Pitterle D M, Johnson J L, Rajagopalan K V
Department of Biochemistry, Duke University Medical Center, Durham, North Carolina 27710.
J Biol Chem. 1993 Jun 25;268(18):13506-9.
The pterin component of the molybdenum cofactor, termed molybdopterin, is synthesized in Escherichia coli by enzymes encoded at the chl loci. A late step in the biosynthetic pathway, the conversion of a molybdopterin intermediate, precursor Z, to molybdopterin, requires the activity of a two-subunit protein, the converting factor. Precursor Z has many of the features of molybdopterin but lacks the dithiolene function essential for molybdenum ligation. Conversion of precursor Z to molybdopterin is accomplished by transfer of sulfur to produce the dithiolene. The present study describes an in vitro system for molybdopterin biosynthesis comprised of purified precursor Z and purified converting factor. It is established that these components are sufficient to yield molybdopterin, identified by conversion to its characteristic products, Form A, Form B, and dicarboxamidomethylmolybdopterin. Under conditions of precursor excess, the formation of molybdopterin was stoichiometric with converting factor, as would be expected in the absence of a sulfur-regenerating system. The labile product of the reaction, molybdopterin, remained associated with the converting factor large subunit. These results establish that the source of sulfur for molybdopterin biosynthesis is the converting factor and suggest that in vivo a novel sulfur cycle must function to resupply sulfur to the converting factor.
钼辅因子的蝶呤成分，即钼蝶呤，在大肠杆菌中由chl位点编码的酶合成。生物合成途径的最后一步，即钼蝶呤中间体前体Z转化为钼蝶呤，需要一种双亚基蛋白即转化因子的活性。前体Z具有许多钼蝶呤的特征，但缺乏与钼结合所必需的二硫烯功能。前体Z向钼蝶呤的转化是通过硫的转移以产生二硫烯来完成的。本研究描述了一种由纯化的前体Z和纯化的转化因子组成的钼蝶呤生物合成体外系统。已确定这些成分足以产生钼蝶呤，通过转化为其特征产物A形式、B形式和二羧酰胺甲基钼蝶呤来鉴定。在前体过量的条件下，钼蝶呤的形成与转化因子呈化学计量关系，这在没有硫再生系统的情况下是可以预期的。反应的不稳定产物钼蝶呤仍与转化因子大亚基结合。这些结果表明钼蝶呤生物合成的硫源是转化因子，并提示在体内必须有一个新的硫循环来为转化因子重新提供硫。
