J. Mol. Riol. (1985) 183, 267-270 Haemoglobin: The Surface Buried Between the a,& and a& Dimers in the Deoxy and Oxy Structures Arthur M. Lesk’t, Jo61 Janin2, Shoshanna Wodak3 and Cyrus Chothia1y4 ‘Medical Research Council, Laboratory of Molecular Biology Hills Road, Cambridge CBZ 2QH, England “Laboratoire de Biologie Physicochimique Universite’ de Paris-Sud, 91405-Orsay, France “Laboratoire de Chimie Biologique, Universite’ Libre de Bruxelles 1640 RhGde-St-Genese, Belgium “Christopher Ingold Laboratories, University College London 20 Gordon Street, London WClH OAJ, England (Received 16 October 1984) Using the newly available refined co-ordinates of deoxy and oxyhaemoglobin, we have re- examined and compared the interfaces between the dimers crlpl and cQ2. The most, extensive monomer-monomer contacts are between ~1~ and j2, and, symmetrically, a2 and PI. In oxyhaemoglobin these interfaces bury 700 A2 less protein surface than in deoxyhaemoglobin. The a1a2 interface involves similar salt bridges in both forms, but in oxyhaemoglobin buries 240 4’ more surface than in deoxyhaemoglobin. There is a loosely packed /Il/IZ interface burying 320 A2 of surface in oxyhaemoglobin; there is no plj!Z2 interface in deoxyhaemoglobin. The greater stability of the deoxy form, in the absence of ligands, can be attributed to a combination of hydrophobic, van der Waals’ and electrostatic interactions. 1. Introduction The co-operative properties of haemoglobin arise from transitions between two forms that differ in their quaternary and tertiary structures, and in their affinities for oxygen (Fermi & Perutz, 1981). Tn the absence of oxygen and any other ligand, the stable form of the protein has the deoxy tertiary and quaternary structure and a low oxygen affinity. Binding oxygen to this form creates strain in the protein, relieved by the transition to the high- affinity oxy form. The origin of the greater stability of the deoxy form in the absence of ligand is of great interest in understanding the mechanism of haemoglobin and because it might reflect a general mechanism of allosteric change. The monomer-monomer interfaces within the ai/?l and a2P2 dimers do not change during the allosteric change, but the dimer-dimer interface does change significantly. In the transition between the deoxy and oxy structures, the MJ~ dimer t Permanent address. Fairleigh Dickinson University, Teaneck-Hackensack Campus, Teaneck. NJ 07666, 1’S.A. rotates by 15” relative to the cr2fiZ dimer. Perutz (1970) observed that several salt bridges present across the dimer-dimer interfaces in the deoxy structure are not present in the oxy structure because the rotation moves the charged groups apart. We showed later that the dimer-dimer interface buries a larger surface area in the deoxy than in the liganded form (Chothia et al., 1976). This increase in the area of the protein surface inaccessible to the solvent implies that differences in hydrophobic interactions should, like the salt bridges, stabilize the deoxy form. We also showed that the subunit interfaces are close-packed in both deoxy and methaemoglobin; that is, that the packing density of atoms buried in these interfaces is the same as in amino acid crystals. These calculations were carried out with atomic co-ordinates of human deoxyhaemoglobin, refined to 2.5 A resolution according to Fermi (1975), and of horse methaemo- globin (Ladner et al., 1977); the structure of oxyhaemoglobin was not then known. Recently, Shaanan (1983) has determined and refined the structure of human oxyhaemoglobin at 2.1 A resolution. It closely resembles the structure of horse methaemoglobin, except at the amino and ~M)22-d836~85/1oOP67-04 $03.00/O 267 0 1985 Academic Press Inc. (London) Ltd.