Purification and Mn2+ activation of Rhodospirillum rubrum nitrogenase activating enzyme

The Fe protein activating enzyme for Rhodospirillum rubrum nitrogenase was purified to approximately 90% homogeneity, using DE52-cellulose chromatography and sucrose density gradient centrifugation. Activating enzyme consists of a single polypeptide of molecular weight approximately 24,000. ATP was required for catalytic activity, but was relatively ineffective in the absence of Mg2+. When the concentration of MgATP2- was held in excess, there was an additional requirement for a free divalent metal ion (Mn2+) for enzyme activity. Kinetic experiments showed that the presence of Mg2+ influenced the apparent binding of Mn2+ by the enzyme, resulting in a lowering of the concentration of Mn2+ required to give half-maximum activity (K alpha) as the free Mg2+ concentration was increased. A low concentration of Mn2+ had a sparing effect on the requirement for free Mg2+. There is apparently a single metal-binding site on activating enzyme which preferentially binds Mn2+ as a positive effector, and free Mg2+ can compete for this site.