Hydrolysis nitrogenase reaction

Figure 1. The Nitrogenase Reaction. The electron transfer proteins ferredoxin (Fd) and flavodoxin (Fid) serve to couple the nitrogenase reaction to metabolically generated reducing equivalents. Ammonia synthesis requires 8 electrons 6 for the reduction of dinitrogen and 2 for the coupled, obligatory synthesis of H2. These reactions are catalyzed by the terminal component in the complex, the MoFe-protein. The electrons are transferred to the MoFe-protein from the Fe-protein in a process coupled to the hydrolysis of 2ATP/electron (Howard and Rees, 1994,1996).

At present, much attention is devoted to enzymes that utilize the energy of ATP hydrolysis for realization of energy-rich mechanics (myosin), transport (Na+,K+-ATPase, Ca2+-ATPase, chemical processes (nitrogenase), polymerases, topoisomerases, GTPases, and for creation of electrochemical gradients in biomembranes (H+-ATPase, ATP synthase ). In this section we focus on the latter process. The coupling mechanism in the nitrogenase reaction is discussed in Section 3.1. 

Figure 3.7. The energy profile of a nitrogenase reaction. Eo is the standard redox potential of the reactants, intermediates and products of the reaction Fd = ferredoxin FeP = Fe protein FeMo = FeMo protein. The arrow indicates the increase of the reduction potential upon ATP hydrolysis (Likhtenshtein 1988a). Reproduced with permission.

MgATP hydrolysis and electron transfer between the two proteins seems not to be direct and the order of reactions may depend on the precise conditions of the experiment at low temperature, electron transfer seems to be reversible (see Ref. 12) for a discussion). One innovation is incorporation of data in which the release of inorganic phosphate was monitored. With other MgATP hydrolyzing enzymes, this step is often the work step in which the energy released by MgATP hydrolysis is utilized. With nitrogenase this step takes place before the dissociation of the two proteins 106). 

MgATP hydrolysis and, 47 189-191 nitrogenase complex, 47 186-189 substrates, 47 192-202 molybdenum iron proteins, 47 161, 166-174, 176-183, 191-192 structure, 47 162-164, 166-170 nitrogen fixation role, 36 78 in nitrogen fixation systems, 27 265-266 noncomplementary reactions with Sn", 10 215... 

Although the hypothesis of Egumi may be an oversimplification, it is certainly true that Fe /Feu is widely used in redox systems. Zn " in hydrolysis, esterification, and similar reactions, and molybdenum in nitrogenase. xanthine oxkkise. nitnite reductase, etc Putting abundance aside, discuss the specific chemical properties of these metals this make them well suited for their tasks. 

Pre-steady-state stopped-flow and rapid quench techniques applied to Mo nitrogenase have provided powerful approaches to the study of this complex enzyme. These studies of Klebsiella pneumoniae Mo nitrogenase showed that a pre-steady-state burst in ATP hydrolysis accompanied electron transfer from the Fe protein to the MoFe protein, and that during the reduction of N2 an enzyme-bound dinitrogen hydride was formed, which under denaturing conditions could be trapped as hydrazine. A comprehensive model developed from a computer simulation of the kinetics of these reactions and the kinetics of the pre-steady-state rates of product formation (H2, NH3) led to the formulation of Scheme 1, the Thorneley and Lowe scheme (50) for nitrogenase function. 

Contrary to hard and soft acid-base predictions, the first example of a vanadyl-thioether complex has been made (151).693 The crown thioether 1,4,7-trithiacyclononane forms a stable 1 1 coordination complex upon reaction with VC13 the presence of adventitious water likely explains the hydrolysis/oxidation of V111 to form VlvC)21. Complexes with a variety of bi- and tetradentate S-donor functionalities have been prepared as model complexes for nitrogen-ase.498,694 These complexes have some structural similarities to the binding site of nitrogenase, however, in contrast to the cofactor they fail to convert N2 to NH3. 

Nitrogenase catalyzes the hydrolysis of ATP ADP + P in a reaction that is reductant-dependent 15,29, 30) and results in electron transfer and activation to provide a low-potential species capable of reducing a unique range of substrates see Reducible Substrate section). The ratedetermining step of ATP utilization may be bimolecular (25, 31). The product ADP is an inhibitor of ATP utilization 32), and thus an ATP-generating system is used in vitro to reconvert ADP to ATP (15, 33). 

The basic outlines of the electron flux through the nitrogenase system have been established, and involve the initial reduction of Fe-protein by low potential carriers such as ferredoxin or flavodoxin. The Fe-protein then transfers electrons to the MoFe-protein in a process that is coupled to the hydrolysis of ATP. After the appropriate numbers of electrons and protons are present in the MoFe-protein, substrate reduction can take place on the FeMo-cofactor. Despite extensive studies, die overall stoichiometry of the nitrogenase catalyzed reaction has still not been definitively established. The uncertainties are expressed in the following equation for the overall enzyme reaction (8) ... 

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