Guanylic acid hydrolysis

On neutral or slightly alkaline hydrolysis, - or by treatment with bone phosphatase, guanylic acid is dephosphorylated to guanosine. Hence Levene perceived that guanylic acid is a phospho-guanosine, but many years elapsed before he was able to show the position of attachment of the phosphoryl group. 

Bolomey and Allen found that a non-specific phosphatase preparation (Bredereck ) containing a small amount of ribonuclease hydrolyzes ribosenucleic acid in such a manner that guanosine is liberated faster than adenosine, in the early stages of the hydrolysis the equivalent amount of free phosphoric acid is simultaneously formed. After hydrolysis of the purine nucleotide constituents has reached a maximum, hydrolysis of the pyrimidine nucleotides becomes appreciable. (If the ribosenucleic acid is subjected to the action of ribonuclease before treatment with the phosphatase, the reaction is much more rapid.) They therefore tentatively suggested that guanylic acid is the first mononucleotide liberated and adenylic acid the second. Hence, provided that... 

It would be of interest to halt the hydrolysis when liberation of guanosine and adenosine approaches a maximum, and determine whether the pyrimidine nucleotides are present as a dinucleotide or as the two mononucleotides. It is not clear whether the action of the non-specific phosphatase on an artificially-prepared, equimolecular mixture of the four mononucleotides has been studied (although the individual mononucleotides have been so examined by Bredereck, Beuchelt and Richter ), but Kobayashi has found that guanylic acid is hydrolyzed more readily than adenylic acid which, in turn, is hydrolyzed more readily than the pyrimidine nucleotides. Furthermore, Bredereck, et oZ. have shown that mild chemical hydrolysis of ribosenucleic acid with aqueous pyridine at 100° gives guanylic acid (G) plus a trinucleotide composed of adenylic (A), cytidylic (C), and uridylic (U) acids. On further hydrolysis in aqueous pyridine, adenylic acid is split off. Hence, in ribosenucleic acid, (G) is at one end of the molecule and, in the trinucleotide, (A) is at one end of the molecule. Possible formulas for the tetranucleotide are therefore... 

It is now known that mild, alkaline hydrolysis of ribonucleic acids yields mixtures of the 2 - and 3 -phosphates of ribonucleosides, although it had earlier been believed that only the 3 -phosphate derivatives are produced. For example, it was reported that guanylic acid (6), obtained from ribonucleic acid hydrolysates, is deaminated with nitrous acid to xanthylic acid (7), which was hydrolyzed at pH 1.9 to a ribose phosphate... 

Purine nucleotides are modified by a variety of enzymes, including those that attack the phosphate group and those that split the glycosidic bond. In addition to these, there are changes in the bases in which the amino groups of adenylic acid and guanylic acid are removed by specific deaminases. With these cases excepted (i.e., the formation of inosinic and xanthylic acids), degradation of purines occurs after hydrolysis to yield the free base. 

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. 

Deoxy-D-ribose 5-phosphate has been obtained by the action of a nucleoside phosphorylase and a mutase on hypoxanthine , by enzymic condensation of triose phosphate and acetaldehyde and by enzymic phosphorylation of D-2-deoxyribose . Acid hydrolysis of deoxy-adenylic or deoxy-guanylic acid yields deoxyribose 5-phosphate . A chemical s)mthesis is available . 

This substance forms salts with acids, and was first isolated in the form of its nitrate. The nitrate is not detonated by shock but undergoes a rapid decomposition with the production of light when it is heated. The picrate and the perchlorate explode violently from heat and from shock. Guanyl azide is not decomposed by boiling water. On hydrolysis with strong alkali, it yields the alkali metal salt of hydrazoic acid. It is hydrolyzed by am-moniacal silver nitrate in the cold with the formation of silver azide which remains in solution and of silver cyanamide which appears as a yellow precipitate. By treatment with acids or weak bases it is converted into 5-aminotetrazole. 

In summary, GTP cyclohydrolase II appears to catalyze an ordered reaction that starts with the formation of a covalent guanyl adduct (15). This is followed by the hydrolytic opening of the imidazole ring (16, 17) and the hydrolysis of the resulting formamide-type intermediate (18, 19) the latter two reactions depend on the Zn ion acting as a Lewis acid, which sequentially activates two water molecules (17, 19) that attack first the imidazole carbon atom 8 of the covalent guanyl adduct 17 and then the formamide motif of the covalent intermediate 19. 

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