Glycogen Phosphorylase: Difference between revisions
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[[Image:Enzyme cascade.jpg|thumb|alt=enzyme cascade|Figure 1: Diagram illustrating the enzyme cascade that occurs when hormones(and neural signals)catalyze the activation of phosphorylase through kinase activity and thus inactivating glycogen synthetase. Phosphatases inactivate phosphorylase and is regulated by phosphorylase itself as well as inhibitor proteins. High glucose concentratios also lead to the inactivation of phosphorylase (inactivation represented by open arrows). Diagram adapted from <ref name="structureandfunction"/>.]]In muscle, glycogen phosphorylase is activated by hormones and neural signals such as epinephrine, that stimulate phosphorylase kinase which phosphorylates the Ser-14 residue of the protein. A second messenger of cyclic AMP (cAMP) increases in concentration due to epinephrine or glucagon, and this increase results in an enzyme cascade <ref name="structureandfunction"/>. Activation of phosphorylase kinase is due to increased concentrations of Ca<sup>2+</sup> or by the phosphorylation by protein kinase A which is cAMP dependent. The activated kinase in turn activates the glycogen phosphorylase enzyme by phosphorylating the Ser-14 residue. In the liver, glucagon is the primary signal which catalyzes this enzyme cascade<ref name="structureandfunction"/><ref name="allosteric"/>. | [[Image:Enzyme cascade.jpg|thumb|alt=enzyme cascade|Figure 1: Diagram illustrating the enzyme cascade that occurs when hormones(and neural signals)catalyze the activation of phosphorylase through kinase activity and thus inactivating glycogen synthetase. Phosphatases inactivate phosphorylase and is regulated by phosphorylase itself as well as inhibitor proteins. High glucose concentratios also lead to the inactivation of phosphorylase (inactivation represented by open arrows). Diagram adapted from <ref name="structureandfunction"/>.]]In muscle, glycogen phosphorylase is activated by hormones and neural signals such as epinephrine, that stimulate phosphorylase kinase which phosphorylates the Ser-14 residue of the protein. A second messenger of cyclic AMP (cAMP) increases in concentration due to epinephrine or glucagon, and this increase results in an enzyme cascade <ref name="structureandfunction"/>. Activation of phosphorylase kinase is due to increased concentrations of Ca<sup>2+</sup> or by the phosphorylation by protein kinase A which is cAMP dependent. The activated kinase in turn activates the glycogen phosphorylase enzyme by phosphorylating the Ser-14 residue. In the liver, glucagon is the primary signal which catalyzes this enzyme cascade<ref name="structureandfunction"/><ref name="allosteric"/>. | ||
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<ref name="allosteric"/>. 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<ref name="structureandfunction"/> and create the perfect environment to phosphohydrolyze the glycosidic bond<ref name="allosteric"/>. 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 glucose-1-phosphate.<ref name="PLP"/>. | 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<ref name="allosteric"/>. 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<ref name="structureandfunction"/> and create the perfect environment to phosphohydrolyze the glycosidic bond<ref name="allosteric"/>. 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 glucose-1-phosphate.<ref name="PLP"/>. | ||
==Reaction== | |||
(written by Jaime Prilusky, Max Lein, Eran Hodis) | |||
Glycogen phosphorylase (GP) catalyzes the degradation of the reducing end of glycogen into glucose-1-phosphate. It employs a cofactor called pyridoxal-5’ –phosphate, that is located in the active site and bound to a K681 residue with a Schiff base linkage. PLP shuttles the phosphate group onto the substrate. | |||
==History== | |||
(written by Jaime Prilusky, Max Lein, Eran Hodis) | |||
This protein comes from the muscle tissue of [http://en.wikipedia.org/wiki/Oryctolagus_cuniculus 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. | |||
Glycogen phosphorylase was the first phosphorylase enzyme to be discovered, and the first example of regulation via covalent modification. | |||
In the 1930s, 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 1947 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. | |||
==Activity and Regulation of GP== | |||
(written by Jaime Prilusky, Max Lein, Eran Hodis) | |||
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. | |||
2. AMP activation / Glc-6-P inhibition site. | |||
3. Catalytic site that binds glycogen, Glc-1-P | |||
4. Inhibitor site, 12Å from catalytic site, binds caffeine and related compounds. | |||
5. 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. This represents the final step in a signal transduction cascade in response to the hormone epinephrine, associated with the 'fight-or-flight' response and causing an increase in available energy to the organism as a whole. The N-terminus contains a high percentage of basic residues, which interact favorably with a pocket of acidic residues (Asp109, Glu110, Glu120, Glu501, Glu505 and Glu509) in the 'B' form. Once Ser14 is phosphorylated, the N-terminus is forced ~50Å away from the acidic residues, settling into a region with R69 and R45' (prime denotes a residue from the adjacent subunit). In summary, the conformatino change causes an ordering of the N-terminal chain and a disordering of residues at the C-terminal. Once disordered, the C-terminal residues are no longer able to block substrate entry into the active site. The enzyme phosphatase is able to remove the phosphate and return GP to form 'B'. | |||
In addition, the 'A' and 'B' forms can be regulated futher by small molecules in the cell. This allows individual cells to ignore the hormonal signal if they already have enough available energy (at high concentrations of glucose derivatives or ATP, designated as the 'T' state for low substrate affinity), or activate GP without a hormonal signal when energy for the individual cell is needed (high concentrations of AMP, designated as the 'R' state for high substrate affinity). | |||
==References to the last 3 sections== | |||
<ref group='xtra'>PMID:1900534</ref><ref group='xtra'>PMID:10873856</ref><ref group='xtra'>PMID:7849589</ref><references group='xtra'/> | |||
==Links== | |||
1GPA is a [[Single protein]] structure of sequence from [http://en.wikipedia.org/wiki/Oryctolagus_cuniculus Oryctolagus cuniculus]. Additional information on 1GPA is available in a page on [http://pdb.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb24_1.html Glycogen Phosphorylase] at the RCSB PDB [http://pdb.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/index.html Molecule of the Month]. Full crystallographic information is available from [http://oca.weizmann.ac.il/oca-bin/ocashort?id=1GPA OCA]. | |||
==3D structures of glycogen phosphorylase== | ==3D structures of glycogen phosphorylase== | ||