Regulation of cobamide-dependent ribonucleotide reductase by allosteric effectors and divalent cations.
Regulation of cobamide-dependent ribonucleotide reductase by allosteric effectors and divalent cations.
复制标题
变构效应子和二价阳离子对钴酰胺依赖性核糖核苷酸还原酶的调节。
DOI：
10.1016/s0021-9258(18)95945-4
复制
发表时间：
1967
期刊：
影响因子：
--
通讯作者：
W. S. Beck
中科院分区：
文献类型：
--
作者：
W. S. Beck
本文详细研究了影响莱氏乳杆菌钴酰胺依赖性核糖核苷酸还原酶速率行为和底物特异性的因素。四种常见的三磷酸核糖核苷（三磷酸胞苷、三磷酸尿苷、三磷酸腺苷和三磷酸鸟苷）的还原都受到不同的三磷酸脱氧核糖核苷的最大刺激。这些脱氧核糖核苷酸，方便地称为“主要”效应物，是：用于CTP还原的脱氧腺苷三磷酸;用于UTP还原的脱氧胞苷三磷酸;用于ATP还原的脱氧鸟苷三磷酸;和用于GTP还原的胸苷三磷酸。只有ATP和GTP在没有添加效应子的情况下被主动减少;然而，在这两种情况下，减少被引发效应子增强。脱氧核糖核苷酸在低浓度（即10- 4 m）下有活性;它们不能被相应的二磷酸和单磷酸所取代;它们相互竞争酶结合位点而不是催化位点。因此，它们是变构效应物。底物混合物的动力学研究表明，存在一个单一的催化位点，但由于一种底物的还原产物二价阳离子（或底物本身）可以是用于还原另一底物的正效应物或负效应物。（Mg++、Ca++和Mn++）也具有显著的刺激和抑制作用，其随底物、效应物，以及阳离子和效应物添加的顺序。结果表明，这种酶，以前显示，当无叶酸细胞在无胸腺嘧啶培养基中生长时，（并且可能被细胞内dTTP抑制），也受到正反馈和负反馈效应的复杂网络的影响，其中变构效应核苷酸和二价阳离子，推测是通过改变酶的物理状态，确定哪些核糖核苷酸被还原。这些作用可能是维持体内脱氧核糖核苷酸合成平衡的一种机制。
Factors regulating the rate behavior and substrate specificity of purified cobamide-dependent ribonucleotide reductase ofLactobacillus leichmanniihave been examined in detail. Reduction of each of the four common ribonucleoside triphosphates (cytidine triphosphate, uridine triphosphate, adenosine triphosphate, and guanosine triphosphate) is maximally stimulated by a different deoxyribonucleoside triphosphate. These deoxyribonucleotides, conveniently termed “prime” effectors, are: deoxyadenosine triphosphate for CTP reduction; deoxycytidine triphosphate for UTP reduction; deoxyguanosine triphosphate for ATP reduction; and thymidine triphosphate for GTP reduction. Only ATP and GTP are actively reduced in the absence of added effector; in both cases, however, reduction is enhanced by prime effector. Deoxyribonucleotides that are not prime effectors for a given reduction may be negative or weakly positive effectors.Effector nucleotides are active at low concentrations (viz. 10-4m); they are not replaceable by the corresponding diphosphates and monophosphates; and they are in competition with one another for an enzymatic binding site other than the catalytic site. Thus, they are allosteric effectors. Kinetic studies with substrate mixtures suggest the presence of a single catalytic site, but are inconclusive because the reduction product of one substrate (or the substrate itself) may be a positive or negative effector for reduction of the other substrate.Divalent cations (Mg++, Ca++, and Mn++) also have significant stimulatory and inhibitory effects that vary with the substrate, the effector, and the sequence of cation and effector addition.The results indicate that this enzyme, previously shown to be derepressed when folate-free cells are grown in thymine-free medium (and probably repressed by intracellular dTTP), is also subject to a complex network of positive and negative feedback effects in which allosteric effector nucleotides and divalent cations, presumably through modifications of the physical state of the enzyme, determine which ribonucleotides are reduced. It is concluded that these effects may maintain the balance of deoxyribonucleotide synthesisin vivo.