MgADP inhibition

Maritano, S., Fairhurst, S.A., and Eady, R.R. (2001) Novel EPR signals associated with FeMoco centres of MoFe protein in MgADP-inhibited turnover of nitrogenase, FEBS Lett. 505, 125-128. 

Many of the crystals were prepared in the presence of azide, which is known to enhance the MgADP inhibition [17], and azide has been resolved in a crystal [38]. Thus, the mutually similar structures, including that of EFi [39] which was also prepared in the presence of azide, must all represent, basically, the MgADP-inhibited state. In this state, 7 is orientated at 80° [18], as in the 80° intermediate during active rotation. Fluorescence energy transfer between probes on a / and rotating 7 has indicated that, of the two active rotation intermediates at 0° and 80°, the crystal structures are much closer to the 80° intermediate [40]. At 80°, we expect one catalytic site to be open to the medium (asterisks in Fig. 14.2B), which is consistent with the idea that the crystal structures with a fully open catalytic site mimic the 80° state rather than the ATP-waiting state. 

In the reverse reaction, free ADP gives competitive substrate inhibition versus MgADP. MgATP and MgADP do not show substrate inhibition as they are too large to fit into the site for AMP or ADP. 

Three mies can be formulated to predict product or dead-end inhibition patterns in such mechanisms. (1) When an inhibitor occupies the same portion of an active site as the variable substrate, the inhibition is competitive. (2) When an inhibitor combines in a different portion of the same active site as the variable substrate, the inhibition is noncompetitive. Thus, with pyruvate carboxylase, MgADP and P are both noncompetitive inhibitors versus bicarbonate, although in accordance with Rule 1 they are competitive versus MgATP. (3) When an... 

Kerrick WGL, Hoar PE (1981) Inhibition of smooth muscle tension by cyclic AMP-de-pendent protein kinase. Nature 292 253-255 Khromov A, Somlyo AV, Trentham DR, Zimmermann B, Somlyo AP (1995) The role of MgADP in force maintenance by dephosphorylated cross-bridges in smooth muscle a flash photolysis study. Biophys J 69 2611-2622 Khromov AS, Somlyo AV, Somlyo AP (1996) Nucleotide binding by actomyosin as a determinant of relaxation kinetics of rabbit phasic and tonic smooth muscle. J Physiol (Lond) 492 669-673... 

It may be useful to describe some general properties of nitrogenase systems before the more detailed accounts of the above-mentioned evidence. First, nitrogenase is a very versatile enzyme and will reduce several other substrates, particularly triple-bonded isosteres of the natural substrate, N (Table IV). Protons, presumably from water, are a second natural substrate the enzyme evolves H2 whether or not a reducible substrate is added. Carbon monoxide (CO) inhibits the reduction of all reducible substrates except ATP-dependent H2 evolution. MgADP, a product of the reaction inhibits all nitrogenase activity. 

Other lines of evidence indicate that ATP hydrolysis by nitrogenase is not solely associated with electron transfer between the component proteins. For instance, nitrogenase is capable of a reductant-independent ATPase activity (Jeng et al., 1970) and the kinetics of inhibition by MgADP of H2 evoluticm and electron transfer between the component proteins are different (Thorne-ley and Cornish-Bowden, 1977). 

The reduction of isolated oxidized Fe protein is clearly inhibited by MgADP to a rate which, though faster than the fastest enzyme turnover time, is very similar to the rate of reduction of Oa-inactivated Fe protein (Yates et al., 1975). A simple possibility is that MgADP binds the oxidized Fe protein and, after reduction, the resulting complex fails to transfer electrons to the MoFe protein. However, ADP release from the reduced protein in the presence of MgATP is as rapid as interprotein electron transfer (ThiNmeley and... 

MgATP to Fe protein Gel equilibration in Sephadex G-50, not, or only slightly, inhibited by MgADP 13... 

Different inhibition kinetics of e transfer and Hs evolution with MgADP 21... 

MgADP to enzyme during disulfonate Kinetics of inhibition of H2 evolution and 21... 

There is little doubt that MgATP both causes a conformational change in the Fe protein to lower its redox potential and is hydrolyzed during inter-protein electron transfer. However, its possible roles in substrate reduction remain obscure. There is also uncertainty about the site of inhibition by MgADP. Some evidence suggests that it may be associated with the MoFe... 

Nucleotide requirements for IE. Like DRAG, IE requires a nucleotide to carry out its activity on Rr2 (Table 1). MgADP is much preferred over MgATT. Our hypothesis is that MgADP is required as an effector that binds to Rr2. Justification for this hypothesis is based on the observation that ADP is not hydrolyzed in the reaction, that analogues that inhibit the electron transfer activity of Rr2 also allow its ADP-ribosylation by IE (for... 

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