Coenzymes & Enzyme Regulation

Enzymes are biological catalysts whose activity is finely tuned by accessory molecules (coenzymes/cofactors) and by an elegant hierarchy of regulatory mechanisms. This is a perennial NEET PG favourite because it ties vitamins → coenzymes → metabolic blocks → clinical deficiencies, and it overlaps directly with pharmacology (enzyme induction/inhibition).

Definitions & basic vocabulary

• Apoenzyme — the catalytically inactive protein part alone.
• Cofactor — a non-protein component required for activity. May be a metal ion or an organic molecule.
• Coenzyme — an organic, often vitamin-derived cofactor that is loosely bound and frequently acts as a co-substrate (e.g., NAD⁺ diffuses on and off).
• Prosthetic group — a cofactor that is tightly/covalently bound and stays with the enzyme (e.g., FAD in succinate dehydrogenase, biotin in carboxylases, haem in cytochromes).
• Holoenzyme = apoenzyme + cofactor = the complete, active enzyme.

High-yield: Coenzyme = loosely bound, recycled as co-substrate (NAD⁺, NADP⁺, CoA). Prosthetic group = tightly/covalently bound (FAD, biotin, lipoic acid, PLP in glycogen phosphorylase, haem). Examiners love asking which one is "covalently attached."

Major coenzymes and their vitamins

Most water-soluble (B-complex) vitamins are precursors of coenzymes — hence multi-enzyme failures in deficiency states.

Vitamin	Coenzyme	Group transferred / reaction	Classic enzyme example
B1 (thiamine)	TPP (thiamine pyrophosphate)	Aldehyde/2-carbon transfer; oxidative & non-oxidative decarboxylation	Pyruvate DH, α-KG DH, transketolase, branched-chain α-keto acid DH
B2 (riboflavin)	FMN, FAD	Redox (2 H, 1 or 2 e⁻)	Succinate DH, acyl-CoA DH, Complex I & II
B3 (niacin)	NAD⁺, NADP⁺	Hydride (2 e⁻ + H⁺) transfer	Lactate DH, glucose-6-P DH (NADP⁺)
B5 (pantothenate)	Coenzyme A, ACP	Acyl group transfer	Pyruvate DH, fatty acid synthase
B6 (pyridoxine)	PLP (pyridoxal phosphate)	Transamination, decarboxylation, transsulfuration	AST/ALT, ALA synthase, glycogen phosphorylase
B7 (biotin)	Biocytin	CO₂ (carboxylation)	Pyruvate carboxylase, acetyl-CoA carboxylase
B9 (folate)	THF (tetrahydrofolate)	One-carbon units (methyl, formyl)	Thymidylate synthase, AICAR transformylase
B12 (cobalamin)	Methylcobalamin, deoxyadenosylcobalamin	Methyl transfer; methylmalonyl-CoA mutase	Methionine synthase, MM-CoA mutase
Lipoic acid	Lipoamide	Acyl transfer + redox	Pyruvate DH, α-KG DH

High-yield: NAD⁺/NADH dominate catabolic (energy-yielding, oxidative) pathways feeding the ETC, while NADPH is the currency of reductive biosynthesis (fatty acid, cholesterol, steroid synthesis) and antioxidant defence (glutathione reductase). Remember: NADPH "Produces" and "Protects."

Key niacin vs. NADP point

NAD⁺ accepts a hydride at C4 of the nicotinamide ring (one substrate gives 2 e⁻ + H⁺ → NADH + H⁺). The only structural difference between NAD⁺ and NADP⁺ is a 2′-phosphate on the adenosine ribose of NADP⁺ — that single phosphate is how enzymes discriminate catabolic from anabolic cofactors.

TPP and the "ThDP-dependent" cluster

TPP-requiring reactions are a classic single-best-answer trap. The four are: pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, branched-chain α-keto acid dehydrogenase, and transketolase (HMP shunt). All except transketolase share the same multi-subunit architecture and use the same five cofactors.

High-yield mnemonic — cofactors of pyruvate & α-KG dehydrogenase: "Tender Loving Care For Nancy" → TPP, Lipoic acid, CoA, FAD, NAD⁺. (Vitamins B1, B5/lipoate, B5, B2, B3.)

