Centre for Molecular and Structural Biochemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, UK.
Biochem J. 2011 May 1;435(3):743-53. doi: 10.1042/BJ20101920.
The denitrifying bacterium Paracoccus denitrificans can grow aerobically or anaerobically using nitrate or nitrite as the sole nitrogen source. The biochemical pathway responsible is expressed from a gene cluster comprising a nitrate/nitrite transporter (NasA), nitrite transporter (NasH), nitrite reductase (NasB), ferredoxin (NasG) and nitrate reductase (NasC). NasB and NasG are essential for growth with nitrate or nitrite as the nitrogen source. NADH serves as the electron donor for nitrate and nitrite reduction, but only NasB has a NADH-oxidizing domain. Nitrate and nitrite reductase activities show the same Km for NADH and can be separated by anion-exchange chromatography, but only fractions containing NasB retain the ability to oxidize NADH. This implies that NasG mediates electron flux from the NADH-oxidizing site in NasB to the sites of nitrate and nitrite reduction in NasC and NasB respectively. Delivery of extracellular nitrate to NasBGC is mediated by NasA, but both NasA and NasH contribute to nitrite uptake. The roles of NasA and NasC can be substituted during anaerobic growth by the biochemically distinct membrane-bound respiratory nitrate reductase (Nar), demonstrating functional overlap. nasG is highly conserved in nitrate/nitrite assimilation gene clusters, which is consistent with a key role for the NasG ferredoxin, as part of a phylogenetically widespread composite nitrate and nitrite reductase system.
脱氮副球菌可以利用硝酸盐或亚硝酸盐作为唯一氮源进行有氧或无氧生长。负责这一生物化学途径的基因簇由硝酸盐/亚硝酸盐转运蛋白（NasA）、亚硝酸盐转运蛋白（NasH）、亚硝酸盐还原酶（NasB）、铁氧还蛋白（NasG）和硝酸盐还原酶（NasC）组成。NasB 和 NasG 是利用硝酸盐或亚硝酸盐作为氮源生长所必需的。NADH 是硝酸盐和亚硝酸盐还原的电子供体，但只有 NasB 具有 NADH 氧化结构域。硝酸盐和亚硝酸盐还原酶的活性对 NADH 的 Km 值相同，可以通过阴离子交换层析分离，但只有含有 NasB 的部分保留氧化 NADH 的能力。这意味着 NasG 将电子通量从 NasB 的 NADH 氧化位点分别传递到 NasC 和 NasB 的硝酸盐和亚硝酸盐还原位点。NasA 介导细胞外硝酸盐向 NasBGC 的传递，但 NasA 和 NasH 都有助于亚硝酸盐的摄取。在厌氧生长过程中，NasA 和 NasC 的作用可以被生化上不同的膜结合呼吸硝酸盐还原酶（Nar）取代，这表明存在功能重叠。NasG 在硝酸盐/亚硝酸盐同化基因簇中高度保守，这与 NasG 铁氧还蛋白作为广泛存在的硝酸盐和亚硝酸盐复合还原酶系统的一部分的关键作用是一致的。
