Effect of the ε-subunit on nucleotide binding to Escherichia coli F1-ATPase catalytic sites

The influence of the epsilon-subunit on the nucleotide binding affinities of the three catalytic sites of Escherichia coli F-1-ATPase was investigated, using a genetically engineered Trp probe in the adenine-binding subdomain (beta-Trp-331). The interaction between epsilon and F-1 was not affected by the mutation. K-d for binding of epsilon to beta Y331W mutant F-1 was similar to 1 nM, and epsilon inhibited ATPase activity by 90%. The only nucleotide binding affinities that showed significant differences in the epsilon-depleted and epsilon-replete forms of the enzyme were those for MgATP and MgADP at the high-affinity catalytic site 1. K-d1(MgATP) and K-d1(MgADP) were an order of magnitude higher in the absence of epsilon than in its presence. In contrast, the binding affinities for MgATP and MgADP at sites 2 and 3 were similar in the epsilon-depleted and epsilon-replete enzymes, as were the affinities at all three sites for free ATP and ADP. Comparison of MgATP binding and hydrolysis parameters showed that in the presence as well as the absence of epsilon, K-m equals K-d3. Thus, in both cases, all three catalytic binding sites have to be occupied to obtain rapid (V-max) MgATP hydrolysis rates.