High-yield: Transketolase activity in RBCs is the functional test for thiamine deficiency. The TPP effect = % rise in transketolase activity after adding TPP in vitro; >20–25% indicates deficiency. Asked repeatedly.

Metal ions as cofactors

• Zn²⁺ — carbonic anhydrase, carboxypeptidase, alcohol dehydrogenase, matrix metalloproteinases, DNA/RNA polymerases.
• Mg²⁺ — kinases (stabilises ATP), enolase.
• Fe²⁺/Fe³⁺ — catalase, peroxidase, cytochromes (haem).
• Cu²⁺ — cytochrome c oxidase, tyrosinase, lysyl oxidase, superoxide dismutase (Cu-Zn).
• Se — glutathione peroxidase (as selenocysteine, the "21st amino acid").
• Mo — xanthine oxidase, sulfite oxidase.

Enzyme regulation — the big picture

Cells regulate flux through pathways at several time-scales. A useful flow is:

Fastest → slowest: Substrate/product availability → allosteric effectors (milliseconds–seconds) → covalent modification (seconds–minutes) → proteolytic activation of zymogens (irreversible) → induction/repression of enzyme synthesis (hours, transcriptional) → degradation of enzyme protein.

1. Allosteric regulation

Allosteric enzymes have separate regulatory and catalytic sites, are usually multi-subunit, and give a sigmoidal (S-shaped) v vs. [S] curve (cooperativity), unlike the hyperbolic Michaelis–Menten curve.

• Positive cooperativity — substrate binding to one subunit raises affinity of others (e.g., haemoglobin–O₂; ATCase).
• Feedback (end-product) inhibition — the final product of a pathway inhibits the committed/first step, an allosteric mechanism that conserves resources.

Feature	Michaelis–Menten enzyme	Allosteric enzyme
Curve shape	Hyperbolic	Sigmoidal
Subunits	Often monomeric	Multimeric (regulatory + catalytic)
Effectors	None (classic)	Activators/inhibitors at separate site
Kinetic parameter altered	—	K-type (changes apparent Km) or V-type (changes Vmax)
Models	—	Concerted (MWC) / Sequential (KNF)

High-yield: Classic feedback-inhibition examples — CTP inhibits aspartate transcarbamoylase (ATCase) in pyrimidine synthesis; isoleucine inhibits threonine deaminase; cholesterol/sterols + mevalonate inhibit HMG-CoA reductase; long-chain acyl-CoA inhibits acetyl-CoA carboxylase; G6P inhibits hexokinase.

High-yield: Phosphofructokinase-1 (PFK-1) is THE rate-limiting, committed step of glycolysis: activated by AMP and fructose-2,6-bisphosphate, inhibited by ATP and citrate. F-2,6-BP is the single most important physiological allosteric activator and the master glycolysis/gluconeogenesis switch.

2. Covalent (reversible) modification — phosphorylation

The dominant reversible covalent modification is phosphorylation/dephosphorylation on Ser/Thr/Tyr residues, catalysed by kinases (add phosphate from ATP) and reversed by phosphatases.

Whether phosphorylation activates or inactivates depends on the enzyme:

Enzyme	Phosphorylated (= fed/stressed state)	Active form favours
Glycogen phosphorylase	Active	Glycogen breakdown
Glycogen synthase	Inactive	(Dephospho = active, synthesis)
Pyruvate dehydrogenase	Inactive	(Dephospho = active)
HMG-CoA reductase	Inactive	(Dephospho/insulin = active)
Hormone-sensitive lipase	Active	Lipolysis
Acetyl-CoA carboxylase	Inactive	(Dephospho = active, FA synthesis)

High-yield: A reliable rule — glucagon/adrenaline (cAMP→PKA) phosphorylate; insulin (via phosphatases) dephosphorylates. In glycogen metabolism, phosphorylation simultaneously activates breakdown (phosphorylase) and inactivates synthesis (synthase) — reciprocal regulation prevents a futile cycle.

Other covalent modifications tested occasionally: adenylylation (E. coli glutamine synthetase), methylation, acetylation, ADP-ribosylation, ubiquitination, and γ-carboxylation (vitamin K-dependent clotting factors II, VII, IX, X).

