| Vitamin | Impaired Reaction | Enzyme Affected | Reaction Catalyzed | Pathway/Consequence |
| Vitamin B6 (PLP) | Transamination of amino acids | Aminotransferases (e.g., ALT, AST) | Alanine + α-Ketoglutarate ↔ Pyruvate + Glutamate | Impaired amino acid catabolism and gluconeogenesis |
| Conversion of glutamate to γ-aminobutyric acid (GABA) | Glutamate decarboxylase | Glutamate → GABA + CO₂ | Impaired neurotransmitter synthesis | |
| Conversion of homocysteine to cystathionine and cysteine | Cystathionine β-synthase | Homocysteine + Serine → Cystathionine | Homocysteine accumulation | |
| Thiamine (B1) | Oxidative decarboxylation of branched-chain α-keto acids | Branched-chain α-keto acid dehydrogenase | Valine, Leucine, Isoleucine α-keto acids + NAD⁺ → Acyl-CoA + NADH + CO₂ | Impaired branched-chain amino acid catabolism |
| Folate (B9) | Conversion of homocysteine to methionine (methyl group transfer) | Methionine synthase (with Vitamin B12) | Homocysteine + 5-Methyl-THF → Methionine + THF | Homocysteine accumulation, disrupted methylation |
| Vitamin B12 | Conversion of methylmalonyl-CoA to succinyl-CoA | Methylmalonyl-CoA mutase | Methylmalonyl-CoA → Succinyl-CoA | Impaired odd-chain fatty acid and amino acid metabolism |
| Conversion of homocysteine to methionine | Methionine synthase | Homocysteine + 5-Methyl-THF → Methionine + THF | Homocysteine accumulation, impaired SAM synthesis | |
| Riboflavin (B2) | Oxidation of L-amino acids (e.g., proline, lysine) | L-amino acid oxidase | L-Amino acid + FAD → α-Imino acid + FADH₂ | Impaired amino acid degradation |
| Biotin | Carboxylation in amino acid metabolism | Propionyl-CoA carboxylase | Propionyl-CoA + CO₂ + ATP → Methylmalonyl-CoA + ADP + Pi | Impaired catabolism of valine, isoleucine, threonine |
Key Reactions Explained:
- Vitamin B6 (PLP):
- Aminotransferases (ALT, AST): Vital for interconversion of amino acids and keto acids in transamination reactions.
- Cystathionine β-synthase: Converts homocysteine and serine to cystathionine, and then to cysteine, preventing homocysteine buildup.
- Glutamate decarboxylase: Critical for GABA synthesis, affecting neurotransmission.
- Thiamine (B1):
- Branched-chain α-keto acid dehydrogenase: Catalyzes oxidative decarboxylation in branched-chain amino acid catabolism (e.g., valine, leucine).
- Folate (B9) and Vitamin B12:
- Methionine synthase: Facilitates methylation cycles by regenerating methionine from homocysteine.
- Biotin:
- Propionyl-CoA carboxylase: Handles catabolism of amino acids generating propionyl-CoA (e.g., valine, isoleucine).
