Aspartate receptors of Escherichia coli and Salmonella typhimurium bind ligand with negative and half-of-the-sites cooperativity.
The aspartate receptors of Escherichia coli and Salmonella typhimurium which mediate chemotactic responsiveness to aspartate have 79% amino acid sequence identity but exhibited apparently quite different aspartate binding plots. The Scatchard plot of the Salmonella receptor was concave upward whereas the E. coli receptor gave a straight line. Because the two binding sites in the Salmonella receptor lacking aspartate have a 2-fold crystallographic symmetry axis and do not overlap, the observation of more than one class of binding sites must be due to a ligand-induced conformational change giving negative cooperativity. The closely related E. coli receptor was found to bind with only one class of sites but with a stoichiometry of one aspartate per dimer. The E. coli receptor thus binds with half-of-sites reactivity, an extreme form of negative cooperativity in which the second ligand is not observed to bind at all. Comparison of the X-ray crystal structure of the ligand binding domain with and without bound aspartate revealed ligand-induced conformational changes that explain the two examples of negative cooperativity.
Biemann
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HP
Department of Molecular and Cell Biology, University of California at Berkeley 94720.
Koshland
D E
DE
D000494:Allosteric Regulation; D001224:Aspartic Acid / Q000378:metabolism*; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008958:Models, Molecular; D011487:Protein Conformation; D017459:Receptors, Amino Acid / Q000378:metabolism*; D012486:Salmonella typhimurium / Q000378:metabolism*
D008024:Ligands; D017459:Receptors, Amino Acid; C031959:aspartic acid receptor; D001224:Aspartic Acid