3. Zymogens (proenzymes) — irreversible activation

Zymogens are synthesised as inactive precursors and activated by irreversible proteolytic cleavage, protecting the cell from premature, self-destructive activity (especially proteases).

Zymogen	Active enzyme	Activator
Trypsinogen	Trypsin	Enteropeptidase (enterokinase)
Chymotrypsinogen	Chymotrypsin	Trypsin
Proelastase	Elastase	Trypsin
Pepsinogen	Pepsin	H⁺ (autocatalytic), then pepsin
Prothrombin	Thrombin	Factor Xa
Procaspases	Caspases	Apoptotic cleavage

High-yield: Enteropeptidase (enterokinase) activates trypsinogen → trypsin → trypsin then activates ALL other pancreatic zymogens (a cascade amplifier). Premature intrapancreatic trypsinogen activation underlies acute pancreatitis; protective brakes include trypsin inhibitor (SPINK1) and trypsin autolysis (the site mutated in hereditary pancreatitis, PRSS1).

4. Induction & repression (gene-level, slow)

Cells change the amount of enzyme by altering transcription — relevant to nutrition and, critically, pharmacology.

• Induction increases enzyme synthesis; repression decreases it. These take hours to days.
• Metabolic examples: high-carbohydrate/insulin induces glucokinase, ACC, FAS, G6PD; glucagon/glucocorticoids induce PEPCK and gluconeogenic enzymes.

High-yield (pharmacology overlap): Cytochrome P450 inducers — "PCBRAS / CRAP-GPS": Phenytoin, Carbamazepine, Barbiturates, Rifampicin, Alcohol (chronic), St. John's wort, Griseofulvin, Phenobarbitone, Smoking. Inhibitors — "GO DEVICES" / cimetidine-erythromycin-ketoconazole group: cimetidine, ketoconazole, erythromycin/clarithromycin, isoniazid, grapefruit juice, valproate, ritonavir, fluconazole, omeprazole, ciprofloxacin. Inducers lower drug levels (warfarin failure with rifampicin); inhibitors raise them (toxicity).

Enzyme kinetics quick recap (frequently bundled in)

• Km = substrate concentration at ½ Vmax; inversely related to affinity (low Km = high affinity).
• Competitive inhibitor: Km ↑, Vmax unchanged; reversible by ↑[S]. (e.g., statins vs. HMG-CoA reductase; methotrexate vs. DHFR; ethanol/fomepizole vs. alcohol dehydrogenase.)
• Non-competitive: Vmax ↓, Km unchanged.
• Uncompetitive: both Km and Vmax ↓ (ratio constant); classic example lithium on inositol monophosphatase.
• Irreversible/suicide inhibitors: aspirin (COX), allopurinol (xanthine oxidase), penicillin (transpeptidase), omeprazole (H⁺/K⁺ ATPase), DFP/organophosphates (acetylcholinesterase).

High-yield: Allopurinol is a substrate analogue that is converted to oxypurinol, a tight-binding inhibitor of xanthine oxidase — a beautiful "suicide-substrate" example. Methotrexate inhibits dihydrofolate reductase (competitive), depleting THF.

Clinical correlations (deficiency → enzyme failure)

• Thiamine (B1) deficiency → impaired PDH/α-KG-DH → lactic acidosis, Wernicke–Korsakoff, beriberi (wet = high-output cardiac failure; dry = neuropathy). Give thiamine before glucose.
• Pyridoxine (B6) deficiency → ↓ALA synthase, ↓transamination → sideroblastic anaemia, peripheral neuropathy (classic with isoniazid, which is a B6 antagonist), seizures in neonates.
• Niacin (B3) deficiency → pellagra (the 3 Ds: dermatitis, diarrhoea, dementia); seen in Hartnup disease and carcinoid (tryptophan diversion).
• Biotin deficiency → from raw egg white (avidin) or biotinidase deficiency → dermatitis, alopecia, lactic acidosis.
• Folate/B12 → megaloblastic anaemia; B12 additionally causes subacute combined degeneration (methylmalonyl-CoA pathway).

