Roles of Conserved Residues of the Glycine Oxidase GoxA in Controlling Activity, Cooperativity, Subunit Composition, and Cysteine Tryptophylquinone Biosynthesis.

Abstract GoxA is a glycine oxidase which possesses a cysteine tryptophylquinone (CTQ) cofactor that is formed by posttranslational modifications that are catalyzed by a modifying enzyme GoxB. It is the second known tryptophylquinone enzyme to function as an oxidase; the other being the lysine epsilon-oxidase, LodA. All other enzymes containing CTQ or tryptophan tryptophylquinone (TTQ) cofactors are dehydrogenases. Kinetic analysis of GoxA revealed allosteric cooperativity for its glycine substrate, but not O2. This is the first CTQ- or TTQ-dependent enzyme to exhibit cooperativity. It is shown that cooperativity and homodimer stabilization are strongly dependent on the presence of Phe237. Conversion of this residue, which is a Tyr in LodA, to Tyr or Ala eliminates the cooperativity and destabilizes the dimer. These mutations also significantly affect the kcat and Km values for the substrates. On the basis of structural and modeling studies a mechanism by which Phe237 exerts this influence is presented. Two active site residues, Asp547 and His466, were also examined and shown by site-directed mutagenesis to be critical for CTQ biogenesis. This result is compared to results of similar studies of mutagenesis of structurally conserved residues of other tryptophylquinone enzymes. These results provide insight into the roles of specific active-site residues in catalysis and CTQ biogenesis, as well as describing an interesting mechanism by which a single residue can dictate whether or not an enzyme exhibits cooperative allosteric behavior towards a substrate.