?

No — liver lacks succinyl-CoA transferase, so it cannot oxidise ketone bodies itself; only exports them.

Where does fatty acid synthesis occur (which tissues, and where in the cell)?

Cytosol (NOT mitochondria — opposite compartment from beta-oxidation). Tissues: liver, adipose

tissue, CNS, lactating mammary gland.

Where does the acetyl-CoA (carbon source) for fatty acid synthesis come from?

Citrate is exported from mitochondria to cytosol (via the citrate shuttle) then cleaved back to acetyl-CoA

+ OAA by citrate lyase in the cytosol.

Where does the NADPH (reducing power) for fatty acid synthesis come from?

Pentose Phosphate Pathway (PPP) and malic enzyme (via malate -> pyruvate + NADPH).

What is the committed/rate-limiting step of fatty acid synthesis, and what

enzyme/cofactor?

Acetyl-CoA -> Malonyl-CoA, via Acetyl-CoA carboxylase, using biotin as a prosthetic group/CO2 carrier;

costs 1 ATP. Stimulated by citrate; inhibited by palmitoyl-CoA (end-product feedback)

How does malonyl-CoA prevent a futile cycle?

Malonyl-CoA (fat synthesis intermediate) INHIBITS CAT I / CPT1, blocking fatty acid entry into

mitochondria for beta-oxidation — ensures synthesis and breakdown don't run simultaneously.

What's the role of Fatty Acid Synthase (FAS) and its two key activities to know?

A multi-enzyme complex (dimer) that 'knits' the fatty acid chain. Key activities: Acyl Carrier Protein

(ACP) — carries the growing chain (via a phosphopantetheine arm, related to Coenzyme A/vitamin B5);

Condensing Enzyme (CE) — condenses the acyl and malonyl groups (via a cysteine thiol). The growing

chain shuttles between ACP and CE thiols of the two antiparallel subunits.

What's the overall stoichiometry of palmitate synthesis?

Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH -> Palmitate + 7 CO2 + 14 NADP+ + 8 CoA. (Accounting for

malonate synthesis: 8 acetyl-CoA + 14 NADPH + 7 ATP -> palmitate + 14 NADP+ + 8 CoA + 7 ADP + 7 Pi)

Compare beta-oxidation vs fatty acid synthesis (location, carriers, cofactors, carbon

unit).

Location: mitochondrial matrix vs cytosol. Acyl carrier: Coenzyme-A vs phosphopantetheine & cysteine

(on FAS). Electron acceptor/donor: FAD & NAD+ (accept e-) vs NADPH (donates e-). Carbon unit:

acetyl-CoA (product/donor) vs malonyl-CoA (& acetyl-CoA) as donor.

Is fatty acid synthesis literally beta-oxidation in reverse?

Chemically similar steps reversed, but biochemically NO — different cellular compartment, different

enzymes, different cofactors (NADPH vs FAD/NAD+), different acyl carriers.