Key differentials / "don't confuse" pairs

• Coenzyme vs. prosthetic group — binding tightness (loose, co-substrate vs. tight/covalent).
• NAD⁺ vs. NADP⁺ — catabolism vs. anabolism; one 2′-phosphate apart.
• Allosteric vs. covalent regulation — non-covalent reversible effector binding (instant) vs. covalent phosphorylation (kinase/phosphatase) vs. irreversible proteolysis (zymogen).
• Isozymes vs. zymogens — isozymes are different molecular forms catalysing the same reaction (LDH1–5, CK-MB/MM/BB) used as diagnostic markers; zymogens are inactive precursors.
• Competitive vs. uncompetitive inhibition — Km changes opposite ways; Vmax behaviour differs.

Recently asked / exam angle

• "Which coenzyme is required by transketolase?" → TPP (and the RBC transketolase/TPP-effect test for thiamine).
• "Cofactors of the pyruvate dehydrogenase complex?" → TPP, lipoic acid, CoA, FAD, NAD⁺ (5 cofactors, 3 enzymes).
• "Rate-limiting enzyme of glycolysis and its activator?" → PFK-1, activated by fructose-2,6-bisphosphate/AMP, inhibited by ATP/citrate.
• "Enzyme activated by phosphorylation?" → glycogen phosphorylase / hormone-sensitive lipase (synthase is inactivated).
• "Activator of trypsinogen?" → enteropeptidase (enterokinase).
• "P450 inducer/inhibitor affecting warfarin/OCP?" → rifampicin (induces, ↓effect) vs. cimetidine/ketoconazole (inhibit, toxicity).
• "Type of inhibition by statins / methotrexate / allopurinol?" → competitive (statins, MTX) and suicide/irreversible (allopurinol→oxypurinol).
• "Coenzyme form of vitamin B6?" → pyridoxal phosphate (PLP), also a prosthetic group of glycogen phosphorylase.
• "Which vitamin/cofactor in carboxylation reactions?" → biotin (CO₂ fixation); vitamin K for γ-carboxylation of clotting factors.
• "NADPH-generating pathway?" → HMP shunt (G6PD), malic enzyme, isocitrate DH.

Rapid revision

1. Holoenzyme = apoenzyme + cofactor; coenzyme is loosely bound (co-substrate), prosthetic group is tight/covalent (FAD, biotin, haem, PLP-in-phosphorylase).
2. NAD⁺/FAD → catabolism & ETC; NADPH → reductive biosynthesis + antioxidant (glutathione) defence. NADP⁺ differs from NAD⁺ by one 2′-phosphate.
3. TPP serves pyruvate-DH, α-KG-DH, branched-chain keto-acid DH, and transketolase; RBC transketolase + TPP-effect tests thiamine.
4. PDH/α-KG-DH cofactors: TPP, Lipoate, CoA, FAD, NAD⁺ ("Tender Loving Care For Nancy").
5. PFK-1 is glycolysis rate-limiter: ↑ by AMP & fructose-2,6-BP, ↓ by ATP & citrate.
6. Feedback inhibition hits the committed step: CTP→ATCase, cholesterol→HMG-CoA reductase, isoleucine→threonine deaminase.
7. Phosphorylation activates glycogen phosphorylase & HS-lipase but inactivates glycogen synthase, PDH, ACC — glucagon/adrenaline phosphorylate, insulin dephosphorylates.
8. Zymogens are activated by irreversible proteolysis; enteropeptidase triggers the pancreatic trypsin cascade; defective braking → pancreatitis.
9. P450 inducers (rifampicin, phenytoin, carbamazepine, barbiturates, chronic alcohol, St John's wort) lower drug levels; inhibitors (cimetidine, ketoconazole, erythromycin, grapefruit) raise them.
10. Competitive inhibitor: Km↑, Vmax unchanged (statins, MTX). Uncompetitive: both ↓ (lithium). Non-competitive: Vmax↓, Km same.
11. Suicide/irreversible inhibitors: allopurinol→oxypurinol (xanthine oxidase), aspirin (COX), omeprazole (proton pump), penicillin (transpeptidase).
12. Allosteric enzymes give sigmoidal kinetics and have separate regulatory sites (MWC concerted vs. KNF sequential models).