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Protein Science, Vol 4, Issue 3 387-393, Copyright © 1995 by Cold Spring Harbor Laboratory Press

ARTICLE

Conservative and nonconservative mutations in proteins: Anomalous mutations in a transport receptor analyzed by free energy and quantum chemical calculations

W. R. CANNON, J. M. BRIGGS, J. SHEN, J. A. MCCAMMON and F. A. QUIOCHO
Department of Chemistry, University of Houston, Houston, Texas 77204-5641

Experimental studies on a bacterial sulfate receptor have indicated anomalous relative binding affinities for the mutations Ser(130) -> Cys, Ser(130) -> Gly, and Ser(130) -> Ala. The loss of affinity for sulfate in the former mutation was previously attributed to a greater steric effect on the part of the Cys side chain relative to the Ser side chain, whereas the relatively small loss of binding affinity for the latter two mutations was attributed to the loss of a single hydrogen bond. In this report we present quantum chemical and statistical thermodynamic studies of these mutations. Qualitative results from these studies indicate that for the Ser(130) -> Cys mutation the large decrease in binding affinity is in part caused by steric effects, but also significantly by the differential work required to polarize the Cys thiol group relative to the Ser hydroxyl group. The Gly mutant cobinds a water molecule in the same location as the Ser side chain resulting in a relatively small decrease in binding affinity. Results for the Ala mutant are in disagreement with experimental results but are likely to be limited by insufficient sampling of configuration space due to physical constraints applied during the simulation.
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H. Tian, L. Yu, M. W. Mather, and C.-A. Yu
The Involvement of Serine 175 and Alanine 185 of Cytochrome b of Rhodobacter sphaeroides Cytochrome bc1 Complex in Interaction with Iron-Sulfur Protein
J. Biol. Chem., September 19, 1997; 272(38): 23722 - 23728.
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