Evidence for the existence of discrete activator and substrate sites for CO2 on ribulose-1,5-bisphosphate carboxylase.

When incubated with CO2 and Mg2+, ribulose-1,5-bis-phosphate carboxylase forms a ternary complex of enzyme . CO2 . Mg. This complex was prepared using high specific activity [14C]O2 and injected into a solution containing a large (50- to 112-fold) molar excess of [12C]O2 and sufficient ribulose 1,5-bisphosphate to permit the catalytic site to turn over several times. The enzyme was then rapidly separated from the other components by gel filtration and its radiospecific activity was determined to be 30 to 60 times that of the medium. If the CO2 activator and the CO2 substrate sites were one and the same, then, following turnover, the enzyme should have been in isotopic equilibrium with the medium. The finding that this was not the case, by a factor of about 40, indicates that the CO2 activator site is physically distinct from the CO2 substrate site.