Phosphorolysis, enzymic

The first group is comprised that of compounds which potentially could be degraded to form G-l-P by phosphorolysis by beta-linkage specific enzymes. They include IPTGlu, cellobiose, sophorose, salicin, and sucrose. Addition of these compounds, or exogenous G-l-P, to Solka Floe fermentations improved maximum cellulase yields from 171 to 309%, and the time period for enzyme synthesis was reduced from 95 to 59% compared with using Solka Floe only. 

Fig. 5 Catalytic mechanism of glycogen phosphorylases. The reaction scheme accounts for the reversibility of phosphorolysis of oligosaccharides (R) in the presence of orthophosphate (upper half) and primer-dependent synthesis in the presence of glucose-l-phosphate (lower half). PL enzyme-bound pyridoxal BH-f a general base contributed by the enzyme protein. Reprinted with permission from [109]. Copyright 1990 American Chemical Society...

Phosphorolysis of the glycosyl esters of sugar nucleotides leads to the corresponding nucleoside 5 -pyrophosphate and glycosyl phosphate. A wheat-germ enzyme specific for adenosine 5 -(a-D-glucopy-ranosyl pyrophosphate),462 and a yeast enzyme specific for guanosine 5 -(a-D-mannopyranosyl pyrophosphate),463,464 are known. 

Some phosphoric acid derivatives of 2-desoxy-D-ribose have been obtained by enzymic methods of preparation. A reaction analogous to the phosphorolysis of glycogen to D-glucose 1-phosphate241 has been effected with either hypoxanthine- or guanine-D-riboside, both of which could be split by enzymic phosphorolysis with the formation of D-ribose 1-phosphate.242 The successful conclusion of these experiments prompted similar investigations with desoxyribonucleosides. 

Manson and Lampen243 reported that they obtained the phosphorolysis and arsenolysis of hypoxanthine desoxyriboside by enzyme preparations from calf-thymus gland and rat liver. An acid-stable phosphate ester was isolated as a product of phosphorolysis. Results to be outlined suggested that this ester was 2-desoxy-D-ribose 5-phosphate and evidence was obtained for its formation by a mutase type reaction from 2-desoxy-D-ribose 1-phosphate. This evidence was extended and reinforced when Manson and Lampen244 obtained indications for the formation of desoxy-D-ribose 1-phosphate during the phosphorolysis of thymidine. Consequently the conversions outlined may be depicted as shown. 

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. 

One possible way to solve this problem is to combine another enzyme, like SP, which produces a-GlP for GP. Waldmann and colleagues reported the combined use of SP and GP for the production of amylose from sucrose (1986). In this system, SP catalyzes the phosphorolysis of sucrose to produce a-GlP and fructose, and the a-GlP is next used as a substrate of GP to produce amylose. An interesting feature of this SP-GP system is that Pi produced in the second GP reaction is used as a substrate for the first SP reaction. The cooperative action by the two phosphorylases proceeds continuously with a constant Pi concentration, without any inhibition caused by an accumulation of Pi. Based on this SP-GP system, we have now established the process to manufacture essentially linear amylose with controlled molecular size, by using thermostable variants of SP and GP (Yanase et al., 2007 Ohdan et al., 2007). 

Dissociation of NADH prior to phosphorolysis is a feature common to many mechanisms for GAPDH (cf. 13) since this is necessary to explain the NAD+ requirement for phosphorolysis of the acyl enzyme (196). Indeed, apart from the earlier steady-state kinetic studies, the only report at variance with this concept is that of Smith (197), who examined the ternary acyl enzyme-NADH complex fluorimetrically and was unable to displace the NADH even at high concentrations of NAD. It has, however, been suggested (117) that phosphate is required for this displacement to occur. 

Glycogen phosphorylase catalyzes the sequential phosphorolysis of glycogen to release glucose 1-phosphate it is thus the key enzyme in the utilization of tissue glycogen reserves. 

Studies have been made of the variation of K and V with pH for the synthetic reaction, and for the phosphorolysis and arsenolysis of amylopectin and glycogen, respectively. Values of pK of groups ionizing in the enzyme and enzyme—substrate complex are shown in Table XIX. 

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