Guanosine-5 -phosphate hydrolysis

Most of the properties of RNase Ti are summarized in Tables II and IV. It is a very acidic protein, active between pH 4 and 8.5 it is most active at pH 7.5 for RNA digestion (12) and at pH 7.2 for the hydrolysis of guanosine 2, 3 -cyclic phosphate (18). The purified enzyme possesses a specific activity of about 1.6 X 10 units/mg of protein. The molecular activity (standard units/jumole enzyme) has not been determined for the cleavage of a definite dinucleoside monophosphate such as GpC or for the hydrolysis of guanosine 2, 3 -cyclic phosphate. 

The optimum pH values of RNase Ta for the hydrolysis of guanosine cyclic phosphate and xanthosine cyclic phosphate are pH 7.2 and 4.5, respectively. The xanthosine cyclic phosphate may have the lactam form (-NH-CO-) susceptible to the enzyme only in the acidic medium (18). 

Figure 9.133 HPLC separation of (/4) guanosine nucleotides and guanosine, injected amount 5 to 10 nmol each in 5 /xL, and (B) neopterin phosphates. The mixture of neopterin phosphates injected was produced by partial hydrolysis of 16 pmol of N IP with alkaline phosphatase and addition of 2, 3 -cNMP. (From Blau and Nieder-wieser, 1983.)...

STEPS s-6 Hydrolysis and dehydrogenation of succinyl CoA. Succinyl CoA is hydrolyzed to succinate in step 5. The reaction is catalyzed by succinyl CoA synthetase and is coupled with phosphorylation of guanosine diphosphate (GDP) to give guanosine triphosphate (GTP). The overall transformation is similar to that of step 8 in glycolysis (Figure 29.4), in which a thiol ester is converted into an acyl phosphate and a phosphate group is then transferred to ADP. 

Even more pronounced general acid catalysis by nucleoside monophosphates in the hydrolysis of diol epoxides 80 and 81 is observed.90 In Fig. 3 are plots of obsd versus total concentrations of guanosine (G), ribose S -phosphate, 5Ccytosine monophosphate (5 -CMP), 5 -adenosine monophosphate (5 -AMP) and 5Cguanosine... 

A uridine phosphate (5), obtained by treatment of uridine 5 -(a-D-glucopyranosyl pyrophosphate) with ammonia, imdergoes hydrazinolysis to D-ribose 5-phosphate (2) and 3-pyrazolone, which establishes the structure of (S) as uridine 5 -phosphate. - (Hydrazinolysis of uridine to pyrazolone, with the liberation of the sugar moiety, had been described, and has served as a useful tool in the determination of the position of attachment of the sugar moiety to the aglycon. ) The 5 -phosphates of adenosine, guanosine, cytidine, and uridine were obtained by enzymic hydrolysis of ribonucleic acid with phosphodiesterases from snake venom and from other sources (such as Streptomyces aureu ). 

Adenylic acids o and b were each converted by trifluoroacetic anhydride into the same 2 3 -cyclic phosphate (13, R = adenin-9-yl) by Brown, Magrath, and Todd. These authors also applied this method to the synthesis of 2 3 -cyclic phosphates (13) of cytidine, uridine, and guanosine. Hydrolysis of these cyclic phosphates gave the corresponding a and b nucleotide mixtures. Markham and Smith found that, during hydrolysis of ribonucleic acid with pancreatic ribonuclease, the 2 3 -cyclic phosphates of the pyrimidine nucleosides are formed as intermediates leading to the ribonucleoside phosphates b. The also showed that 2 3 -cyclic phosphates (13) are formed by very mild, alkaline hydrolysis of ribonucleic acid. The discoveries of these a and b isomers of mononucleotides from... 

In any event, it is clear that further study of the hydrolysis of nucleoside 3 5 -cyclic phosphates is necessary. It also remains to be explained why the 3 5 -cyclic phosphates of adenosine and guanosine are more resistant to glycosyl cleavage in acid than are the corresponding 5 -phosphates or nucleosides. 

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