  the enzyme ornithine decarboxylase (odc) catalyzes the decarboxylation of ornithine (a product of the urea cycle) to form putrescine. this reaction is the committed step in polyamine synthesis. in humans, this protein has 461 amino acids and forms a homodimer. == reaction mechanism == lysine 69 on ornithine decarboxylase binds the cofactor pyridoxal phosphate to form a schiff base. ornithine displaces the lysine to form a schiff base attached to ornithine, 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 ornithine decarboxylase. 900px|border|alt=ornithine decarboxylase mechanism|hypothesized ornithine decarboxylase mechanism. this is the first step and the rate-limiting step in humans for the production of polyamines, compounds required for cell division. ==structure== [[image:ornithine decarboxylase publication view.png|270px|thumb|right|alt = colored dice with checkered background|3d crystal structure of ornithine decarboxylase.; {{cite journal | author = almrud jj, oliveira ma, kern ad, grishin nv, phillips ma, hackert ml. | title = crystal structure of human ornithine decarboxylase at 2.1 a resolution: structural insights to antizyme binding | journal = j. mol. biol. | volume = 295 | issue = 1 | pages = 7â16 |date=january 2000 | pmid = 10623504| doi = 10.1006/jmbi.1999.3331| url = http://www.sciencedirect.com/science?_ob=articleurl&_udi=b6wk7-45f4v3c-5p&_user=483702&_coverdate=01%2f07%2f2000&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchstrid=1468011589&_rerunorigin=google&_acct=c000022720
EC_4.1.1 Enzymes Genes_on_chromosome_2