Inosine phosphorolysis

Fig. 16 A schematic mechanism of PNP-catalyzed inosine phosphorolysis showing electrophile migration. This mechanism is based on cocrystal structures of PNP and TS analyses of inosine hydrolysis and arsenolysis. A large number of direct and water-mediated enzyme-substrate contacts hold the leaving group hypoxanthine and the nucleophile phosphate almost immobile while the ribosyl ring (the electrophile), because of the small number of contacts with the enzyme, is able to migrate from leaving group to nucleophile.

Arsenate similarly replaces phosphate in various phosphorolysis reactions, so that sucrose phosphorylase catalyzes the hydrolysis of sucrose in its presence (23), potato phosphorylase can hydrolyze amylose and amylopectin (24), nucleoside phosphorylase can hydrolyze inosine... 

Kalckar117118119 has shown that the enzymatic phosphorolysis of inosine (hypoxanthine 9-D-ribofuranoside) may give rise to the formation of a pentose phosphate, isolable as its barium salt. The phosphate was found to be non-reducing although easily hydrolyzed by either acid or alkali to equimolar quantities of phosphate and pentose. In view of these properties and the fact that it could be used for the enzymatic synthesis of purine ribosides, Kalckar has tentatively assigned to it the D-ribose 1-phosphate structure its ring structure and configuration at carbon 1 remain undetermined. 

A close look at this reaction reveals that in the opposite direction, the reaction is of the phosphorolysis type. For this reason, the enzymes catalyzing the reaction with ribose-l-phosphate are called phosphorylases, and they also participate in nucleic acid degradation pathways. Purine nucleoside phosphorylases thus convert hypoxanthine and guanine to either inosine and guanosine if ribose-l-phosphate is the substrate or to deoxyinosine and deoxyguanosine if deoxyribose-1-phosphate is the substrate. Uridine phosphorylase converts uracil to uridine in the presence of ribose-l-phosphate, and thymidine is formed from thymine and deoxyribose-l-phosphate through the action of thymidine phosphorylase. 

Purine nucleosides are cleaved by the action of purine nucleoside phosphorylase with the liberation of ribose 1-phosphate (Kl, PI). The enzyme is apparently specific for purines. The material from erythrocytes catalyzes the phosphorolysis of purine but not pyrimidine nucleosides (T6.) Purine phosphorylase activity is found widespread in nature and in many animal tissues (FIO). Friedkin and Kalckar investigated an enzyme capable of cleaving purine deoxynucleosides to the aglycone and deoxy-ribose 1-phosphate. They concluded that the enzyme was identical to that which splits purine ribonucleosides (F8, F9). This enzyme is capable of degrading inosine, xanthosine, and guanosine to forms readily attacked by other enzymes. In so doing, it permits living cells to retain the ribose and deoxyribose moieties. 

Bovine PNP is involved in the mammalian purine salvage pathway, catalyzing phosphorolysis of inosine, guanosine or their 2 -deoxy analogues (equation 9). It is a trimeric enzyme with three identical subunits. It displays third-of-sites activity, with only one subunit active at any time. KIEs were measured for phosphorolysis of inosine by bovine spleen PNP. The 1 - H KIE = 1.047 0.002 for the overall reaction was equal to the previously measured binding KIE = 1.047 0.02 for formation of the Michaelis complex, PNP pho-sphatednosine. This, along with primary KIE = 1.000 0.001 for the... 

T.b. gambiense bloodstream forms have APRTase, HGPRTase, adenosine kinase and adenylosuccinate synthetase but lack adenosine deaminase. Two phosphorylase activities have been described for the bloodstream forms of T.b. brucei (42,50). One catalyzes the reversible phosphorolysis of adenosine, inosine and guanosine and the other is specific for adenosine and methylthioadenosine. Guanine deaminase is present whereas both adenosine and adenine deaminase are absent (8). Similar results have been reported for T. congolense (51). T. vivax is unique among the other trypanosomes in that it has an adenine deaminase (51). 

Another compound which undergoes phosphorolysis is nicotinamide riboside, a component part of the pyridino-coenzymes. It is apparently split by the same enzyme which acts on inosine. Thus, the ratio of enzymatic activities against inosine and nicotinamide riboside has been found to remain constant during the course of purification of hog liver fractions, and inosine inhibits the splitting of nicotinamide riboside competitively. The phosphorolysis can be represented as follows ... 

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