Guanosine, 3 ,5 -cyclic phosphate, hydrolysis

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). 

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. 

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. 

This compound has been isolated after subjecting yeast ribonucleic acid to brief acid hydrolysis. When it is treated with alkali there is cleavage at (1), with the formation and subsequent hydrolysis of a cyclic anhydride of guanylic acid. The final products are guanosine-2 -phosphate, guanosine-3 -phosphate, and cytidine-3 -phosphate. The last two would be split by 3-nucleotidase from barley. 

RNase Tl cleaves P-05 ester bonds in ssRNA, specifically at the 3 -P of the guanylic acid residues. As in RNase A (see Fig. 3.3), the catalysis occurs by a two-step mechanism, i.e., the formation of a terminal guanosine 2, 3 -cyclic phosphate intermediate (transesterification step) and the hydrolysis of the cyclic ester to guanosine 3 -monophosphate (hydrolysis step). The transesterification step involves a general acid—base catalysis. 

Cells also contain nucleotides with phosphate groups in positions other than on the 5 carbon (Fig. 8 6). Ribonucleoside 2, 3 -cyclic monophosphates are isolatable intermediates, and rihonucleoside 3 -monophosphates are end products of the hydrolysis of RNA by certain ribonucleases. Other variations are adenosine 3, 5 -cyclic monophosphate (cAMP) and guanosine 3, 5 cyclic monophosphate (cGMP), considered at the end of this chapter. 

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