A novel dCMP methylase by engineering thymidylate synthase

Area of Science:

• Biochemistry
• Enzyme kinetics
• Structural biology

Background:

• Thymidylate synthase (TS) is crucial for DNA synthesis, catalyzing the methylation of deoxyuridine monophosphate (dUMP).
• Understanding TS substrate specificity is key for developing targeted therapeutics and advancing enzyme engineering.
• Previous studies identified steric clashes in dCMP methylation by a Lactobacillus casei TS N229D mutant.

Purpose of the Study:

• To engineer Lactobacillus casei TS for enhanced dCMP methylation activity.
• To investigate the structural and kinetic basis for improved dCMP specificity.
• To overcome steric hindrance in the active site for dCMP processing.

Main Methods:

• Site-directed mutagenesis to create TS variants (N229D, H199A).
• X-ray crystallography to determine binary complex structures (TS-dUMP, TS-dCMP).
• Steady-state kinetic analysis to measure enzyme parameters (specificity, catalytic efficiency).

Main Results:

• The TS H199A/N229D double mutant exhibited a 10^11-fold increase in specificity for dCMP methylation compared to dUMP.
• Structural analysis revealed that the H199A mutation corrects the displacement of dCMP in the active site caused by the N229D mutation.
• The optimized binding of dCMP in the H199A/N229D mutant approaches that of dUMP in the wild-type enzyme, while dUMP binding is disrupted.

Conclusions:

• Modifying key active site residues (His199) can resolve steric clashes and enhance substrate specificity in TS.
• The H199A/N229D double mutant demonstrates a successful strategy for engineering dCMP methylase activity.
• This work provides a structural and kinetic foundation for designing novel TS inhibitors and engineered enzymes.