Most B vitamins are coenzyme precursors for specific reaction types
Mnemonic
Each B vitamin becomes a named cofactor, and the pairing is worth learning as a set:
- B1 thiamine - thiamine pyrophosphate. Pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, transketolase
- B2 riboflavin - FAD and FMN
- B3 niacin - NAD and NADP
- B5 pantothenate - coenzyme A
- B6 pyridoxine - pyridoxal phosphate. Transamination and decarboxylation
- B7 biotin - carboxylation. Pyruvate carboxylase, acetyl-CoA carboxylase
- B9 folate - tetrahydrofolate, one carbon transfer
- B12 - methylmalonyl-CoA mutase and methionine synthase
“Thiamine for the dehydrogenases, B6 for the transaminases, biotin for the carboxylases.”
That explains the biochemistry of Wernicke’s: without thiamine, pyruvate dehydrogenase fails, so pyruvate accumulates and is converted to lactate, and giving glucose before thiamine consumes the last of it and precipitates the syndrome.
Expansion
| Vitamin | Coenzyme | Reaction type |
|---|---|---|
| B1 thiamine | Thiamine pyrophosphate | Oxidative decarboxylation (pyruvate and alpha-ketoglutarate dehydrogenase), transketolase |
| B2 riboflavin | FAD, FMN | Oxidation and reduction |
| B3 niacin | NAD, NADP | Oxidation and reduction |
| B5 pantothenate | Coenzyme A | Acyl transfer |
| B6 pyridoxine | Pyridoxal phosphate | Transamination, decarboxylation, ALA synthase |
| B7 biotin | Biotin | Carboxylation (pyruvate carboxylase, acetyl-CoA carboxylase) |
| B9 folate | Tetrahydrofolate | One carbon transfer |
| B12 cobalamin | Methylcobalamin, adenosylcobalamin | Methyl transfer, methylmalonyl-CoA mutase |
This table explains several clinical observations. Thiamine deficiency blocks pyruvate dehydrogenase, so pyruvate accumulates and is converted to lactate, giving a lactic acidosis and explaining why glucose without thiamine is dangerous in the malnourished.
B6 deficiency with isoniazid causes peripheral neuropathy and can cause sideroblastic anaemia, since ALA synthase requires it.
Erythrocyte transketolase activity is the functional test for thiamine status, and it increases markedly when thiamine pyrophosphate is added in vitro if the patient is deficient.