Magnesium-dependent ATPase Activity and Cooperativity of Magnesium Chelatase from Synechocystis sp. PCC6803*□S

The first committed step in chlorophyll biosynthesis is catalyzed by magnesium chelatase, a complex enzyme with at least three substrates, cooperative Mg activation, and free energy coupling between ATP hydrolysis and metal-ion chelation. A detailed functional study of the behavior of the intact magnesium chelatase has been performed, including characterization of magnesium cooperativity and the stoichiometry of ATP consumption in relation to the magnesium porphyrin produced. It is demonstrated that, in vitro, this catalyzed reaction requires hydrolysis of 15 MgATP and that the chelation partial reaction is energetically unfavorable, under our assay conditions, with a G° of 25–33 kJ mol . Given the likely metabolite concentrations in vivo, this results in the chelatase reaction operating far from equilibrium. We have also determined the steadystate kinetic behavior of the intact enzyme and have compared the kinetic parameters obtained with those observed for the partial reactions of individual subunits. KDIX (where DIX represents deuteroporphyrin IX) is estimated to be 3.20 M, and KMgATP 2 is 0.45 mM. kcat for chelation is estimated to be 0.8 min , suggesting that the ATP hydrolysis catalyzed by the isolated ChlI subunit is substantially slower in the intact chelatase. The magnesium-rich form of the chelatase is a more effective catalyst of the chelation reaction; magnesium activation of the chelatase increases V, as well as the specificity constant for the reaction of MgATP and DIX, possibly as a result of a magnesium-triggered conformational change.