Understanding the Allosteric Transition in Escherichia coli Aspartate Transcarbamoylase through a Novel R-State Structure

Abstract

A full understanding of an enzyme's catalytic mechanism and a crystal structure representative of its in vivo form are powerful tools in computational drug screening and design. In the case of aspartate transcarbamoylase (ATCase), an allosteric enzyme, the mechanism and allosteric transition are still being explored. The crystallization of the ATCase mutant Asp236 to alanine, a T-state destabilized mutant, in the presence of phosphonoacetamide (PAM) by microdialysis was successful at pH 5.7. The enzyme crystallized in the R-state in the presence of only one substrate analogue. Globally the enzyme had converted to R, but the active site domains are more open than previously observed. Due to the ordered nature of the reaction, the R-state active site exists with a variety of small molecules bound at different times through out the course of the reaction. This structure shows an R-state active site with only one substrate analogue bound, and may therefore represent the R active site after catalysis has occurred and the active site is binding new substrates to perform its reaction again. Docking studies of small molecules can be conducted using this more open, emptier active site as it may be more representative of an in vivo conformation of the enzyme just before catalysis. Additionally, Arg296, previously unobserved as part of the active site, makes a hydrogen bonding interaction with the PAM molecule. The role of this residue will require further investigation.