Roth R, Hägerhäll C
Department of Biochemistry, Lund University, Box 124, 22100, Lund, Sweden.
Biochim Biophys Acta. 2001 Apr 2;1504(2-3):352-62. doi: 10.1016/s0005-2728(00)00265-6.
quinone oxidoreductase, or Complex I, is a multi-subunit membrane-bound enzyme in the respiratory chain of many pro- and eukaryotes. The enzyme catalyzes the oxidation of NADH and donates electrons to the quinone pool, coupled to proton translocation across the membrane, but the mechanism of energy transduction is not understood. In bacteria the enzyme consists of 14 subunits, seven membrane spanning and seven protruding from the membrane. The hydrophobic NuoH (NQO8, ND1, NAD1, NdhA) subunit is seemingly involved in quinone binding. A homologous, structurally and most likely functionally similar subunit is also found in F(420)H2 oxidoreductases and in complex membrane-bound hydrogenases. We have made theoretical analyses of NuoH and NuoH-like polypeptides and experimentally analyzed the transmembrane topology of the NuoH subunit from Rhodobacter capsulatus by constructing and analyzing alkaline phosphatase fusion proteins. This demonstrated that the NuoH polypeptide has eight transmembrane segments, and four highly conserved hydrophilic sequence motifs facing the inside, bacterial cytoplasm. The N-terminal and C-terminal ends are located on the outside of the membrane. A topology model of NuoH based on these results is presented, and implications from the model are discussed.
醌氧化还原酶，即复合体I，是许多原核生物和真核生物呼吸链中的一种多亚基膜结合酶。该酶催化NADH的氧化，并将电子传递给醌池，同时伴随着质子跨膜转运，但能量转导机制尚不清楚。在细菌中，该酶由14个亚基组成，其中7个跨膜亚基，7个膜外突出亚基。疏水的NuoH（NQO8、ND1、NAD1、NdhA）亚基似乎参与醌的结合。在F(420)H2氧化还原酶和复合膜结合氢化酶中也发现了一个同源的、结构和功能上很可能相似的亚基。我们对NuoH和类NuoH多肽进行了理论分析，并通过构建和分析碱性磷酸酶融合蛋白，对荚膜红细菌NuoH亚基的跨膜拓扑结构进行了实验分析。结果表明，NuoH多肽有8个跨膜区段，4个高度保守的亲水序列基序面向细菌细胞质内部。N端和C端位于膜外。基于这些结果提出了NuoH的拓扑模型，并讨论了该模型的意义。
