Protein Mass-Modulated Effects in Alkaline Phosphatase

Recent experimental studies engaging isotopically substituted protein (heavy protein) have revealed that many, but not all, enzymatic systems exhibit altered chemical steps in response to an altered mass. The results have been interpreted as femtosecond protein dynamics at the active site being linked (or not) to transition-state barrier crossing. An altered enzyme mass can influence several kinetic parameters (kcat, Km, and kchem) in amounts of ≤30% relative to light enzymes. An early report on deuterium-labeled Escherichia coli alkaline phosphatase (AP) showed an unusually large enzyme kinetic isotope effect on kcat. We examined steady-state and chemical step properties of native AP, [2H]AP, and [2H,13C,15N]AP to characterize the role of heavy enzyme protein dynamics in reactions catalyzed by AP. Both [2H]- and [2H,13C,15N]APs showed unaltered steady-state and single-turnover rate constants. These findings characterize AP as one of the enzymes in which mass-dependent catalytic site dynamics is dominated by reactant-linked atomic motions. Two catalytic site zinc ions activate the oxygen nucleophiles in the catalytic site of AP. The mass of the zinc ions is unchanged in light and heavy APs. They are essentially linked to catalysis and provide a possible explanation for the loss of linkage between catalysis and protein mass in these enzymes.