Ornithine decarboxylase
The enzyme ornithine decarboxylase (PDB 7odc, EC 4.1.1.17) is a homodimer of 461 amino acids (in humans, at least) that catalyzes the decarboxylation of ornithine producing, as a result, diamine putrescine. This is the first step and the rate limiting step in humans for the production of polyamines, compounds required for cell division.