硝酸盐对氮代谢至关重要，但亚硝酸盐在细胞中会是有害的，因为它会被还原成对细胞有毒的一氧化氮。因此细胞亚硝酸盐会被相关通道和运输因子从细胞中迅速清除，或被吸收酶还原成铵或双氮。令人吃惊的是，我们对硝酸盐运输知之甚少，但现在，细菌硝酸盐/亚硝酸盐运输蛋白NarK在有基质和没有基质两种情况下的X-射线晶体结构已被确定。这些结构显示了一个没有“可质子化”残迹的、带正电的基质-转位通道，说明NarK起一个硝酸盐/亚硝酸盐交换器的作用，而且质子不大可能被一起运输。（生物谷Bioon.com）
生物谷推荐英文摘要：
Nature doi:10.1038/nature12139
Crystal structure of a nitrate/nitrite exchanger
Hongjin Zheng, Goragot Wisedchaisri & Tamir Gonen
Mineral nitrogen in nature is often found in the form of nitrate (NO3-). Numerous microorganisms evolved to assimilate nitrate and use it as a major source of mineral nitrogen uptake. Nitrate, which is central in nitrogen metabolism, is first reduced to nitrite (NO2-) through a two-electron reduction reaction. The accumulation of cellular nitrite can be harmful because nitrite can be reduced to the cytotoxic nitric oxide. Instead, nitrite is rapidly removed from the cell by channels and transporters, or reduced to ammonium or dinitrogen through the action of assimilatory enzymes. Despite decades of effort no structure is currently available for any nitrate transport protein and the mechanism by which nitrate is transported remains largely unknown. Here we report the structure of a bacterial nitrate/nitrite transport protein, NarK, from Escherichia coli, with and without substrate. The structures reveal a positively charged substrate-translocation pathway lacking protonatable residues, suggesting that NarK functions as a nitrate/nitrite exchanger and that protons are unlikely to be co-transported. Conserved arginine residues comprise the substrate-binding pocket, which is formed by association of helices from the two halves of NarK. Key residues that are important for substrate recognition and transport are identified and related to extensive mutagenesis and functional studies. We propose that NarK exchanges nitrate for nitrite by a rocker switch mechanism facilitated by inter-domain hydrogen bond networks.
