31P nuclear magnetic resonance study of alkaline phosphatase: the role of inorganic phosphate in limiting the enzyme turnover rate at alkaline pH

Area of Science:

• Biochemistry
• Enzymology
• Biophysical Chemistry

Background:

• Alkaline phosphatase is a crucial enzyme involved in various biological processes.
• Understanding the mechanism of substrate binding and enzyme turnover is essential for enzyme kinetics and drug development.

Purpose of the Study:

• To directly observe and characterize the binding of inorganic phosphate to alkaline phosphatase using 31P NMR.
• To elucidate the different forms of bound phosphate and their pH-dependent behavior.
• To identify the rate-limiting step in alkaline phosphatase turnover at high pH.

Main Methods:

• 31P nuclear magnetic resonance (NMR) spectroscopy was employed to study inorganic phosphate binding.
• Enzyme kinetic assays were performed to determine the rate-limiting step.

Main Results:

• Direct evidence for tight binding of 1.5-2.0 moles of inorganic phosphate per dimer of alkaline phosphatase was obtained.
• Two distinct forms of bound phosphate were identified: a non-covalent E-P1 complex (predominant above pH 7) and a covalent E-P1 complex (predominant below pH 5).
• 31P NMR line width analysis indicated that the dissociation of noncovalent phosphate is the rate-limiting step in enzyme turnover at high pH.

Conclusions:

• The study provides direct spectroscopic evidence for the binding of inorganic phosphate to alkaline phosphatase.
• The findings reveal the existence of distinct non-covalent and covalent phosphate-enzyme complexes.
• Noncovalent phosphate dissociation is identified as the rate-limiting step in alkaline phosphatase activity at physiological pH.