The "Gln-Type" Thiol Dioxygenase from Azotobacter vinelandii Is a 3-Mercaptopropionic Acid Dioxygenase

Cysteine dioxygenase (CDO) is a non-heme iron enzyme that catalyzes the O-2-dependent oxidation of L-cysteine to produce cysteinesulfinic acid. Bacterial CDOs have been subdivided as either "Arg-type" or "Gln-type" on the basis of the identity of conserved active site residues. To date, "Gln-type" enzymes remain largely uncharacterized. It was recently noted that the "Gln-type" enzymes are more homologous with another thiol dioxygenase [3-mercaptopropionate dioxygenase (MDO)] identified in Variovorax paradoxus, suggesting that enzymes of the "Gln-type" subclass are in fact MDOs. In this work, a putative "Gln-type" thiol dioxygenase from Azotobacter vinelandii (Av) was purified to homogeneity and characterized. Steady-state assays were performed using three substrates [3-mercaptopropionic acid (3mpa), L-cysteine (cys), and cysteamine (ca)]. Despite comparable maximal velocities, the "Gln-type" Av enzyme exhibited a specificity for 3mpa (k(cat)/K-M = 72000 M-1 s(-1)) nearly 2 orders of magnitude greater than those for cys (110 M-1 s(-1)) and ca (11 M-1 s(-1)). Supporting X-band electron paramagnetic resonance (EPR) studies were performed using nitric oxide (NO) as a surrogate for O-2 binding to confirm obligate-ordered addition of substrate prior to NO. Stoichimetric addition of NO to solutions of 3mpa-bound enzyme quantitatively yields an iron-nitrosyl species (Av ES-NO) with EPR features consistent with a mononuclear (S = 3/2) {FeNO}(7) site. Conversely, two distinct substrate-bound conformations were observed in Av ES-NO samples prepared with cys and ca, suggesting heterogeneous binding within the enzymatic active site. Analytical EPR simulations are provided to establish the relative binding affinity for each substrate (3map > cys > ca). Both kinetic and spectroscopic results presented here are consistent with 3mpa being the preferred substrate for this enzyme.