Ligand-induced conformational changes of betaine aldehyde dehydrogenase from Pseudomonas aeruginosa and Amaranthus hypochondriacus L. leaves affecting the reactivity of the catalytic thiol.

The reaction catalyzed by betaine aldehyde dehydrogenase (BADH) involves the nucleophilic attack of a catalytic cysteinyl residue on the aldehyde substrate. As a possible mechanism of regulation, we have studied the modulation by ligands of the reactivity and/or accessibility of the essential thiol of the enzyme from the human pathogen Pseudomonas aeruginosa and the leaves of the plant Amaranthus hypochondriacus (amaranth). In the absence of ligands, the kinetics of inactivation by thiol modifying reagents of both enzymes were biphasic, suggesting the existence of two enzyme conformers differing in the reactivity of their catalytic thiolate. Preincubation of P. aeruginosa BADH with the coenzymes or the aldehyde prior to the chemical modification brought about active site rearrangements that resulted in an important decrease in the inactivation rate. Amaranth BADH responded similarly to the preincubation with NADH or betaine aldehyde but NAD(+) elicited opposite changes, increasing the rate of inactivation after prolonged preincubation. In amaranth BADH, the different behavior of both coenzymes, and the observed biphasic inactivation kinetics are consistent with the previously proposed iso kinetic mechanism, characterized by the existence of two interconvertible apoenzyme forms, one able to bind NAD(+) and the other NADH. Taken together, our results suggest that ligand-induced conformational changes in BADH from the two sources studied might be important for both proper enzyme function and protection against oxidation.