Ammonia assimilation control

Finally, what areas remain important for future research Prediction is always a dangerous business but to us the key questions of ammonia assimilation all revolve around carbon metabolism—what are the sources for the carbon skeletons of amino acids that are synthesized in the chloroplast How do the anaplerotic sequences that provide a ready supply of 2-oxoglutarate and oxoloacetate operate and how are they controlled What are the critical steps that ensure, in times of plentiful ammonia supply, that sufficient carbohydrates are transported and oxidised And perhaps most interestingly of all, how are the electrons needed to reduce and assimilate N2 in nodules and NO in roots generated from carbohydrate transported from the shoot ... 

Certain organisms, for example fungi of the higher orders, possess two GDH enzymes, one specific for NAD, the other for NADP. A. variety of evidence (Smith et al., 1975) has indicated that in these cases, the NADP-linked enzyme is predominantly concerned with ammonia assimilation, while the NAD-linked enzyme is predominantly concerned with deamination of glutamate to 2-oxcglutarate. Control of the relative rates of amination and deamination to reactions in the cell has been observed to be exerted through control of the relative levels of the biosynthetic NADP enzyme and the catabolic NAD enzyme. This control mechanism therefore bypasses the necessity for the individual enzymes to possess elaborate allosteric control mechanisms. 

However, in organisms which have only one GDH enzyme, which is responsible for both ammonia assimilation and deamination of glutamate, there has been a necessity for the enzyme to evolve complex regulatory properties so that the relative rates of amination and deamination reactions can be finely controlled by the concentrations of certain metabolites, and by the energy status of the cell. This is seen in the complex regulatory properties of GDH enzymes from bovine liver and from the Phycomycetes (fungi). 

The reduction of nitrate to ammonia is a rather simple chemical process under very complex control in biological systems. In higher plants the assimilation of nitrate involves the acquisition and subsequent reduction of nitrate to ammonia by nitrate and nitrite reductase. Although our understanding of the biochemistry, genetics, and molecular aspects of nitrate assimilation has advanced, a great deal remains to be learned, especially about regulation and... 

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