A catalytic mechanism for the dual-specific phosphatases

Area of Science:

• Biochemistry
• Enzymology
• Protein tyrosine phosphatases

Background:

• Dual-specific protein-tyrosine phosphatases share a conserved active-site motif HCXXGXXRS(T).
• The role of the conserved hydroxyl group in this motif is not fully understood.
• The VHR phosphatase is a key enzyme in cellular signaling pathways.

Purpose of the Study:

• To investigate the role of the conserved hydroxyl group at serine-131 in the catalytic mechanism of the VHR phosphatase.
• To determine how mutating serine-131 to alanine (S131A) affects enzyme kinetics and the rate-limiting step.

Main Methods:

• Site-directed mutagenesis was used to create the S131A VHR mutant.
• Enzyme kinetics were analyzed by measuring kcat/Km and kcat values across a range of pH.
• Stopped-flow spectrophotometry was employed to observe reaction intermediates and kinetics.

Main Results:

• The S131A mutation did not alter the pH profile of kcat/Km but reduced kcat by 100-fold.
• The pH-independent profile of kcat for the S131A mutant indicated a shift in the rate-limiting step.
• Stopped-flow analysis revealed a burst of product formation, confirming that intermediate breakdown is rate-limiting in the mutant.

Conclusions:

• The hydroxyl group at serine-131 is essential for efficient hydrolysis of the phosphoenzyme intermediate.
• Mutation of serine-131 to alanine shifts the rate-limiting step from intermediate formation to intermediate breakdown.
• This highlights the critical role of the active-site hydroxyl in the phosphatase catalytic cycle.