# Coenzyme Functions > Coenzyme functions in MBBS Biochemistry: thiamine, FAD, NAD, CoA, pyridoxal phosphate, biotin and cobalamin as enzyme partners with deficiency correlations. - Canonical URL: https://prepelephant.com/topics/mbbs/biochemistry/coenzyme-functions - Exam / course: MBBS · Subject: Biochemistry - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Coenzyme Functions", PrepElephant, https://prepelephant.com/topics/mbbs/biochemistry/coenzyme-functions ## 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.