Phosphorylase polysaccharide degradation

The rate of synthesis, or degradation, of amylose depends on the degree of polymerization of the polysaccharide the action is faster on the species of lower molecular weight. It is thought that inactive enzyme—substrate complexes can form between phosphorylase and internal sections of D-glucosidic chains, and the result of this is an effective diminution in the concentration of enzjune free to attack chain ends. As the inactive complexes are more likely to form with the longer molecules of amylose, the rate of reaction decreases with increasing molecular size of the polysaccharide. 

The maximum velocity of a phosphorylase reaction depends on the temperature and the pH, but is only obtained when the enzyme is saturated with both substrates that is, polysaccharide and inorganic phosphate for degradation, or primer and D-glucosyl phosphate for synthesis. Activities at 30° vary from 2.8 to 14 X 10 moles of D-glucosyl phosphate used up per min. per 10 g, of protein. ... 

Because, under any set of conditions, phosphorylase catalyzes synthesis and degradation of polysaccharides, equilibrium is eventually reached. The value of the ratio [inorganic phosphate]/[D-glucosyl phosphate] at equilibrium is remarkably constant over a wide range of concentration of polysaccharide, but varies with pH, because both substances can ionize it varies from 5.7 at pH 6 to 2.7 at pH 7.6. 

The continuing interest of Bourne in the chemistry of polysaccharides and associated enzymes originated from the work of Haworth and Peat directed towards the enzymic synthesis and degradation of starch. The impetus for this work was given by the discovery, made by C. S. Hanes in 1940, that a phosphorylase isolated from the potato and pea effects the synthesis, from D-glucosyl phosphate, of starch, later shown (by Haworth, Heath, and Peat) to be amylose. In his first paper (with Haworth and Peat) in 1944, Bourne described the isolation of the Q-enzyme which, in conjunction with phosphorylase, effects the conversion of D-glucosyl phosphate into the major component of whole starch, namely, amylopectin. He had discovered the Q-enzyme in a fraction discarded by previous workers. Already, the quintessence of his mind was revealed in this work meticulous attention to detail, and perception of essentials. 

The linkages of amylopectin and glycogen provide the cell with a glucose polysaccharide that upon action by degradative enzymes, such as amylases, isoamylases, debranching enzymes, or phosphorylases are easily converted into glucose and or a-Glc-1-P. 

Oligo- and polysaccharides are degraded in organisms by specific enzymatic hydrolysis (by hydrola ) or phosphorolysis (by phosphorylases). 

Starch phosphorylase incorporates phosphate, rather than water, across the a-1,4 linkage between the penultimate and last glucose at the non-reducing end of the polysaccharide chain. One molecule of glucose-1-phosphate is released. This enzyme attacks amylose repetitively and amylopectin can be degraded to within two or three glucose residues of a a-1,6 branch linkage. 

The most important enzyme, which is eifective in the synthesis as well as in the degradation of the starchlike polysaccharides, is phosphorylase. This enzyme is known to be widespread in nature. It is present in animal tissues, such as muscle, liver, and brain in higher plants potato, waxy maize, barley, and beans in yeast and in bacteria, namely. 

The conversion of an unbranched linear 1,4-a-linked polysaccharide (amylose) can be carried practically to completion if the polysaccharide is treated with phosphorylase in the presence of a sufficiently large excess of inorganic phosphate to insure that the equilibrium ratio of the bivalent ions is not attained before all the polysaccharide is degraded. - On the... 

Arsenate can replace phosphate in the degradation of these polysaccharides by phosphorylase. However, the glucose-l-arsenate that is formed is unstable and is immediately hydrolyzed to glucose and arsenate. This accounts for the fact that no arsenate-glucose-l-arsenate equilibrium can be established when arsenate is used. The arsenolysis reaction results in the complete degradation of linear polysaccharide containing only... 

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