THE CRYSTAL-STRUCTURE OF HORSE DEOXYHEMOGLOBIN TRAPPED IN THE HIGH-AFFINITY (R) STATE

Go-operative oxygen binding by the vertebrate haemoglobins arises from an equilibrium between a quaternary structure with low affinity (T), favoured in the absence of ligand, and a high affinity form (R) adopte d by the fully ligated protein. While R state haemoglobin has an oxyge n affinity close to that of isolated subunits, the affinity of the T s tate is roughly 300-fold lower. The mechanism by which the T state res trains ligand binding, and the pathway of the quaternary transition, h ave been largely revealed by detailed crystallographic analyses of a n umber of haemoglobin molecules in the equilibrium states, as well as i ntermediate forms of the T state including partially ligated species. The ligation intermediates of the R state, however, have not been as w ell characterized structurally. We report here the crystal structure o f one such intermediate species, namely, horse deoxyhaemoglobin in the R state, at 1.8 Angstrom resolution. While ligand binding in the T st ate may result in unfavourable stereochemistry in and around the haem- ligand complex, the more plastic R structure appears to accommodate eq ually well both liganded and ligand-free haem. toss of ligand at the R state haem results in movements of the haem and shifts of the FG corn ers, which form characteristic intersubunit contacts that distinguish the quaternary states. The shifts are comparable in magnitude to the c orresponding movements associated with de-ligation in the T state, alt hough they differ in direction. These and other differences illustrate how the structural changes in the haem pocket are communicated to the subunit interfaces and how the movements that can occur in the R stat e may be impeded in the T state. (C) 1996 Academic Press Limited