Thermodynamics of nucleotide interactions with the Azotobacter vinelandii nitrogenase iron protein

The nitrogenase iron (Fe) protein binds two molecules of MgATP or MgADP, which results in protein conformational changes that are important for subsequent steps of the nitrogenase reaction mechanism. In the present work, isothermal titration calorimetry has been used to deconvolute the apparent binding constants (K'a1 and K'a2) and the thermodynamic terms (delta H' degree and delta S' degree) for each of the two binding events of MgATP or MgADP to either the reduced or oxidized states of the Fe protein from Azotobacter vinelandii. The Fe protein was found to bind two nucleotides with positive cooperativity and the oxidation state of the [4Fe-4S] cluster of the Fe protein was found to influence the affinity for binding nucleotides, with the oxidized ([4Fe-4S]2+) state having up to a 15-fold higher affinity for nucleotides when compared to the reduced ([4Fe-4S]1+) state. The first nucleotide binding reaction was found to be driven by a large favorable entropy change (delta S' degree = 10-21 cal mol-1 K-1), with a less favorable or unfavorable enthalpy change (delta H' degree = +1.5 to -3.3 kcal mol-1). In contrast, the second nucleotide binding reaction was found to be driven by a favorable change in enthalpy (delta H' degree = -3.1 to -13.0 kcal mol-1), with generally less favorable entropy changes. A plot of the associated enthalpy (-delta H' degree) and entropy terms (-T delta S' degree) for each nucleotide and protein binding reaction revealed a linear relationship with a slope of 1.12, consistent with strong enthalpy-entropy compensation. These results indicate that the binding of the first nucleotide to the nitrogenase Fe protein results in structural changes accompanied by the reorganization of bound water molecules, whereas the second nucleotide binding reaction appears to result in much smaller structural changes and is probably largely driven by bonding interactions. Evidence is presented that the total free energy change (delta G' degree) derived from the binding of two nucleotides to the Fe protein accounts for the total change in the midpoint potential of the [4Fe-4S] cluster.