Serine hydroxymethyltransferase: origin of substrate specificity.

Area of Science:

• Biochemistry
• Enzyme kinetics
• Protein engineering

Background:

• Serine hydroxymethyltransferase (SHMT) is vital for one-carbon metabolism.
• Five threonine residues are conserved near the pyridoxal phosphate-binding lysine in known SHMT structures.
• The role of these threonine residues in SHMT mechanism and catalysis is not fully understood.

Purpose of the Study:

• To investigate the functional significance of five conserved threonine residues in Escherichia coli serine hydroxymethyltransferase.
• To determine the impact of site-directed mutagenesis of these threonine residues on enzyme kinetics and spectral properties.

Main Methods:

• Site-directed mutagenesis was used to replace five threonine residues with alanine in E. coli SHMT.
• Purification and characterization of five mutant enzymes.
• Kinetic assays (Km, kcat) and spectral analyses were performed.
• Rapid reaction studies were employed to analyze reaction intermediates.

Main Results:

• Mutants T224A, T227A showed minimal changes in kinetic and spectral properties compared to wild-type.
• Mutants T225A and T230A exhibited altered Km and kcat values but retained wild-type spectral properties.
• Mutant T226A displayed near-normal substrate affinity but only 3% of wild-type catalytic activity.
• Spectral analysis of T226A revealed altered substrate-induced spectral shifts, with a prominent 343 nm peak instead of the wild-type 425 nm peak.

Conclusions:

• Threonine residues at positions 224, 225, 227, and 230 are not critical for the catalytic mechanism of E. coli SHMT.
• Threonine 226 plays a pivotal role in SHMT catalysis, as its mutation severely impairs catalytic activity and alters reaction intermediate formation.
• The findings highlight the specific importance of T226 in the SHMT reaction pathway.