硝酸鹽對氮代謝至關(guān)重要,，但亞硝酸鹽在細(xì)胞中會是有害的，因?yàn)樗鼤贿€原成對細(xì)胞有毒的一氧化氮,。因此細(xì)胞亞硝酸鹽會被相關(guān)通道和運(yùn)輸因子從細(xì)胞中迅速清除,，或被吸收酶還原成銨或雙氮。令人吃驚的是,，我們對硝酸鹽運(yùn)輸知之甚少,，但現(xiàn)在，細(xì)菌硝酸鹽/亞硝酸鹽運(yùn)輸?shù)鞍譔arK在有基質(zhì)和沒有基質(zhì)兩種情況下的X-射線晶體結(jié)構(gòu)已被確定,。這些結(jié)構(gòu)顯示了一個沒有“可質(zhì)子化”殘跡的,、帶正電的基質(zhì)-轉(zhuǎn)位通道，說明NarK起一個硝酸鹽/亞硝酸鹽交換器的作用,，而且質(zhì)子不大可能被一起運(yùn)輸,。（生物谷Bioon.com）
生物谷推薦英文摘要：
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.
