Phosphorylase starch degradation

Since then, phosphorylase activity has been found to be ubiquitous in plant extracts, but its role in vivo is now believed to be starch degradation rather than synthesis. 

Many starch-degrading enzymes have been investigated electro-phoretically in attempts to confirm their homogeneity, for example, alpha-amylase, heta-amylase, phosphorylase, and pullulanase. ... 

Another exo-acting starch-degrading enzyme is phosphorylase. It is produced by many plants and may be a primary enzyme involved in starch degradation in plants, although many plants... 

Similarly, the choice of enzymes in pathway design is of importance. In the case of the conversion of cellulose to starch, cellobiose phosphorylase and a-glucan phosphorylase are responsible for reversibly eonverting from eello-biose to amylose or vice versa. It was found that a-gluean phosphorylase from potato is a key enzyme to drive the reaction toward starch synthesis. In contrast, the same enzyme from Clostridium thermocellum eannot generate amylase from cellobiose because it prefers the starch degradation direetion. ... 

The second enzymic mechanism of starch degradation involves the enzyme phosphorylase which in the presence of inorganic phosphate... 

Because a degraded starch chain is the product of the degradation reaction, the synthetic phosphorylase catalyzed reaction would require a starch chain or a maltodextrin chain to be a substrate in the synthetic reaction, as shown above. This was the origin for the primer required reaction for starch bios)uithesis from the non-reducing-end of the primer, and it has pretty much been retained for 60 years [119,120,121]. 

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. 

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