Investigation of the substrate binding and catalytic groups of the PC bond cleaving enzyme, phosphonoacetaldehyde hydrolase

Kinetic studies with substrate analogs and group-directed chemical modification agents were carried out for the purpose of identifying the enzyme-substrate interactions required for phosphonoacetaldehyde (P-Ald) binding and catalyzed hydrolysis by P-Ald hydrolase (phosphonatase). Malonic semialdehyde ( K i = 1.6 mM), phosphonoacetate ( K i = 10 mM), phosphonoethanol ( K i = 10 mM), and fluorophosphate ( K i = 20 mM) were found to be competitive inhibitors of the enzyme but not substrates. Thiophosphonoacetaldehyde and acetonyl phosphonate underwent phosphonatase-catalyzed hydrolysis but at 20-fold and 140-fold slower rates, respectively, than did P-Ald. In the presence of NaBH 4, acetonylphosphonate inactivated phosphonatase at a rate exceeding that of its turnover. Sequence analysis of the radiolabeled tryptic peptide generated from [3- 3H]acetonylphosphonate/NaBH 4-treated phosphonatase revealed that Schiff base formation had occurred with the catalytic lysine. From the V m K m and V m pH profiles for phosphonatase-catalyzed P-Ald hydrolysis, an optimal pH range of 6–8 was defined for substrate binding and catalysis. The pH dependence of inactivation by acetylation of the active site lysine with acetic anhydride and 2,4-dinitrophenyl acetate evidenced protonation of the active site lysine residue as the cause for activity loss below pH 6. The pH dependence of inactivation of an active site cysteine residue with methyl methanethiolsulfonate indicated that deprotonation of this residue may be the cause for the loss of enzyme activity above pH 8.