Mitochondrial ATP synthase -: Crystal structure of the catalytic F1 unit in a vanadate-induced transition-like state and implications for mechanism

ATP synthesis from ADP, P-i, and Mg2+ takes place in mitochondria on the catalytic F-1 unit (alpha 3 beta 3 gamma delta epsilon) of the ATP synthase complex ( F0F1), a remarkable nanomachine that interconverts electrochemical and mechanical energy, producing the high energy terminal bond of ATP. In currently available structural models of F1, the P- loop ( amino acid residues (156)GGAGVGKT(163)) contributes to substrate binding at the beta subunit catalytic sites. Here, we report the first transition state- like structure of F-1 ( ADP center dot V-i center dot Mg center dot F-1) from rat liver that was crystallized with the phosphate ( Pi) analog vanadate ( VO43- or V-i). Compared with earlier " ground state" structures, this new F1 structure reveals that the active site region has undergone significant remodeling. P- loop residue alanine 158 is located much closer to Vi than it is to Pi in a previous structural model. No significant movements of P- loop residues of the alpha subunit were observed at its analogous but noncatalytic sites. Under physiological conditions, such active site remodeling involving the small hydrophobic alanine residue may promote ATP synthesis by lowering the local dielectric constant, thus facilitating the dehydration of ADP and Pi. This new crystallographic study provides strong support for the catalytic mechanism of ATP synthesis deduced from earlier biochemical studies of liver F-1 conducted in the presence of V-i.