Dihydroorotase from Escherichia coli: loop movement and cooperativity between subunits

Area of Science:

• Biochemistry
• Structural Biology
• Enzymology

Background:

• Escherichia coli dihydroorotase is a key enzyme in pyrimidine biosynthesis.
• Previous structural studies reported unusual dimeric forms with bound substrate or product.
• The functional implications of these structures remained unclear.

Purpose of the Study:

• To elucidate the structural basis for the observed dimeric forms of E. coli dihydroorotase.
• To understand the role of a specific loop in enzyme activity and regulation.
• To investigate potential communication mechanisms between enzyme subunits.

Main Methods:

• X-ray crystallography of E. coli dihydroorotase with L-dihydroorotate (L-DHO).
• Structural analysis and comparison with previously reported structures.
• Enzyme kinetics assays to study substrate conversion and cooperativity.

Main Results:

• The crystal structure of E. coli dihydroorotase in complex with L-DHO was determined at 1.9A resolution.
• A flexible loop (residues 105-115) exhibits distinct conformations in the presence of substrate (N-carbamyl-D,L-aspartate) versus product (L-DHO).
• Kinetic analysis revealed positive cooperativity in L-DHO conversion, suggesting inter-subunit communication.

Conclusions:

• The flexible loop plays a critical role in modulating the active site environment and potentially substrate binding.
• Conformational changes in this loop may facilitate communication between enzyme subunits in the dimer.
• These findings provide insights into the allosteric regulation of E. coli dihydroorotase.