Coenzyme Functions
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Direct answer
Apoenzyme plus coenzyme equals holoenzyme: the protein contributes specificity, the coenzyme contributes chemistry that amino-acid side chains cannot perform — electron transfer, one-carbon shuffling, amino-group swap or carbon dioxide fixation. Nearly every coenzyme is a vitamin derivative: thiamine pyrophosphate for oxidative decarboxylation and transketolation, FAD and FMN from riboflavin for redox, NAD from niacin for dehydrogenations, coenzyme A from pantothenate for acyl-group traffic, pyridoxal phosphate for transamination and decarboxylation, biotin for carboxylation and cobalamin for methyl transfer and rearrangements. Name the coenzyme and the deficiency syndrome follows — which is precisely how exams pair them.
What you must remember
- Thiamine pyrophosphate (B1): cofactor of pyruvate and alpha-ketoglutarate dehydrogenase, transketolase and branched-chain ketoacid dehydrogenase; deficiency gives beri-beri and Wernicke encephalopathy; the erythrocyte transketolase activation assay confirms deficiency.
- FAD and FMN (B2): succinate dehydrogenase, acyl-CoA dehydrogenases, glutathione reductase; FMN is complex I's prosthetic group.
- NAD and NADP (B3): hydride carriers; NAD feeds electrons to the respiratory chain, NADPH drives reductive synthesis and keeps glutathione reduced; niacin can be synthesised from tryptophan (roughly 60 mg tryptophan per mg niacin, the number exams quote).
- Coenzyme A (B5): carries acyl groups through a thioester bond; the same phosphopantetheine chemistry powers the acyl carrier protein of fatty acid synthase.
- Pyridoxal phosphate (B6): transaminases (AST, ALT), amino-acid decarboxylases including glutamate decarboxylase for GABA; more than a hundred PLP enzymes exist; isoniazid antagonises it — the reason pyridoxine is co-prescribed in Indian tuberculosis regimens.
- Biotin: pyruvate carboxylase, acetyl-CoA carboxylase, propionyl-CoA carboxylase; raw egg-white avidin binds it, causing deficiency.
- Cobalamin (B12): methylcobalamin for methionine synthase, adenosylcobalamin for methylmalonyl-CoA mutase — deficiency shows a megaloblastic anaemia with neurological fallout.
- Non-vitamin coenzymes: lipoic acid and coenzyme Q are the classic answers to "which coenzyme is not a vitamin derivative".
The transamination bench test
Serum AST and ALT are measured millions of times a day in India, and each value rests on PLP chemistry. In the ALT reaction, alanine hands its amino group to alpha-ketoglutarate through a Schiff base with pyridoxal phosphate, forming pyruvate and glutamate; the glutamate is then oxidised in a coupled reaction that reduces NADP to NADPH, which the autoanalyser reads at 340 nanometres. One coenzyme, one Schiff base intermediate, two enzymes — and the laboratory number that decides hepatitis monitoring. The same PLP logic explains isoniazid-induced peripheral neuropathy: the drug forms a hydrazone with pyridoxal phosphate, functionally starving decarboxylases, so national tuberculosis guidance supplements pyridoxine throughout therapy. Connect the bench to the bedside this way and the vitamin table stops being a memory load.
High-yield viva angles
Examiners probe three fault lines. Which coenzyme serves the greatest number of enzymes — pyridoxal phosphate, the safe answer. NAD versus NADP: NAD's phosphate-free state suits catabolism and ATP harvesting, while NADPH's extra phosphate is a binding tag for anabolic and detoxification enzymes; do not merge them into "nicotinamide does everything". Finally, lipoic acid and coenzyme Q trip students who recite "all coenzymes are vitamins" — both are synthesised endogenously, and lipoic acid is the accepted answer for a coenzyme that is not a vitamin. A sharp corollary worth stating: lipoamide is covalently attached (prosthetic group) while NAD dissociates freely (cosubstrate) — the prosthetic-versus-cosubstrate distinction earns marks.
Frequently asked questions
Which coenzyme is required for transamination reactions?
Pyridoxal phosphate, the active B6 vitamin, forming a Schiff base intermediate with the amino acid before transfer to the keto acid.
Which enzyme assay confirms thiamine deficiency?
Erythrocyte transketolase activity with and without added TPP — a marked activation on supplementation indicates deficiency.
Which vitamin gives rise to FAD and FMN?
Riboflavin (B2), phosphorylated first to FMN and then adenylated to FAD.
Which coenzyme carries carbon dioxide in carboxylation reactions?
Biotin, serving pyruvate carboxylase, acetyl-CoA carboxylase and propionyl-CoA carboxylase as a CO2 carrier between its valeric acid carboxyl and the enzyme's lysine.
Why is pyridoxine given with isoniazid?
Isoniazid forms an inactive hydrazone with pyridoxal phosphate, inducing B6 deficiency neuropathy, so supplementation protects the nervous system during antitubercular therapy.