COMPUTER SIMULATION OF HEMOGLOBIN ELECTROSTATICS

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Computer simulations will be used to investigate structure/function relationships in human hemoglobin. The calculations will make use of modern simulation methods of estimating free energy changes, and continuum descriptions of some electrostatic interactions, to study the interaction of hemoglobin with protons and anions, the energetics of dissociation of tetramers into dimers, and the structural forces following ligand binding to the T quaternary state. These detailed investigations are made feasible by recent advances in computing power and in simulation algorithms, and by corresponding advances in high- resolution X-ray diffraction studies of mutant and partially liganded hemoglobins. These developments, plus a large body of evidence collected over the past decade on the properties of mutant hemoglobins, make possible a variety of tests of our ability to understand subunit association and allosteric interactions at an atomic level of detail. Specific interactions to be studied included: (a) binding of protons and anions to the R and T quaternary states; (b) energetics of tetramer/dimer dissociation in normal and mutant hemoglobins; (c) structural and energetic consequences of binding ligands to T state structures; and (d) properties of the alternate R structure seen in hemoglobin Ypsilanti and in normal hemoglobin under certain crystallization conditions. The calculations will make use of solvated molecular dynamics techniques as well as Poisson-Boltzmann calculations to estimate electrostatic interactions. In addition to helping to gain an understanding of structure/function relations in hemoglobin, the project will provide new insight into the ability of these sorts of theoretical methods to describe the details of complex protein interactions. Studies on hemoglobin and mutant hemoglobins have important health implications for several reasons. Modified hemoglobins are currently of great interest as potential blood substitutes, and successful predictions of such properties would aid in their design. More generally, hemoglobin offers an excellent opportunity for basic studies in protein engineering, since hundreds of mutants have been isolated, and over 60 of these have already been subjected to extensive physical characterization. Hemoglobin is also an excellent test case for studies of subunit/subunit association; such interactions are involved in a great many regulatory events, and a deeper understanding of the principles involved in protein- protein recognition could be of major benefit in understanding biochemical control mechanisms.