Structure of Salmonella typhimurium OMP synthase in a complete substrate complex

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Orotate phosphoribosyltransferase (OMP synthase) is essential for de novo pyrimidine nucleotide synthesis.
• Previous structures of OMP synthase showed basic topology but lacked detail on dynamic substrate-induced changes.

Purpose of the Study:

• To determine the high-resolution crystal structure of dimeric Salmonella typhimurium OMP synthase in a complete substrate complex.
• To elucidate the structural rearrangements and catalytic mechanism of OMP synthase.

Main Methods:

• Cocrystallization of Salmonella typhimurium OMP synthase with MgPRPP and orotate.
• X-ray crystallography to determine the complex structure at 2.2 Å resolution.

Main Results:

• A dramatic, asymmetric reorganization of the active site upon substrate binding was observed.
• The catalytic loop of one subunit closes the active site in the other subunit, sequestering it from solvent.
• Interactions of specific residues and a single catalytic Mg(2+) ion with the substrate were identified, stabilizing the transition state.

Conclusions:

• Loop movement is critical for OMP synthase catalysis, with specific movements identified for each catalytic cycle.
• The structure provides insights into the enzyme's catalytic mechanism, supporting a model of substrate-assisted transition state stabilization.