A light-scattering study of the effect of sodium chloride on the molecular weight of human adult hemoglobin

A light-scattering study of the effect of calcium chloride on the molecular weight of Busycon hemocyanin

Solutions of Busycon canaliculatum have been studied by light scattering. In 0.05 M Trizma buffer +0.1 M NaCl at pH 7.0 at 14 degrees, the weight-average molecular weight is 8.9 X 10(6). In the presence of added CaCl2 (0.02 M), the molecular weight of the protein increases to 10.7 X 10(6), and the second virial coefficient is reduced. At pH 9.95, the molecular weights with and without 0.02 M CaCl2, are 3.7 X 10(6) and 1.3 X 10(6), respectively; and the effect of Ca++ in reducing the second virial coefficient is much greater than at pH 7.0. These results can be understood on the basis that at pH 7.0, ca++ increases the association of hemocyanin, by binding and intermolecular linkage through the carboxyl groups of protein side chains. At pH 9.95, amino groups are deprotonated and therefore also become available for Ca++ binding. The relative effect of Ca++ in enhancing the association of hemocyanin therefore becomes greater at the higher pH.

The binding of organic phosphates to human methaemoglobin A. Perturbation of the polymerization of proteins by effectors

Theory is presented relating to the binding of an effector to two states of a protein acceptor coexisting in equilibrium. The problem is treated in terms of the four possible cases which specify relations between numbers of binding sites and intrinsic binding constants relevant to the acceptor states. It is shown that a distinction between these cases may be possible on the basis of the form of a plot of unbound effector concentration versus the constituent equilibrium coefficient which may be calculated from the sedimentation coefficient of the protein constituent. Particularly noteworthy in this respect is the finding that a turning point may exist in this plot for defined conditions with systems in which binding sites are not conserved (and binding affinities are altered) on polymer formation. The latter type of system is exemplified by studies on methaemoglobin A in 0.25 M sodium acetate, pH 5.4. In the absence of added organic phosphate effectors, a dimer-tetramer equilibrium operates governed by an association constant of 4.15 +/- 0.06 X 10(3) 1/mol, determined from sedimentation equilibrium results. Correlation of sedimentation velocity and equilibrium results shows that addition of adenosine 5'-triphosphate (ATP) results in its binding to one site on each of the dimeric (alpha beta) and tetrameric (alpha beta)2 species with intrinsic binding constants 1.03-10(3)-1.20-10(3) and 1.1-10(4)-2.1-10(4) 1/mol, respectively. It is also shown that 2,3-diphosphoglycerate perturbs the dimer-tetramer equilibrium in a similar way to ATP.

A light scattering investigation of the propylurea dissociation of human hemoglobin A

The subunit dissociation of human hemoglobin A by propylurea in several liganded and chemically modified states was investigated by light scattering molecular weight methods. The dissociation data were analyzed by means of the equation developed in our earlier studies: deltaF degrees D EQUALS TO DELTAF degrees D, W - 2N'RTKB[D], where deltaF degrees D and deltaf degreees D,W represent the free energy of dissociation of hemoglobin tetramers into half-molecules consisting of alpha beta dimers in the presence and in the absence of propylurea, KB is the binding constant of the urea to the average peptide unit, [D] is its concentration, and N' is the number of amino acid sites exposed per half-molecule on dissociation. It is found that the dissociation of oxyhemoglobin, cyanmethemoglobin, and N-ethylmaleimide oxyhemoglobin is characterized by essentially the same N' value of 15 to 21 plus or minus 3, that are close to the 19 amino acid residues per surface which comprise the smaller alpha beta contact area, seen in the X-ray crystallographic model of horse hemoglobin of Perutz and coworkers. Due to the very low degree of dissociation of deoxyhemoglobin, only a very approximate estimate of N' of about the same order of magnitude could be obtained for this form of the protein. In contrast, a significantly lower value of N' was obtained with bis(maleimidomethyl) ether modified oxyhemoglobin of 8 plus or minus 3, that is cross-linked at cysteine residue F9 (93)beta and histidine residue FG4 (97) beta in the same beta chains. Our results suggest that alterations caused by the presence of the cross-linking reagent reflect both the loss in amino acid residues that can interact with the urea at the blocked segments of the polypeptide chains in the dissociated state of hemoglobin and the changes in accessibility of some of the amino acid residues perturbed by the introduction of the reagent in the parent tetrameric form.