Abstract
The sulfosugar sulfoquinovose (SQ) is catabolized through the sulfoglycolytic Entner-Doudoroff pathway, beginning with the oxidation of SQ to sulfogluconolactone by SQ dehydrogenase. We present a comprehensive structural and kinetic characterization of Pseudomonas putida SQ dehydrogenase (PpSQDH). PpSQDH is a tetrameric enzyme belonging to the short-chain dehydrogenase/reductase (SDR) superfamily with a strong preference for NAD+ over NADP+. Kinetic analysis revealed a rapid equilibrium ordered mechanism in which the NAD+ cofactor is the first substrate to bind, and NADH is the last product to dissociate. Structural studies revealed a homotetrameric structure in solution and crystals, involving cross-subunit interactions in which the C-terminus residue (Gln260) inserts into the diagonally opposite subunit to form part of the second shell of residues lining the active site. Complexes of PpSQDH with SQ or NAD+ provide insight into the recognition of SQ and together with the kinetic analysis allow the proposal of a catalytic reaction mechanism. Our findings illuminate the mechanism of SQ degradation and the evolution of the SDR superfamily for organosulfonate catabolism.