Coenzymes and B vitamins
Coenzymes are small organic helpers that carry groups or electrons for enzymes. Every B vitamin works as, or is made into, a coenzyme.
B vitamins as coenzymes
| Vitamin | Coenzyme | Reaction type | Example enzyme |
|---|---|---|---|
| Thiamine (B1) | Thiamine pyrophosphate (TPP) | Oxidative decarboxylation; transketolase reaction | Pyruvate dehydrogenase, α-ketoglutarate dehydrogenase, transketolase |
| Riboflavin (B2) | FMN, FAD | Oxidation–reduction (carry hydrogen) | Succinate dehydrogenase (FAD), NADH dehydrogenase (FMN) |
| Niacin (B3) | NAD+, NADP+ | Oxidation–reduction (carry hydrogen) | Lactate dehydrogenase (NAD+), glucose-6-phosphate dehydrogenase (NADP+) |
| Pantothenic acid (B5) | Coenzyme A (CoA) | Acyl (e.g. acetyl) group transfer | Pyruvate dehydrogenase (forms acetyl-CoA) |
| Pyridoxine (B6) | Pyridoxal phosphate (PLP) | Amino acid reactions: transamination, decarboxylation | ALT, AST; glycogen phosphorylase |
| Biotin (B7) | Biocytin (enzyme-bound biotin) | Carboxylation (adds CO2) | Pyruvate carboxylase, acetyl-CoA carboxylase |
| Folic acid (B9) | Tetrahydrofolate (THF) | One-carbon transfer | Thymidylate synthase (DNA synthesis) |
| Cobalamin (B12) | Methylcobalamin, deoxyadenosylcobalamin | Methyl transfer; rearrangement | Methionine synthase, methylmalonyl-CoA mutase |
Grouping coenzymes by job
- Carry hydrogen or electrons: NAD+, NADP+ (niacin); FMN, FAD (riboflavin); coenzyme Q.
- Carry other groups: TPP (2-carbon aldehyde units), CoA (acyl groups), PLP (amino groups), biotin (CO2), THF (one-carbon units), B12 coenzymes (methyl groups).
- Some coenzymes, such as ATP, coenzyme Q and lipoic acid, are made in the body, so they are not vitamins.
Two kinds of cofactor
A coenzyme held tightly or covalently, such as FAD in succinate dehydrogenase or biotin in carboxylases, is called a prosthetic group.
B-vitamin deficiency is really coenzyme deficiency. No TPP → beriberi; no NAD+ → pellagra (dermatitis, diarrhoea, dementia), seen in maize- and jowar-eating areas; low folate or B12 → megaloblastic anaemia. Vitamin B12 comes only from animal foods (milk, curd, eggs, meat), so strict vegetarians are at risk. Raw egg white contains avidin, which binds biotin. Good sources: hand-pounded rice and pulses (B1), milk and green leafy vegetables (B2), groundnut (niacin), green leafy vegetables (folate).
10 marks: “Discuss the role of B-complex vitamins as coenzymes.” Tabulate each B vitamin with its coenzyme form, reaction type and one enzyme; group them by function and end with the deficiency diseases.
5 marks: “Explain the coenzyme role of niacin and riboflavin.” Niacin forms NAD+ and NADP+; riboflavin forms FMN and FAD. Both carry hydrogen in dehydrogenase reactions and in the electron transport chain.
2 marks: “What is a coenzyme? Give two examples.” A non-protein organic cofactor, mostly made from B vitamins, that helps an enzyme by carrying groups or electrons. Examples: NAD+, TPP.
The first letter often gives it away: Niacin → NAD, riboFlavin → FAD and FMN, Pyridoxine → PLP, Thiamine → TPP.