Biochemical Reactions - Nitrate Assimilation

Nitrate is the most readily available and preferred source of nitrogen for growth. Assimilatory reduction of NO^-₃ to NH₃ is known to occur in two steps as shown below:

The first step is catalysed by nitrate reductase (NR) which reduces NO^-₃ to NO^-₂ at the expense of two electrons. The second step is catalysed by nitrite reductase which - converts NO^-₂ into NH₃ at the expense of six electrons. Nitrate reductase is Mo-enzyme like dinitrogenase and nitrite reductase is Fe-protein. The physiological source of reductant for the two reductive processes could be reduced ferredoxin - Fd_(red) or reduced pyridine nucleotides (NADH or NADPH) depending upon the system. It is important to point out here that NO^-₃ is known to undergo dissimilation in which it is reduced to N₂ gas. Such a process of nitrate metabolism is called nitrate respiration or denitrification and occurs exclusively in certain bacterial forms under anaerobic condition. The enzymes of denitrification of nitrate are called dissimilatory nitrate reductases.