The properties of hybrid F₁-ATPase enzymes suggest that a cyclical catalytic mechanism involving three catalytic sites occurs

Maximal rates of ATP hydrolysis catalyzed by F₁-ATPase enzymes are known to involve strong positive catalytic site cooperativity. There are three potential catalytic nucleotide-binding sites on F₁. Two important and unanswered questions are (i) whether all three potential catalytic sites must interact cooperatively to yield maximal rates of ATP hydrolysis and (ii) whether a cyclical three-site mechanism operates as suggested by several authors. We have studied these two questions here by measuring the ATPase activities of hybrid enzymes containing normal β-, γ-, δ-, and ε-subunits together with different combinations of mutant and normal α-subunits. The mutant α-subunits were derived from uncA401, uncA447, and uncA453 mutant E. coli F₁-ATPase, in which positive cooperativity between catalytic sites is strongly attenuated by defined mis-sense mutations. Our data show that three normal catalytic sites are required to interact in order to achieve maximal ATPase rates and suggest that a cyclical mechanism does operate. Hybrid enzyme containing one-third mutant α-subunit and two-thirds normal α-subunits had substantial but submaximal activity, showing that cooperativity between three sites in a noncyclical fashion, or between pairs of sites, can achieve effective catalysis.