The second domain has the polysaccharide binding domain where phosphorylase is able to attach to the glycogen substrate [5]. The R states of GP a and GP b are almost identical; the difference lays in the modification of the Ser residue where GP a has a covalently linked phosphate group whereas GP b has a non-covalently linked sulfide group.
GP a is activated by phosphorylation of the serine residue whereas GP b can be activated by the binding of AMP to the that are present within the molecule [4]. Glycogen phosphorylase is different from other enzymes that require the cofactor PLP because instead of utilizing the pyrimidine ring, phosphorylase uses the phosphate group [6].
The binding sites in glycogen phosphorylase include: a catalytic, inhibiting, AMP, glycogen and new allosteric site [1]. The residues that make up the site are [6].
The heterocyclic rings of the compounds bind to the inhibiting site, stablilizing it and blocking access to the catalytic center [6]. The can be accessed once the Ser residue has been phosphorylated and conformational changes in glycogen phosphorylation have been observed [5] [6]. The structure and function of glycogen phosphorylase is complex, though the function of the enzyme is due to the structure.
Glycogen phosphorylase is regulated by phosphorylation, binding of allosteric effectors and by the catalytic mechanism; phosphorylation takes glycogen phosphorylase from a disordered state to an ordered one, allosteric effector provide changes in the structure of the enzyme and when coupled with phosphorylation allow access to the buried catalytic site [6].
The catalytic mechanism itself is dependent upon the proximity of PLP and the substrate phosphate which is directed by the surrounding groups which stabilize the interactions [5] and create the perfect environment to phosphohydrolyze the glycosidic bond [6].
The environment is established by the phosphate compound making a hydrogen bond with the 5'-phosphate of PLP and being stable enough to successfully cleave the bond yielding the product of glucosephosphate. Glycogen phosphorylase GP catalyzes the degradation of the reducing end of glycogen into glucosephosphate. This protein comes from the muscle tissue of Oryctolagus cuniculus. There is an isozyme from liver tissue that is regulated by glucagon instead of epinephrine, with a different gene that encodes it and different regulation properties written by Jaime Prilusky, Max Lein, Eran Hodis.
Glycogen phosphorylase was the first phosphorylase enzyme to be discovered, and the first example of regulation via covalent modification. In the s, the first work done by Carl and Gerty Cori. They proved that the enzyme exists in 'A' and 'B' forms, and they showed that the reverse reaction produced glycogen.
They won the Nobel Prize in along with Bernardo Housay of Argentina for their work on carbohydrate metabolism. This was also the first example of a polymerizing enzyme, inspiring others to look for other polymerizing enzymes.
Subsequently, Earl Sutherland found that the 'B' form predominates in resting muscle and epinephrine triggers activation to form 'A'. Since then, many groups have worked on this enzyme, both to understand its mechanism and to discover drug targets. Crystal structures have been obtained for the protein in the 'A' and 'B' form, in the presence of natural substrates, inhibitors, and transition state analogs.
Please see the end of this article for links to crystallographic information. In its active form, GP is a dimer of two identical subunits. The subunits make interactions that stabilize the final structure.
Each Sub-unit contains 5 potential effector sites: 1. Ser14 phosphate-recognition site. Catalytic site that binds glycogen, GlcP 4. Glycogen storage site. There are two forms of the enzyme, designated as 'A' and 'B', that are controlled hormonally. The 'B' form is converted into the 'A' form by phosphorylase kinase, which catalyzes the addition of phosphate from ATP to Ser14 near the N-terminus.
The phosphatase and kinase that catalyze the interconversion of phosphorylase a and b are themselves regulated, and this is also shown in the figure. The mechanism of phosphorylase prominently features general acid - base catalysis.
The active site contains a pyridoxal phosphate PLP cofactor, which acts in the mechanism as a general acid and a general base. This is an atypical role for PLP, which - derived from vitamin B6 - is most notable for its catalysis in transformations of amines and amino acids.
In the s, Carl and Gerty Cori discovered phosphorylase, showing it to be the enzyme that degrades glycogen to glucose 1-phosphate the latter, which they had isolated from minced frog muscle, was historically called the "Cori ester".
Furthermore, the Coris found that phosphorylase activity could be affected by small molecules AMP, ATP, glucose and that phosphorylase could be purified in two different stable forms. One - called phosphorylase a - was active without addition of AMP. The other, termed phosphorylase b , was inactive unless AMP was present. This was the first description of what we now recognize as allosteric regulation of an enzyme's activity.
The Coris had even identified an enzyme they dubbed "PR" for "prosthetic group-removing" enzyme, which converted phosphorylase a to phosphorylase b. It was not until the s, when research - principally by Edwin Krebs who had trained with the Coris and Edmond Fischer - elucidated the novel mechanism of regulation of glycogen phosphorylase by reversible phosphorylation. They found that a kinase, phosphorylase kinase [ EC 2.
Fischer and Krebs won the Nobel Prize for Physiology or Medicine for their discoveries of "reversible protein phosphorylation as a biological regulatory mechanism". Page updated Regulation of glycogen phosphorylation by phosphorylation and dephosphorylation, as well as allosteric effects.
Reaction mechanism. Historical development of research on glycogen metabolism and its regulation. Mechanism of glycogen phosphorylase The mechanism of phosphorylase prominently features general acid - base catalysis.
Historical development of research on glycogen metabolism In the s, Carl and Gerty Cori discovered phosphorylase, showing it to be the enzyme that degrades glycogen to glucose 1-phosphate the latter, which they had isolated from minced frog muscle, was historically called the "Cori ester".
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