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Alkaline phosphatase is an almost perfect enzyme

T T Simopoulos1, W P Jencks

  • 1Graduate Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254-9110.

Biochemistry
|August 30, 1994
PubMed
Summary

Alkaline phosphatase activity is limited by substrate encounter and product release, with viscosity affecting reaction rates. Glycerol impacts catalysis through solvent effects and viscosity, unlike sucrose.

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Area of Science:

  • Biochemistry
  • Enzyme kinetics
  • Protein dynamics

Background:

  • Alkaline phosphatase (AP) is a crucial enzyme involved in various biological processes.
  • Understanding the factors influencing AP's catalytic efficiency is vital for its applications.
  • Viscosity and solvent effects can modulate enzyme activity by affecting substrate/product diffusion and enzyme conformation.

Purpose of the Study:

  • To investigate the impact of solution viscosity on the kinetics of 4-nitrophenyl phosphate hydrolysis by alkaline phosphatase.
  • To differentiate between diffusion-limited and chemically limited steps in the AP catalytic cycle.
  • To assess the effects of different viscosity-modifying agents (sucrose, arabinose, glycerol) on AP activity.

Main Methods:

  • Enzyme kinetic assays were performed using alkaline phosphatase and 4-nitrophenyl phosphate as substrate.
  • Reaction rates were measured at varying viscosities induced by sucrose, arabinose, and glycerol at different pH values (6 and 8.0).
  • The second-order rate constant (kcat/Km) and the first-order rate constant (kcat) were determined and analyzed in relation to viscosity changes.

Main Results:

  • At pH 8.0, kcat/Km decreased with increasing viscosity in the presence of sucrose or arabinose, indicating diffusion-limited substrate-enzyme encounter.
  • However, kcat/Km values were below the theoretical diffusion limit, suggesting only a fraction of encounters are productive.
  • At pH 6, kcat was viscosity-independent with sucrose, but at pH 8.0, kcat decreased with viscosity, suggesting rate-limiting product dissociation.
  • Glycerol significantly decreased both kcat/Km and kcat, indicating solvent effects on catalytic activity beyond just viscosity increase.

Conclusions:

  • Alkaline phosphatase operates near the diffusion limit for substrate binding, classifying it as a 'near-perfect enzyme'.
  • Product dissociation is a rate-limiting step at pH 8.0, influenced by viscosity.
  • Glycerol exerts a dual effect, increasing viscosity and altering the enzyme's catalytic efficiency through solvent effects.

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