Role of molybdenum in nitrate reduction by chlorella.
Role of molybdenum in nitrate reduction by chlorella.
复制标题
钼在小球藻还原硝酸盐中的作用。
作者：
J. Vega;J. Herrera;P. Aparicio;A. Paneque;M. Losada
从fusca小球藻中分离的烟酰胺腺嘌呤二核苷酸硝酸还原酶复合体的硝酸还原活性绝对需要钼。整个烟酰胺腺嘌呤二核苷酸硝酸还原酶是由在没有添加钼酸盐的情况下生长的细胞形成的，但只有它的第一个活性（烟酰胺腺嘌呤二核苷酸diaphorase）是功能性的。该复合物的第二种活性，随后也参与电子从烟酰胺腺嘌呤二核苷酸到硝酸盐的酶转移（FNH(2)-硝酸盐还原酶），取决于钼的存在。硝酸还原酶从头合成既不需要钼酸盐也不需要硝酸盐，但氨作为完整酶复合物的营养抑制因子。在排除硝酸还原酶从头合成的条件下，向缺钼细胞中添加钼酸盐明显提高了该酶的活性水平，从而表明微量金属在体内掺入了先前存在的失活脱酶。与钨酸盐的竞争研究证实了这些结论，并表明钼在小球藻中所起的唯一作用与将硝酸盐还原为亚硝酸盐有关。钨似乎通过取代硝酸盐还原酶复合体中的钼而起作用，从而使烟酰胺腺嘌呤二核苷酸硝酸盐还原酶复合体的FNH(2)-硝酸盐还原酶部分失去活性。
Molybdenum is absolutely required for the nitrate-reducing activity of the nicotinamide adenine dinucleotide nitrate reductase complex isolated from Chlorella fusca. The whole enzyme nicotinamide adenine dinucleotide nitrate reductase is formed by cells grown in the absence of added molybdate, but only its first activity (nicotinamide adenine dinucleotide diaphorase) is functional. The second activity of the complex, which subsequently participates also in the enzymatic transfer of electrons from nicotinamide adenine dinucleotide to nitrate (FNH(2)-nitrate reductase), depends on the presence of molybdenum. Neither molybdate nor nitrate is required for nitrate reductase synthesis de novo, but ammonia acts as a nutritional repressor of the complete enzyme complex. Under conditions which exclude de novo synthesis of nitrate reductase, the addition of molybdate to molybdenum-deficient cells clearly increases the activity level of this enzyme, thus suggesting in vivo incorporation of the trace metal into the pre-existing inactive apoenzyme.Competition studies with tungstate corroborate these conclusions and indicate that the only role played by molybdenum in Chlorella is connected with the reduction of nitrate to nitrite. Tungsten seems to act by replacing molybdenum in the nitrate reductase complex, thus rendering inactive the FNH(2)-nitrate reductase portion of the nicotinamide adenine dinucleotide nitrate reductase complex.