Nitrogenase

Nitrogenase is an enzyme complex that is capable to reduce the elemental, molecular nitrogen (N2 ), and thus convert it into a biologically available form. This process is known as nitrogen fixation. Nitrogenase are present in various bacteria and some archaea. Nitrogen-fixing bacteria are also known as cyanobacteria (eg Anabaena ) and Proteobacteria (eg, Azotobacter ).

Structure and Properties

The enzyme complex consists of two proteins, the dinitrogenase ( heterotetrameric α2β2 ) and the dinitrogenase reductase ( homodimer ). The reaction takes place in the dinitrogenase, reductase transmits the electron received from ferredoxin by a [ 4Fe -4S] iron-sulfur cluster. The active center of dinitrogenase consists of a further [ 8Fe -7S ] iron-sulfur clusters, as well as the iron-sulfur molybdenum cofactor ( FeMoco ). For a long time the identity of the central atom of the FeMoco - factor was unclear. Both a carbon, oxygen or nitrogen atom were plausible candidates. By X -ray emission spectroscopy has recently been shown that there probably is a carbon atom. Some bacteria are capable of producing alternative cofactors in molybdenum deficiency, which contain vanadium or molybdenum held only iron.

The nitrogenase of most bacteria, like all enzymes with iron -sulfur cluster extremely sensitive to oxygen. In order to protect the enzyme against oxygen, bacteria have developed various adaptations, such as thick mucus capsules or particularly thick-walled cells. Bacteria that operate oxygenic photosynthesis, separate nitrogen-fixing cells ( heterocysts ) spatially separated from oxygen -releasing cells or assimilate nitrogen only at night when the light reaction of photosynthesis rests. Some bacteria can fix nitrogen only in symbiosis, such as rhizobia ( rhizobia ), ( often leguminous plants) together with the plants. Since the plants are not even able to fix elemental, molecular nitrogen, they are dependent on the product of the bacterial nitrogenase. The ammonia formed is the starting material for the production of glutamic acid and glutamine.

Streptomyces thermoautotrophicus, a thermophilic bacterium that was isolated from charcoal kilns, has an unusual oxygen- insensitive nitrogenase.

Catalyzed reaction

Since N2 is a very stable and inert molecule ( the binding energy of 945 kJ / mol), a large amount of energy in the form of ATP is required to cleave the triple bond between the two nitrogen atoms. The product of this reaction produces ammonia.

The overall equation of the reaction catalyzed by nitrogenase reaction is:

Nitrogenase is not limited to nitrogen, the enzyme reduces other triple bonds, for example ethyne, cyanide, azide or nitrogen oxides. Under natural conditions, this property probably has no meaning. The reduction of ethyne to ethene however, is used to detect nitrogenase experimentally.

Regulation

The whole process of biological nitrogen fixation is relatively complex and requires the interaction of several enzymes, of which the nitrogenase is the most important. The genes of these enzymes are subject to strict regulation. Your transcription is switched off, for example by oxygen and nitrogen compounds such as nitrate and some amino acids. The shutdown by nitrogen compounds is advantageous because the use of nitrogen from these sources consumes far less energy.

The nitrogenase activity is regulated in Klebsiella pneumoniae over the nifLA operon at the transcriptional level. Protein NifL is a sensor for O2, NifA is, inter alia, a transcriptional activator for the nifHDKY operon coding for the structural elements of the nitrogenase. If the bacterium is exposed to oxygen, NifL and NifA form a heterodimer, NifA can result in the activator function not exercise, nitrogenase is not expressed. This effect is useful because nitrogenase would be inhibited by O2 and would be an expression in the presence of O2 waste of energy. The expression of the genes NifL and NifA itself is started by NtrC. This factor indicates the need to fix nitrogen.