Cytidylate phosphorylation

Once orotate is formed, the ribose 5-phosphate side chain, provided once again by PRPP, is attached to yield orotidylate (Fig. 22-36). Orotidylate is then decarboxy-lated to uridylate, which is phosphorylated to UTP. CTP is formed from UTP by the action of cytidylate synthetase, by way of an acyl phosphate intermediate (consuming one ATP). The nitrogen donor is normally glutamine, although the cytidylate synthetases in many species can use NH4 directly. 

Like their parent nucleosides, uridylic and cytidylic acids give only a faint orcinol test owing to the stability of the union between sugar and pyrimidine. In addition, the phosphoryl group adheres to the pentose... 

Phosphorylation of trityl-cytidine, followed by detritylation, allegedly gives cytidylic acid, but this synthesis merely indicates that cytidylic... 

Cytidylic acid, thymidylic acid, and uridylic acid (UMP) are compounds in which the sugar moiety of each of the related nucleosides described above is phosphorylated at the 5 position.The sugar moiety combined with the uracil base of the uridyHc acid used for RNA formation is D-ribose, whereas the sugar combined with the thymine of thymidylic acid used in DNA formation is D-2-deoxyribose. 2 -Deoxycytidyhc acid is used in RNA synthesis, whereas cytidyHc acid is used in the formation of DNA. 

Guanylate kinase has been partly purified from hog brain 10), from Escherichia coli 11), and from a transplantable mouse tumor, Sarcoma 180 12). These preparations show similarities with respect to substrate specificity and cation requirements, but differ with respect to pH optima and molecular weight. All three preparations are highly specific in that guanylate and deoxyguanylate are phosphorylated, but adenylate, ino-sinate, xanthylate, and cytidylate are not substrates. The brain and tumor enzymes also phosphorylate 8-azaguanylate. ATP or dATP are the specific phosphate donors for the three enzymes. 

Sugino el al. 17) have partly purified a deoxycytidylate kinase from calf thymus this preparation also phosphorylates cytidylate and uridylate, but not deoxyuridylate. The deoxyuridylate kinase activity was recovered in another protein fraction and so represents a distinct entity. As phosphate donors, only ATP and dATP were active in the cytidylate reaction. 

Thus, while existence of a cytidylate-deoxycytidylate kinase is established, there remains some uncertainty about the association of uridylate kinase activity with this enzyme. Phosphorylation of deoxyuridylate appears to be catalyzed by a separate enzyme activity, possibly by thymidylate kinase. 

Nelson and Carter 18) have purified thymidylate kinase about 5,000-fold from E. coli B and have shown that thymidylate was phosphorylated about seven times faster than deoxyuridylate. Uridylate was not a substrate, nor were adenylate, guanylate, cytidylate, or their deoxyribosyl counterparts. This enzyme was less specific for the triphosphate phosphoryl donor than the kinases previously discussed, because CTP, GTP, and the corresponding deoxyribonucleotides also served as phosphate donors, although less weU than ATP. 

Fractionation of extracts from calf thymus for kinase activity toward deoxyribonucleoside monophosphates has revealed the presence of at least four separate enzymes the kinase activities for dAMP, dGMP, dCMP, and dTMP are separate entities in this tissue 31). Each has been partly purified and examination of substrate specificities showed that the kinases for deoxyadenylate and deoxyguanylate also phosphorylate the ribosyl homologues, adenylate and guanylate, respectively. The deoxycytidylate kinase accepts as substrates both cytidylate and uridylate, but will not phosphor> late deoxyuridylate. The calf thymus thymidine monophosphate... 

Phosphoryl-choline cytidyl transferase in aleurone 1 y 1-3 glucanase in aleurone cells... 

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