Catalytic Oxidation of Dithiols by a Semisynthetic Enzyme

Article abstract of DOI:10.1021/ja00272a052

The semisynthetic enzyme flavopapain (1C), obtained from the alkylation of Cys-25 of papain with 8α-(bromoacetyl)-10-methylisoalloxazine (1B), was found to be an effective catalyst for the oxidation of dithiols to disulfides.The k₂/K_s values for the oxidation of d,l-dihydrolipoamide and d,l-dihydrolipoic acid determined from anaerobic single-reaction stopped-flow kinetics were 4400 and 3400 M^-1 s^-1, respectively.These values were, respectively, 126 and 200 times larger than the second-order rate constants for oxidation of d,l-dihydrolipoamide and d,l-dihydrolipoic acid by the model flavin 8-acetyl-10-methylisoalloxazine (1A).Under aerobic turnover conditions using the synthetic dye MTT as an electron acceptor, the k_cat and K_m values for the oxidation of d,l-dihydrolipoamide by 1C were in approximate agreement with the k₂ and K_s values, indicating that the rate-limiting step of catalytic cycle is substrate oxidation rather than oxidation of dihydroflavopapain.When compared with flavopapains 2C and 3C , flavopapain 1C is the most efficient catalyst.The circular dichroic spectra of flavopapains 1C, 2C and 3C were recorded, and the dissociation constants of the sulfite addition complexes of 1C and 2C were determined.From these kinetic and physical studies, the differences in catalytic activity of 1C, 2C, and 3C were judged to be due to changes in the flavin orientation within the active site and the ability to fit the substrate into a productive reaction conformation.