From ATP to electron transfer: Electrostatics and free-energy transductionin nitrogenase

Nitrogenase consists of two proteins that work in concert to reduce atmosph eric dinitrogen to a biologically useful form, ammonia (Curr. Opin. Chem. B iol. 2000, 4, 559-566; Chem. Rev. 1996, 96, 2965-2982). The smaller of the proteins (the so-called Fe protein) shuttles high-energy electrons to the l arger subunit (the so-called MoFe protein) where the reduction of dinitroge n molecules takes place. The Fe protein catalyzes the hydrolysis of two MgA TP molecules per electron transferred to the MoFe protein. The physical mec hanism that couples the ATP hydrolysis and electron-transfer reactions in n itrogenase is one of the "great mysteries" of nitrogen fixation. Our goal i s to describe the free-energy transformations that occur in nitrogenase bas ed upon theoretical analysis of structural and electrochemical data. The el ectrostatic and thermodynamic analysis described here, made possible by rec ent X-ray structural data (and motivated by closely related electrochemical studies: Biochemistry 1997, 36, 12976-12983; FEES Lett. 1998, 432, 55-58), shows that the ATP hydrolysis energy in nitrogenase serves the purpose of increasing the driving force of the electron-transfer reaction in the prote in-protein complex. MgATP binding induces conformational changes and protei n-protein association. The protein-protein docking excludes water from the negatively charged [Fe4S4]S-4(cys) redox cofactor that lies near the Fe-pro tein surface, boosting its energy through diminished solvation. We estimate d the induced redox-potential change to be equal to or larger than one-thir d of an electronvolt, which is roughly the energy associated with the hydro lysis of one MgATP molecule. Nitrogenase appears, therefore, to employ a re latively simple ATP hpdrolysis coupled redox cofactor desolvation mechanism to energize, and thus to accelerate, interprotein electron transfer. Our a nalysis also indicates that electrostatic interactions play an important ro le in the substitution of MgADP by MgATP upon reduction of the [Fe4S4]S-4(c ys) cluster in the Fe protein. The nitrogenase scheme of energy conversion may suggest alternative strategies for the design of new molecular devices.