The catalytic transition state in ATP synthase

The catalytic transition state of ATP synthase has been characterized and m odeled by combined use of (1) Mg-ADP-fluoroaluminate, Mg-ADP-fluoroscandium , and corresponding Mg-IDP-fluorometals as transition-state analogs; (2) fl uorescence signals of beta -Trp331 and beta -Trp148 as optical probes to as sess formation of the transition state; (3) mutations of critical catalytic residues to determine side chain ligands required to stabilize the transit ion state. Rate acceleration by positive catalytic site cooperativity is ex plained as due to mobility of alpha -Arg376, acting as an "arginine finger" residue, which interacts with nucleotide specifically at the transition st ate step of catalysis, not with Mg-ATP-or Mg-ADP-bound ground states. We sp eculate that formation and collapse of the transition state may engender ca talytic site alpha/beta subunit-interface conformational movement, which is linked to gamma -subunit rotation.