Mechanistic Analyses of Catalysis in Human Pancreatic α-Amylase: Detailed Kinetic and Structural Studies of Mutants of Three Conserved Carboxylic Acids

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Abstract
The roles of three conserved active site carboxylic acids (D197, E233, and D300) in the catalytic mechanism of human pancreatic α-amylase (HPA) were studied by utilizing site-directed mutagenesis in combination with structural and kinetic analyses of the resultant enzymes. All three residues were mutated to both alanine and the respective amide, and a double alanine mutant (E233A/D300A) was also generated. Structural analyses demonstrated that there were no significant differences in global fold for the mutant enzymes. Kinetic analyses were performed on the mutants, utilizing a range of substrates. All results suggested that D197 was the nucleophile, as virtually all activity (>105-fold decrease in kcat values) was lost for the enzymes mutated at this position when assayed with several substrates. The significantly greater second-order rate constant of E233 mutants on “activated” substrates (kcat/Km value for α-maltotriosyl fluoride = 15 s-1 mM-1) compared with “unactivated” substrates (kcat/Km value for maltopentaose = 0.0030 s-1 mM-1) strongly suggested that E233 is the general acid catalyst, as did the pH−activity profiles. Transglycosylation was favored over hydrolysis for the reactions of several of the enzymes mutated at D300. At the least, this suggests an overall impairment of the catalytic mechanism where the reaction then proceeds using the better acceptor (oligosaccharide instead of water). This may also suggest that D300 plays a crucial role in enzymic interactions with the nucleophilic water during the hydrolysis of the glycosidic bond.