INVOLVEMENT OF GLUTAMATE-268 IN THE ACTIVE-SITE OF HUMAN LIVER MITOCHONDRIAL (CLASS-2) ALDEHYDE DEHYDROGENASE AS PROBED BY SITE-DIRECTED MUTAGENESIS

On the basis of chemical modification studies, it was postulated that glutamate 268 was a component of the active site of liver aldehyde dehydrogenase [Abriola, D.P., Fields, R., MacKerell, A.D., Jr., and Pietruszko, R. (1987) Biochemistry 26, 5679-5684]. To study its role, the residue in human liver mitochondrial (class 2) aldehyde dehydrogenase was mutated to an aspartate, a glutamine, or a lysine, and the enzyme was expressed in Escherichia coli. The mutations did not affect the K-m values for NAD or propionaldehyde, but grossly affected the catalytic activity of the enzymes when compared to recombinantly expressed native enzyme; the mutant enzymes had less that 0.4% of the specific activity of the recombinantly expressed native aldehyde dehydrogenase. The mutations also caused a long lag phase to occur prior to the steady state phase of the reaction. The activity of the mutant enzymes could not be restored by the addition of general bases such as sodium acetate, sodium formate, or imidazole. The K-d for NADH was essentially identical for the E268Q mutant and native enzyme. The three mutant forms of the enzyme possessed less than 0.8% of the esterolytic activity of the recombinantly expressed native enzyme. Pre-steady state analysis showed that there was no burst of NADH formation in the dehydrogenase reaction or of p-nitrophenol formation in the esterase reaction. This can be interpreted as implying that glutamate 268 may function as a general base necessary for the initial activation of the essential cysteine residue (302), rather than being involved in only the deacylation or hydride transfer step. Alternatively, glutamate 268 could function as a component of a charge relay triad necessary to activate the nucleophilic residue. Furthermore, it appears that esterase and dehydrogenase require the same active site components, for both the dehydrogenase activity and esterase activity were essentially abolished when glutamate 268 was changed to another residue.