Pyridoxal phosphate

Pyridoxal phosphate is the active coenzyme form of vitamin B₆. Its pyridine ring and reactive aldehyde group enable enzymes to catalyze reactions involving amino acids.

Schiff base: A reversible covalent linkage formed when an aldehyde or ketone reacts with a primary amine. PLP forms Schiff bases with enzyme lysines and substrate amino groups during catalysis.
Aspartate aminotransferase: An enzyme that interconverts aspartate and oxaloacetate while transferring an amino group to or from glutamate. It uses PLP to link amino-acid metabolism with the tricarboxylic acid cycle.
Vitamin B6: A group of related compounds that serve as precursors to the coenzyme forms pyridoxal phosphate and pyridoxamine phosphate. PLP is produced from vitamin B₆ vitamers through cellular phosphorylation and oxidation.
Vitamin B6 deficiency: A state of inadequate vitamin B₆ that can impair PLP-dependent metabolism and cause neurological or hematological symptoms. Insufficient PLP can disrupt several enzyme systems, including neurotransmitter synthesis.
Transamination: The transfer of an amino group between an amino acid and a keto acid. PLP enables amino-group transfer by alternating between amino-acid and keto-acid forms.
Glutamate decarboxylase: An enzyme that converts glutamate into gamma-aminobutyric acid by removing carbon dioxide. Its PLP-dependent reaction produces the inhibitory neurotransmitter GABA.
Pyridine: A six-membered aromatic heterocycle containing one nitrogen atom. PLP's pyridine ring helps stabilize charged reaction intermediates.
Isoniazid: An antibiotic used to treat tuberculosis that can interfere with vitamin B₆ metabolism. Isoniazid can reduce functional PLP availability, making pyridoxine supplementation relevant in some patients.
External aldimine: A PLP–substrate Schiff base formed after the substrate displaces an enzyme-bound lysine. This intermediate positions the substrate for bond cleavage or rearrangement.
Aromatic L-amino acid decarboxylase: A PLP-dependent enzyme that decarboxylates aromatic amino acids and their hydroxylated derivatives. It helps synthesize dopamine and serotonin precursors from amino acids.

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