Abstract:Hydroxylamine oxidoreductase (HAO) is a multiheme protein,and each monomer consists of seven electron transfer hemes and one catalytic heme.HAO can catalyze the oxidation of hydroxylamine and hydrazin or the reduction of hydroxylamine,nitrite oxide,and nitrite.The optimum temperature,pH,substrate,product specificity,and inhibitor of HAO vary among different nitrifying bacteria.As a key enzyme in nitrification,HAO plays an important role in improving the biological nitrogen removal rate and eliminating the toxicity of nitrification intermediates (hydroxylamine).This article systematically reviewed the distribution,structure,expression regulation,and activity of HAO in denitrifying microorganisms,summarized the biochemical properties of HAO in different nitrifying bacteria,and finally put forward the future research directions of HAO.This review is helpful for understanding the biological denitrification process and the mechanism of hydroxylamine metabolism in microorganisms and will shed new light on the optimization of wastewater treatment.
