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Biochim Biophys Acta. 1981 Jul 28;669(2):244-50.

Human serum albumin as an allosteric two-state protein. Evidence from effects of calcium and warfarin on proton binding behaviour.


The proton binding of human serum albumin, and the influence on it of Ca2+ and warfarin (3-(alpha-acetonylbenzyl)-4-hydroxycoumarin), has been studied in the pH region 6 to 9, in order to get more information on the conformational change occurring in serum albumin around neutral pH, the so-called N-B transition. Some of the results for human serum albumin are compared with bovine serum albumin. A two-state model describing this transition is presented. In this model the two states are assumed to be the N state (found at low pH) and the B state (found at high pH). The ligand to be considered is the proton, having the highest affinity for the N conformation. An allosteric constant, L, governs the equilibrium between the two states. Both Ca2+ and warfarin can act as allosteric effectors, thereby increasing L. The model is used to describe results such as: (a) the cooperativity in proton binding, most clearly observable when Ca2+ is present, and the difficulty of measuring experimentally this cooperativity; (b) the different number of protons bound when Ca2+ is present or absent; (c) the fraction of protein found in one of the two conformations; (d) the correspondence between the increase of L due to addition of Ca2+ or warfarin, as predicted from model calculations, and the experimentally found increase of L.

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