Nitrate reductase (NADH)
Nitrate reductase (NADH) (EC 1.7.1.1) is an enzyme that catalyzes the reduction of nitrate (NO3-) to nitrite (NO2-) using NADH as an electron donor. This reaction is a crucial step in the nitrogen cycle, particularly in bacteria and fungi. It represents the first step in dissimilatory nitrate reduction, also known as denitrification, where nitrate serves as the terminal electron acceptor in anaerobic respiration, ultimately leading to the production of nitrogen gas. This process is important for removing excess fixed nitrogen from the environment.
The general reaction catalyzed by nitrate reductase (NADH) is:
NO3- + NADH + H+ → NO2- + NAD+ + H2O
This enzyme is typically a complex metalloprotein containing molybdenum, iron-sulfur clusters, and heme. The molybdenum cofactor is essential for the catalytic activity of the enzyme. The electron flow through the enzyme proceeds from NADH to FAD, then through iron-sulfur clusters, and finally to molybdenum, where nitrate reduction occurs.
NADH-dependent nitrate reductases are commonly found in bacteria, especially in anaerobic or facultative anaerobic species. They are often membrane-bound and coupled to proton translocation, contributing to the generation of a proton gradient that can be used to synthesize ATP. The enzyme's activity is often regulated by the availability of nitrate and oxygen, as well as by other environmental factors.
In some organisms, alternative nitrate reductases using other electron donors, such as NADPH or formate, may also exist. However, NADH-dependent nitrate reductase specifically utilizes NADH as the reducing agent.