Download MendeleySites of Binding and Orientation in a Four-Location Model for Protein Stereospecificity.
Mesecar, A.D., Koshland Jr., D.E.(2000) IUBMB Life 49: 457-466
- PubMed: 10902579
- DOI: https://doi.org/10.1080/152165400410326
- Primary Citation of Related Structures:
1PB3 - PubMed Abstract:
The stereospecificity of the enzyme isocitrate dehydrogenase was examined by steady-state kinetics and x-ray crystallography. The enzyme has the intriguing property that the apoenzyme in the absence of divalent metal showed a selectivity for the inactive l-enantiomer of the substrate isocitrate, whereas the enzyme containing magnesium showed selectivity for the physiologically active d-enantiomer. The hydrogen atom on the C2 carbon that is transferred during the reaction was, in both the d- and l-isocitrate complexes, in an orientation very close to that expected for delivery of a hydride ion to the cosubstrate NADP+. The beta-carboxylate that is eliminated as a CO2 molecule during the reaction occupied the same site on the protein in both the d- and l-isocitrate complexes. In addition, the C3 carbon was in the same protein site in both the d- and l-enantiomers. Only the fourth group, the OH atom, was in a very different position in the apo enzyme and in the metal-containing complexes. A four-location model is necessary to explain the enantiomeric specificity of IDH in contrast to the conventional three-point attachment model. The thermodynamic and kinetic ramifications of this model are explored.
Organizational Affiliation:
Center for Pharmaceutical Biotechnology and the Department of Medicinal Chemistry and Pharmacognosy, University of Illinois at Chicago, 60607, USA. mesecar@uic.edu
