Ornithine decarboxylase

The enzyme ornithine decarboxylase (ODC) is a homodimer of 461 amino acids (in humans, at least).

Reaction
It 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.

Clinical significance
ODC gene expression is induced by a large number of biological stimuli including seizure activity in the brain.

Ubiquitin-independent proteasomal degradation
ODC is the most well-characterized cellular protein subject to ubiquitin-independent proteasomal degradation. Although most proteins must first be tagged with multiple ubiquitin molecules before they are bound and degraded by the proteasome, ODC degradation is instead mediated by several recognition sites on the protein and its accessory factor antizyme 1. The ODC degradation process is regulated in a negative feedback loop by its reaction products.

Until a report by Sheaff et al. (2000), which demonstrated that the cyclin-dependent kinase (Cdk) inhibitor p21Cip1 is also degraded by the proteasome in a ubiquitin-independent manner, ODC was only clear example of ubiquitin-independent proteasomal degradation.