Allosteric properties of the oxyphenbutazone--human serum albumin complex.
The conformational change of albumin which occurs around physiological pH, the so-called N-B transition, has been studied by measuring the induced circular dichroic signal of the oxyphenbutazone-albumin complex. This N-B transition has been characterized by a set of parameters according to the two-state model of Monod, Wyman and Changeux. The influence of calcium ions on the N-B transition has been interpreted in terms of a change in some of the parameters describing the two-state model, viz. a decrease of the apparent pK value of the histidines and of the apparent allosteric constant of the oxyphenbutazone-albumin complex. This apparent pK change increases with increasing Ca2+ concentration, whereas the apparent allosteric constant approaches a final value at 5 mM Ca2+. From acid-base titration curves of albumin in the presence and in the absence of Ca2+ it could be concluded that in the presence of Ca2+ less histidines are titratable than in the absence of Ca2+. Assuming that these histidines are not involved in the N-B transition it follows that at least four to five histidines are involved in the N-B transition.
D000494:Allosteric Regulation; D002118:Calcium; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002942:Circular Dichroism; D006639:Histidine; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008956:Models, Chemical; D008968:Molecular Conformation; D010113:Oxyphenbutazone / Q000097:blood*; D011485:Protein Binding; D012709:Serum Albumin / Q000032:analysis*
D012709:Serum Albumin; D006639:Histidine; D010113:Oxyphenbutazone; D002118:Calcium