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A three-domain copper-nitrite reductase with a unique sensing loop

机译：具有独特感应环的三畴亚硝酸铜还原酶

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Dissimilatory nitrite reductases are key enzymes in the denitrification pathway, reducing nitrite and leading to the production of gaseous products (NO, N2O and N2). The reaction is catalysed either by a Cu-containing nitrite reductase (NirK) or by a cytochrome cd1 nitrite reductase (NirS), as the simultaneous presence of the two enzymes has never been detected in the same microorganism. The thermophilic bacterium Thermus scotoductus SA-01 is an exception to this rule, harbouring both genes within a denitrification cluster, which encodes for an atypical NirK. The crystal structure of TsNirK has been determined at 1.63 Å resolution. TsNirK is a homotrimer with subunits of 451 residues that contain three copper atoms each. The N-terminal region possesses a type 2 Cu (T2Cu) and a type 1 Cu (T1CuN) while the C-terminus contains an extra type 1 Cu (T1CuC) bound within a cupredoxin motif. T1CuN shows an unusual Cu atom coordination (His2–Cys–Gln) compared with T1Cu observed in NirKs reported so far (His2–Cys–Met). T1CuC is buried at ∼5 Å from the molecular surface and located ∼14.1 Å away from T1CuN; T1CuN and T2Cu are ∼12.6 Å apart. All these distances are compatible with an electron-transfer process T1CuC → T1CuN → T2Cu. T1CuN and T2Cu are connected by a typical Cys–His bridge and an unexpected sensing loop which harbours a SerCAT residue close to T2Cu, suggesting an alternative nitrite-reduction mechanism in these enzymes. Biophysicochemical and functional features of TsNirK are discussed on the basis of X-ray crystallography, electron paramagnetic resonance, resonance Raman and kinetic experiments.
机译：亚硝酸盐异化还原酶是反硝化途径中的关键酶，还原亚硝酸盐并导致生成气态产物（NO，N2O和N2）。由于从未在同一微生物中检测到两种酶的同时存在，因此该反应是通过含铜的亚硝酸盐还原酶（NirK）或通过细胞色素cd1亚硝酸盐还原酶（NirS）催化的。嗜热菌Thermus scotoductus SA-01是该规则的例外，它在反硝化簇中包含两个基因，该簇编码非典型的NirK。 TsNirK的晶体结构已经确定为1.63Å的分辨率。 TsNirK是具有451个残基的亚基的均三聚体，每个残基包含三个铜原子。 N端区域具有2型Cu（T2Cu）和1型Cu（T1CuN），而C端包含结合在铜氧还蛋白基序中的额外1型Cu（T1CuC）。与迄今为止报道的NirKs（His2-Cys-Met）中观察到的T1Cu相比，T1CuN显示出异常的Cu原子配位（His2-Cys-Gln）。 T1CuC埋在距分子表面约5处，并且距T1CuN约14.1。 T1CuN和T2Cu相距约12.6。所有这些距离都与电子传输过程T1CuC→T1CuN→T2Cu兼容。 T1CuN和T2Cu通过典型的Cys-His桥和意外的感应环连接，该感应环在T2Cu附近带有一个SerCAT残基，表明这些酶具有另一种亚硝酸盐还原机制。在X射线晶体学，电子顺磁共振，拉曼共振和动力学实验的基础上讨论了TsNirK的生物物理化学和功能特征。

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