Three different enzymes - all pyridoxal-dependent decarboxylases - seem to
share regions of sequence similarity [1,2,3,4], especially in the vicinity of
the lysine residue which serves as the attachment site for the pyridoxal-phosphate (PLP) group. These enzymes are:
Glutamate decarboxylase (EC 18.104.22.168) (GAD). Catalyzes the decarboxylation
of glutamate into the neurotransmitter GABA (4-aminobutanoate).
Histidine decarboxylase (EC 22.214.171.124) (HDC). Catalyzes the decarboxylation
of histidine to histamine. There are two completely unrelated types of HDC:
those that use PLP as a cofactor (found in Gram-negative bacteria and
mammals), and those that contain a covalently bound pyruvoyl residue (found
in Gram-positive bacteria).
Aromatic-L-amino-acid decarboxylase (EC 126.96.36.199) (DDC), also known as
L-dopa decarboxylase or tryptophan decarboxylase. DDC catalyzes the
decarboxylation of tryptophan to tryptamine. It also acts on 5-hydroxy-
tryptophan and dihydroxyphenylalanine (L-dopa).
Tyrosine decarboxylase (EC 188.8.131.52) (TyrDC) which converts tyrosine into
tyramine, a precursor of isoquinoline alkaloids and various amides.
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