Biochemistry Of Nitrogen Fixation Research Articles

Biology and biochemistry of nitrogen fixation

Biology and biochemistry of nitrogen fixation

Molecular aspects of nitrogen fixation by photosynthetic prokaryotes.

The photosynthetic prokaryotes possess diverse metabolic capabilities, both in carrying out different types of photosynthesis and in their other growth modes. The nature of the coupling of these energy-generating processes with the basic metabolic demands of the cell, such as nitrogen fixation, has stimulated research for many years. In addition, nitrogen fixation by photosynthetic prokaryotes exhibits several unique features; the oxygen-evolving cyanobacteria have developed various strategies for protection of the oxygen-labile nitrogenase proteins, and some photosynthetic bacteria have been found to regulate their nitrogenase (N2ase) activity in a rapid response to fixed nitrogen, thus saving substantial amounts of energy. Recent advances in the biochemistry, physiology, and genetics of nitrogen fixation by cyanobacteria and photosynthetic bacteria are reviewed, with special emphasis on the unique features found in these organisms. Several major topics in cyanobacterial nitrogen fixation are reviewed. The isolation and characterization of N2ase and the isolation and sequence of N2ase structural genes have shown a great deal of similarity with other organisms. The possible pathways of electron flow to N2ase, the mechanisms of oxygen protection, and the control of nif expression and heterocyst differentiation will be discussed. Several recent advances in the physiology and biochemistry of nitrogen fixation by the photosynthetic bacteria are reviewed. Photosynthetic bacteria have been found to fix nitrogen microaerobically in darkness. The regulation of nif expression and possible pathways of electron flow to N2ase are discussed. The isolation of N2ase proteins, particularly the covalent modification of the Fe protein, the nature of the modifying group, properties of the activating enzyme, and regulating factors of the inactivation/activation process are reviewed.

Biochemical genetics of nitrogen fixation.

This chapter will introduce the subject of nitrogen fixation and will stress the biochemical aspects. Subsequent papers on nitrogen fixation in this volume will focus on the molecular biology and regulation of the system in several different organisms. For a more detailed background, the reader may want to consult one or more reviews dealing with the Rhizobium-legume symbiosis1,2,3,4,5,6,7 or those dealing with the biochemistry of nitrogen fixation in free-living bacteria3,6,8,9. Both the biochemistry and molecular biology of nitrogen fixation are quite complex and breakthroughs in one area of nitrogen fixation research frequently impact on other areas.

Progress in the biochemistry of nitrogen fixation.

A discussion of progress in the biochemistry of nitrogen fixation is simplified by the thorough reviews which have been given by Professor P. W. Wilson and Professor J. Chatt. Professor Wilson has outlined the history of the subject and has indicated that a great deal of our current knowledge actually was established by the use of intact organisms. Detailed studies of the enzymology of the process were limited by the necessity for using whole cells, but with intact cells the specific inhibitors of nitrogen fixation, the role of ammonia as the key intermediate in nitrogen fixation, the involvement of molecular hydrogen, the physical constants characteristic of nitrogen fixation, the enhancement of the hydrogen exchange reaction by molecular nitrogen, and the demonstration that nitrous oxide can serve as an alternative substrate to N 2 all had been established. Little was known about the electron donors in nitrogen fixation, the role of ATP in the process had not been shown, and nothing was known about the composition of the nitrogen fixing enzymes although it was assumed that they contained iron and molybdenum. It was of obvious importance to obtain an active preparation which could fix nitrogen in the absence of intact cells. Much work was directed to this end and instances of positive nitrogen fixation were reported in the literature (Burris 1966). However, the fixation obtained was not very helpful for mechanism studies, because it was inconsistent and poorly controlled. In late 1959 we obtained consistent fixation with cell-free extracts from the blue–green algae (Schneider et al 1960), and at approximately the same time Carnahan, Mortenson, Mower & Castle (1960 a ) obtained good cell-free fixation with extracts from Clostridium pasteurianum . Their preparation was considerably more active and convenient than ours, and so most work with cell-free preparations from blue-green algae was abandoned in favour of work with the anaerobic bacteria. Their process of drying the cells on a rotary vacuum evaporator and supplying very high concentrations of pyruvate as the substrate were critical for achieving active fixation.

Biochemistry of nitrogen fixation in free-living organisms.

Biochemistry of nitrogen fixation in free-living organisms.

The biochemistry of nitrogen fixation by leguminosae

A large number of experiments were performed under a variety of conditions to test whether the nodule bacteria are capable of using atmospheric nitrogen apart from the host plant. The results have offered no encouragement to the belief that nitrogen fixation does take place.