Question

Name two enzymes involved in nitrogen reduction in plants. What are the important factors involved in the reduction of nitrate by one of these enzymes?

Answer 1

Some higher plants, and some animals (termites), have formed associations (symbioses) with diazotrophs. Diazotrophs are microbes. They are intensively studied by microbiologists. Biological nitrogen fixation was discovered by the German agronomist Hermann Hellriegel and Dutch microbiologist Martinus Beijerinck. Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas (N₂). There is only one known family of enzymes that accomplishes this process. All nitrogenases have an iron – and sulfur-containing cofactor that includes a heterometal complex in the active site (e.g., FeMoCo). In most species, this heterometal complex has a central molybdenum atom. However, in some species it is replaced by a vanadium or iron atom. Enzymes responsible for nitrogenase action are very susceptible to destruction by oxygen. Many bacteria cease production of the enzyme in the presence of oxygen. Many nitrogen-fixing organisms exist only in anaerobic conditions, respiring to draw down oxygen levels, or binding the oxygen with proteins. Biological nitrogen fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by an enzyme called nitrogenase. The reaction for BNF is: N₂ + 8 H⁺ + 8 e⁻ → 2 NH₃ + H₂. This type of reaction results in N₂ gaining electrons (see above equation) and is thus termed a reduction reaction. The exact mechanism of catalysis is unknown due to the technical difficulties biochemists have in actually visualizing this reaction in vitro, so the exact sequence of the steps of this reaction are not completely understood. Despite this, a great deal is known of the process. While the equilibrium formation of ammonia from molecular hydrogen and nitrogen has an overall negative enthalpy of reaction (i.e. it gives off energy), the energy barrier to activation is very high without the assistance of catalysis, which is done by nitrogenases. The enzymatic reduction of N₂ to ammonia therefore requires an input of chemical energy, released from ATP hydrolysis, to overcome the activation energy barrier.