Analysis of cooperativity observed in pH titrations of proton nuclear magnetic resonances of histidine residues of rabbit cardiac tropomyosin.
We have investigated in detail the cooperativity which we had previously observed in the pH titration profiles of the histidine residues of rabbit tropomyosin [Edwards, B. F. P., & Sykes, B. D. (1978) Biochemistry 17, 684]. Nonpolymerizing tropomyosin was prepared by carboxypeptidase digestion, and the titration profiles of its histidine residues were compared with those of undigested tropomyosin which was fully polymerized (in 0.1 M KCl) throughout the titration. We have concluded that both histidine-153 and histidine-273 have significant cooperativity in their pH titrations only in polymerized tropomyosin, that the cooperativity arises from an intrinsic pH-dependent conformational transition which links the two residues together and not from the known pH dependence of the polymerization, and that the best model for the cooperativity is a biallosteric adaption of the Monod--Wyman--Changeux formalism involving two classes of binding sites for the same ligand (protons). Three other models which postulated either a Hill transition, an interaction with a neighboring residue that also titrates, or a pH-dependent polymerization were also considered.
D000494:Allosteric Regulation; D000818:Animals; D001665:Binding Sites; D006639:Histidine / Q000032:analysis*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D008433:Mathematics; D009206:Myocardium / Q000378:metabolism*; D011487:Protein Conformation; D011817:Rabbits; D014335:Tropomyosin / Q000378:metabolism
D014335:Tropomyosin; D006639:Histidine