Evidence for a synergistic salt-protein interaction -- complex patterns of activation vs. inhibition of nitrogenase by salt.

The molybdenum nitrogenase enzyme system, comprised of the MoFe protein and the Fe protein, catalyzes the reduction of atmospheric N(2) to NH(3). Interactions between these two proteins and between Fe protein and nucleotides (MgADP and MgATP) are crucial to catalysis. It is well established that salts are inhibitors of nitrogenase catalysis that target these interactions. However, the implications of salt effects are often overlooked. We have reexamined salt effects in light of a comprehensive framework for nitrogenase interactions to offer an in-depth analysis of the sources of salt inhibition and underlying apparent cooperativity. More importantly, we have identified patterns of salt activation of nitrogenase that correspond to at least two mechanisms. One of these mechanisms is that charge screening of MoFe protein-Fe protein interactions in the nitrogenase complex accelerates the rate of nitrogenase complex dissociation, which is the rate-limiting step of catalysis. This kind of salt activation operates under conditions of high catalytic activity and low salt concentrations that may resemble those found in vivo. While simple kinetic arguments are strong evidence for this kind of salt activation, further confirmation was sought by demonstrating that tight complexes that have previously displayed little or no activity due to the inability of Fe protein to dissociate from the complex are activated by the presence of salt. This occurs for the combination Azotobacter vinelandii MoFe protein with: (a) the L127Delta Fe protein; and (b) Clostridium pasteurianum Fe protein. The curvature of activation vs. salt implies a synergistic salt-protein interaction.