MgATP

MgATP. The numbers in parenthesis represent the number of electrons required for the reaction shown. is cyclopropene A cyclopropane. 

Substrate reduction is accompHshed by a series of sequential associations and dissociations of the two proteias, and duting each cycle, two molecules of MgATP are hydroly2ed and a single electron is transferred from the Fe proteia to the MoFe proteia (11,133), with the dissociation step being rate-limiting at about 6 (H)- Although the kinetics of aU. the partial reactions have been measured, Httie is known about the physical details of the... 

In most animal, plant, and microbial cells, the enzyme that phosphorylates glucose is hexokinase. Magnesium ion (Mg ) is required for this reaction, as for the other kinase enzymes in the glycolytic pathway. The true substrate for the hexokinase reaction is MgATP. The apparent K , for glucose of the animal... 

As discussed earlier, a decrease in muscle MgATP could inhibit Ca release from the SR, by lowering ATP content and increasing the free Mg. A fall in the ATP/ADP ratio may also inhibit Ca release by slowing Cd reuptake into the SR. A better preserved ATP/ADP ratio in exercising muscle as a result of increased creatine content could counteract the inhibition of Ca " kinetics and delay fatigue. 

Equation (1) demonstrates that, even at its most efficient, one Ha molecule is evolved for every Na molecule reduced to ammonia by ni-trogenase. Perturbation of the enzymic reaction conditions by temperature or protein ratio can lead to this reaction becoming far less efficient, with a large quantity of Ha being produced per molecule of Na reduced emd/or the ratio of MgATP hydrolysed to electrons transferred exceeding two. 

These were relatively low-resolution structures, and with refinement some errors in the initial structural assignments have been detected (4-7). Since the structures were first reported the subject has been extensively reviewed in this series (8) and elsewhere 9-15). This review will focus on the structure, biosynthesis, and function of the met-allosulfur clusters found in nitrogenases. This will require a broader overview of some functional aspects, particularly the involvement of MgATP in the enzymic reaction, and also some reference will be made to the extensive literature (9, 15) on biomimetic chemistry that has helped to illuminate possible modes of nitrogenase function, although a detailed review of this chemistry will not be attempted here. This review cannot be fully comprehensive in the space available, but concentrates on recent advances and attempts to describe the current level of our understanding. 

The purified preparations of the apo-MoFe proteins from both organisms included a small additional polypeptide of around 20 kDa. This was shown to be the nifY product in K. pneumoniae (94) and a non-nif protein denoted y in A. vinelandii (95). These proteins are apparently essential for effective reaction with FeMoco and are associated with the MoFe protein polypeptide through its interaction with the Fe protein and MgATP (96, 97) (see Section IV,C,3). These observations demonstrate a third role for the Fe protein in generating a form of apo-MoFe protein that is capable of accepting FeMoco. 

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

The reduced Fe protein MgATP complex forms a complex with the MoFe protein... 

An electron is transferred from the Fe protein to the MoFe protein with concomitant hydrolysis of MgATP to MgADP and Pi... 

The Fe protein is reduced by Na2S204 and the MgADP is replaced by MgATP... 

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). 

Fe protein MoFe protein complex from Avl + Av2 with MgADP/ MgATP and AlFj. This putative transition state complex is proving extremely useful in the analysis of the interactions between the two proteins and nucleotides. 

Despite intensive efforts by many groups the roles of MgATP hydrolysis in nitrogenase turnover is still enigmatic. MgATP binds to both... 

---