Ornithine decarboxylase
class of enzymes
The enzyme ornithine decarboxylase (EC 4.1.1.17, ODC) catalyzes the decarboxylation of ornithine (a non-proteinogenic amino acid commonly produced from arginine) to form putrescine. Ornithine is part of both the urea cycle and polyamine synthesis, and this reaction is the committed step in that latter polyamine pathway. In the urea cycle, ornithine is instead acted on by the enzyme ornithine transcarbamylase. In humans, this protein has 461 amino acids and forms a homodimer.
In humans, ornithine decarboxylase (ODC) is expressed by the gene ODC1. The protein ODC is sometimes referred to as "ODC1" in research pertaining to humans and mice, but certain species such as Drosophila (dODC2), species of Solanaceae plant family (ODC2), and the lactic acid bacteria Paucilactobacillus wasatchensis (odc2) have been shown to have a second ODC gene.
Reaction mechanism
Lysine 69 on ornithine decarboxylase (ODC) binds the cofactor pyridoxal phosphate to form a Schiff base. Ornithine displaces the lysine to form a Schiff base attached to orthonine, which decarboxylates to form a quinoid intermediate. This intermediate rearranges to form a Schiff base attached to putrescine, which is attacked by lysine to release putrescine product and reform PLP-bound ODC. This is the first step and the rate-limiting step in humans for the production of polyamines, compounds required for cell division.
Spermidine synthase can then catalyze the conversion of putrescine to spermidine by the attachment of an aminopropyl moiety.
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