α-Me-Gly(Propargyl)-OH

The nitrogenase MoFe protein contains the active site metallocluster called FeMo-cofactor [7Fe-9S-Mo-homocitrate] that exhibits an S = 3/2 EPR signal in the resting state. No interaction with FeMo-cofactor is detected when either substrates or inhibitors are incubated with MoFe protein in the resting state. Rather, the detection of such interactions requires the incubation of the MoFe protein together with its obligate electron donor, called the Fe protein, and MgATP under turnover conditions. This indicates that a more reduced state of the MoFe protein is required to accommodate substrate or inhibitor interaction. In the present work, substitution of an arginine residue (alpha-96(Arg)) located next to the active site FeMo-cofactor in the MoFe protein by leucine, glutamine, alanine, or histidine is found to result in MoFe proteins that can interact with acetylene or cyanide in the as-isolated, resting state without the need for the Fe protein, or MgATP. The dithionite-reduced, resting states of the alpha-96(Leu)-, alpha-96(Gln)-, alpha-96(Ala)-, or alpha-96(His)-substituted MoFe proteins show an S = 3/2 EPR signal (g = 4.26, 3.67, 2.00) similar to that assigned to FeMo-cofactor in the wild-type MoFe protein. However, in contrast to the wild-type MoFe protein, the alpha-96-substituted MoFe proteins all exhibit changes in their EPR spectra upon incubation with acetylene or cyanide. The alpha-96(Leu)-substituted MoFe protein was representative of the other alpha-96-substituted MoFe proteins examined. The incubation of acetylene with the alpha-96(Leu) MoFe protein decreased the intensity of the normal FeMo-cofactor signal with the appearance of a new EPR signal having inflections at g = 4.50 and 3.50. Incubation of cyanide with the alpha-96(Leu) MoFe protein also decreased the FeMo-cofactor EPR signal with concomitant appearance of a new EPR signal having an inflection at g = 4.06. The acetylene- and cyanide-dependent EPR signals observed for the alpha-96(Leu)-substituted MoFe protein were found to follow Curie law 1/T dependence, consistent with a ground-state transition as observed for FeMo-cofactor. The microwave power dependence of the EPR signal intensity is shifted to higher power for the acetylene- and cyanide-dependent signals, consistent with a change in the relaxation properties of the spin system of FeMo-cofactor. Finally, the alpha-96(Leu)-substituted MoFe protein incubated with (13)C-labeled cyanide displays a (13)C ENDOR signal with an isotropic hyperfine coupling of 0.42 MHz in Q-band Mims pulsed ENDOR spectra. This indicates the existence of some spin density on the cyanide, and thus suggests that the new component of the cyanide-dependent EPR signals arise from the direct bonding of cyanide to the FeMo-cofactor. These data indicate that both acetylene and cyanide are able to interact with FeMo-cofactor contained within the alpha-96-substituted MoFe proteins in the resting state. These results support a model where effective interaction of substrates or inhibitors with FeMo-cofactor occurs as a consequence of both increased reactivity and accessibility of FeMo-cofactor under turnover conditions. We suggest that, for the wild-type MoFe protein, the alpha-96(Arg) side chain acts as a gatekeeper, moving during turnover in order to permit accessibility of acetylene or cyanide to a specific [4Fe-4S] face of FeMo-cofactor.

Substitution of the MoFe protein alpha-70(Val) residue with Ala or Gly expands the substrate range of nitrogenase, allowing the reduction of larger alkynes, including propargyl alcohol (HC[triple bond]CCH(2)OH). Herein, we report characterization of the alpha-70(Val)(-->)(Ala) MoFe protein with propargyl alcohol trapped at the active site. The alpha-70(Ala) variant MoFe protein was rapidly frozen during reduction of propargyl alcohol, resulting in the conversion of the resting-state FeMo-cofactor EPR signal (S = 3/2 and g = [4.41, 3.60, 2.00]) to a new state (S = 1/2 and g = [2.123, 1.998, 1.986]). This EPR signal of the new state increased in intensity with increasing propargyl alcohol concentration, consistent with the binding of a single substrate. The EPR signal of the propargyl alcohol state showed temperature and microwave power dependencies markedly different from those of the classic FeMo-cofactor EPR signal, consistent with the difference in spin. The new state is analogous to that induced by the binding of the inhibitor CO ("lo CO" state) to FeMo-cofactor in the wild-type MoFe protein. The (13)C ENDOR spectrum of the alpha-70(Ala) MoFe protein with trapped (13)C-labeled propargyl alcohol exhibited three well-resolved (13)C doublets centered at the (13)C Larmor frequency with isotropic hyperfine couplings of approximately 3.2, approximately 1.4, and approximately 0.7 MHz, indicating that the alcohol (or a fragment) is coordinated to the cofactor. The results presented here localize the binding site of propargyl alcohol to one [4Fe-4S] face of FeMo-cofactor and indicate roles for the alpha-70(Val) residue in controlling FeMo-cofactor reactivity.

Site-directed mutagenesis and gene-replacement procedures were used to isolate mutant strains of Azotobacter vinelandii that produce altered MoFe proteins in which the alpha-subunit residue-195 position, normally occupied by a histidine residue, was individually substituted by a variety of other amino acids. Structural studies have revealed that this histidine residue is associated with the FeMo-cofactor binding domain and probably provides an NH-->S hydrogen bond to a central bridging sulfide located within FeMo-cofactor. Substitution by a glutamine residue results in an altered MoFe protein that binds but does not reduce N2, the physiological substrate. Although N2 is not a substrate for the altered MoFe protein, it is a potent inhibitor of both acetylene and proton reduction, both of which are otherwise effectively reduced by the altered MoFe protein. This result provides evidence that N2 inhibits proton and acetylene reduction by simple occupancy of a common active site. N2 also uncouples MgATP from proton reduction catalyzed by the altered MoFe protein but does so without lowering the overall rate of MgATP hydrolysis. Thus, the quasi-unidirectional flow of electrons from the Fe protein to the MoFe protein that occurs during nitrogenase turnover is controlled, in part, by the substrate serving as an effective electron sink. Substitution of the alpha-histidine-195 residue by glutamine also imparts to the altered MoFe protein hypersensitivity of both its acetylene reduction and N2 binding to inhibition by CO, indicating that the imidazole group of the alpha-histidine-195 residue might protect an Fe contained within the FeMo-cofactor from attack by CO. Finally, comparisons of the catalytic and spectroscopic properties of altered MoFe proteins produced by various mutant strains suggest that the alpha-histidine-195 residue has a structural role, which serves to keep FeMo-cofactor attached to the MoFe protein and to correctly position FeMo-cofactor within the polypeptide matrix, such that N2 binding is accommodated.