The enzyme ornithine cyclodeaminase (EC 4.3.1.12) catalyzes the chemical reaction
<div style="text-align:center"><small>L</small>-ornithine <small>L</small>-proline + NH<sub>4</sub><sup>+</sup></div>
This enzyme belongs to the family of lyases, specifically ammonia lyases, which cleave carbon-nitrogen bonds. The systematic name of this enzyme class is <small>L</small>ornithine ammonia-lyase (cyclizing; <small>L</small>-proline-forming). Other names in common use include ornithine cyclase, ornithine cyclase (deaminating), and <small>L</small>-ornithine ammonia-lyase (cyclizing). This enzyme participates in arginine and proline biosynthesis. It employs one cofactor, NAD<sup>+</sup>.
As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and .