Hydrolysis of the 5'-end of the nascent transcript by the capping enzyme

After the capping complex is formed, the RNA triphosphatase activity of the capping enzyme hydrolyzes the 5'-end phosphate group of the nascent mRNA transcript to a diphosphate.<BR>The RNA triphosphatase (RTP) domain of mammalian capping enzyme is a member of a superfamily of phosphatases that include the protein tyrosine phosphatases, some lipid phosphatases, and several nucleic acid phosphatases. This family uses a conserved nucleophilic cysteine residue to attack the target phosphate. A transient phospho-cysteinyl enzyme intermediate is then hydrolyzed to regenerate the enzyme active site. It should be noted that while higher eukaryotic capping enzymes use PTP-like triphosphatase domains, the yeast triphosphatases are a completely different class of enzymes. The yeast RTPs are metal-dependent phosphatases. RNA 5'-triphosphatase (RTP) catalyzed first reaction can be represented as:pppN(pN)n + GTP -> ppN(pN)n + Pi; (n=20-25)<P>

在形成帽状复合物之后，帽酶的RNA三磷酸酶活性将新生mRNA转录本的5'-端磷酸基团水解为二磷酸。哺乳动物帽酶的RNA三磷酸酶（RTP）结构域是包含蛋白质酪氨酸磷酸酶、某些脂质磷酸酶以及数种核酸磷酸酶的磷酸酶超家族的一员。该家族利用保守的亲核半胱氨酸残基攻击目标磷酸。随后，一个短暂的磷酸半胱氨酸酶中间体被水解以再生酶的活性位点。值得注意的是，尽管高等真核生物的帽酶使用PTP样三磷酸酶结构域，但酵母三磷酸酶属于完全不同的酶类。酵母RTPs是金属依赖性磷酸酶。RNA 5'-三磷酸酶（RTP）催化的首次反应可表示为：pppN(pN)n + GTP -> ppN(pN)n + Pi; (n=20-25)