3,4-Dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione 4,5-Dioxygenase from Nocardia restrictus

Kinetic studies with 3,4-dihydroxy-9,10-secoandrosta-1,3,5(10)-triene-9,17-dione-4,5-dioxygenase gave intersecting initial velocity plots that conform to a sequential mechanism. Linear noncompetitive product inhibition patterns were observed with respect to either substrate, indicating the formation of a dead end complex. 4-Isopropyl catechol, a structural analogue of the organic substrate, inhibited the enzyme competitively with respect to the organic substrate as expected, but inhibited the enzyme uncompetitively with respect to molecular oxygen. These results are consistent with an ordered Bi-Uni mechanism where molecular oxygen is added first, followed by organic substrate, and the product is then released. Detailed analysis of the kinetic data suggests the existence of a dead end enzyme-oxygen-product complex.