THE STRUCTURE OF BOVINE MITOCHONDRIAL F1-ATPASE - AN EXAMPLE OF ROTATIONAL CATALYSIS ? A.G.W. Leslie, J.P. Abrahams, M. van Raaij, R. Lutter, J.E. Walker, MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK

ATP is the energy currency of all living cells. It is generated by the enzyme ATP synthase, a complex oligomeric assembly found in mitochondria, chloroplasts and bacteria. This enzyme uses a trans-membrane proton gradient (established by respiration in mitochondria or photosynthesis in chloroplasts) to generate ATP from ADP and inorganic phosphate. There is a high level of sequence homology between the enzymes from these various sources and it is likely that they share a common tertiary structure and catalytic mechanism.

The intact ATP synthase consists of a membrane bound segment (Fo) and a soluble component (F1) linked by a stalk. Fo contains the proton channel, while the catalytic sites are located on F1. In isolation, F1 displays ATPase activity, and is therefore known as F1-ATPase. It consists of five different subunits, with stoichiometry [[alpha]]3[[beta]]3[[gamma]]d[[epsilon]]. In the crystal structure of bovine mitochondrial F1-ATPase, the [[alpha]] and [[beta]] subunits are arranged like the segments of an orange around a central spindle formed by two long alpha helices from the [[gamma]] subunit. The three catalytic sites are on the [[beta]] subunits, but lie at the interface with the adjacent [[alpha]] subunit. The three catalytic interfaces are quite different in nature, and may represent different conformational states on the catalytic pathway. The structure supports the binding change mechanism of catalysis as proposed by P. Boyer, in which the three catalytic sites alternate cyclically between three different states. Several aspects of the structure suggest that during catalysis the interconversion of the catalytic sites is achieved by a relative rotation of the [[alpha]]3-[[beta]]3 assembly and the [[gamma]] subunit. The structures of two enzyme-inhibitor complexes lend support to this hypothesis.