Enzyme
2.4.1.1 - Glycogen phosphorylase
Alternative Name(s)
- Polyphosphorylase.
- Amylophosphorylase.
- Muscle phosphorylase a and b.
Catalytic Activity
[(1->4)-alpha-D-glucosyl](n) + phosphate = [(1->4)-alpha-D-glucosyl](n-1) + alpha-D-glucose 1-phosphate
Cofactors
There are no Cofactors for this Enzyme
Reaction Mechanism
Phophorylases are important allosteric enzymes in the carbohydrate metabolism. Enzymes from different sources differ in their regulatory mechanisms and in their natural substrates. However all known phosphorylases share catalytic and structural properties. For example glycogen phosphorylase catalyses the intracellular degradation of glycogen into glucose-1-phosphate, the first step of glycolysis. It was the first enzyme to be discovered that was controlled by phosphorylation. Other phosphorylases include maltodextrin phosphorylase and starch phosphorylase.
The coenzyme pyridoxal phosphate (PLP) is covalently bound to Lys568 as a Schiff Base. Glycogen substrate binding (shown here as maltose) is followed by a conformational change which brings the substrate close to the coenzyme pyridoxal phosphate (PLP) resulting in the distortion and enhanced electrophilicity of the latter thus labelling the glucosyl phosphate linkage. Bond cleavage then ensues with the coenzyme PLP and its neighbouring basic groups, Gly674, Thr675 and Lys573, essentially sequestering the released inorganic phosphate. The glucosyl carbanion generated may be stabilised as an intermediate by a negatively charged enzymic group, attacked directly in a concerted reaction by the oligosaccharide or trapped by an enzymic group as a glucosyl enzyme intermediate. The latter is the most likely option, the glyucosyl bound via a carboxylate group, Glu671. The glucosyl unit now reacts with the nonreducing end of the oligosaccharide substrate and the reaction proceeds with retention of configuration at the glucose moiety. As a result from this Sn1 reaction, the products glucose-1-phosphate and a n-1 glycogen chain are formed.
Catalytic Residues
| AA | Uniprot | Uniprot Resid | PDB | PDB Resid |
|---|---|---|---|---|
| His | P00489 | 378 | 1gpb | 377 |
| Lys | P00489 | 681 | 1gpb | 680 |
| His | P00489 | 378 | 1gpb | 377 |
| Lys | P00489 | 569 | 1gpb | 568 |
| Arg | P00489 | 570 | 1gpb | 569 |
| Lys | P00489 | 575 | 1gpb | 574 |
| Thr | P00489 | 677 | 1gpb | 676 |
Step Components
cofactor used, proton relay, intermediate formation, native state of cofactor regenerated, intermediate terminated, bimolecular nucleophilic addition, proton transfer, native state of enzyme regenerated, overall reactant used, unimolecular elimination by the conjugate base, overall product formed
Step 1.
The ring oxygen of the substrate initiates an elimination of the alcohol product, which deprotonates a non-covalently bound phosphate, which in turn deprotonates the phosphate of the covalently attached PLP cofactor.
Step 2.
The phosphate of the covalently attached PLP cofactor deprotonates the non-covalently bound phosphate, which in turn attacks the intermediate in a nucleophilic addition that produces the second product.
Products.
The products of the reaction.
Reaction Parameters
-
Kinetic Parameters
Organism KM Value [mM] Substrate Comment Acetivibrio thermocellus 1.1 phosphate pH 7.2, 50°C, degradation reaction -
Temperature
Organism Temperature Range Comment Carassius carassius 5 - 35 in the tissues, except the gill, the basal and total activities of glycogen phosphorylase increase from 5 to 25°C, but less steeply between 5 and 15°C than between 15 and 25°C, inactivation starts above 25°C Dictyostelium discoideum 18 - 35 approx. half-maximal activity at 18°C and 35°C Acetivibrio thermocellus 20 - 90 activity range Cenchrus americanus 34 - 40 50% of maximal activity at 34°C and 40°C, soluble enzyme Thermus thermophilus 50 - 90 50°C: about 50% of maximal activity, 90°C: about 40% of maximal activity -
pH
Organism pH Range Comment Solanum tuberosum 4 - 8 approx. half-maximal activity at pH 4 and pH 8, slow isozyme Sulfurisphaera tokodaii str. 7 4 - 8 pH 4.0: about 60% of maximal activity, pH 8.0: about 50% of maximal activity Acetivibrio thermocellus 4 - 10 activity range Corynebacterium callunae 5 - 8 Klebsiella pneumoniae 5.1 - 7.5 approx. half-maximal activity at pH 5.1 and 7.5
Associated Proteins
Citations
- Glycogen phosphorylase inhibition improves cognitive function of aged mice.
- Identification of glycogen phosphorylase L as a potential target for lung cancer.
- Allosteric activation and inhibition of glycogen phosphorylase share common transient structural features
- Allosteric Regulation of Glycogen Phosphorylase by Order/Disorder Transition of the 250' and 280s Loops.
- Computational insights into novel inhibitor indole-heterocycle specific against glycogen phosphorylase isoenzymes interaction mechanism.
- Protective effects of a novel glycogen phosphorylase inhibitor against cerebral ischemia-reperfusion injury in mice.
- The role of glycogen phosphorylase in glycogen biogenesis in skeletal muscle after exercise.
- Glycogen phosphorylase inhibition alongside taxol chemotherapy synergistically elicits ferroptotic cell death in clear cell ovarian and kidney cancers
- Multidisciplinary docking, kinetics and X-ray crystallography studies of baicalein acting as a glycogen phosphorylase inhibitor and determination of its' potential against glioblastoma in cellular models.
- Phosphorylase phosphatase and flash activation of skeletal muscle glycogen phosphorylase-A tribute to Edmond H. Fischer.
- Glycogen phosphorylase isoenzyme GPbb versus GPmm regulation of ventromedial hypothalamic nucleus glucoregulatory neurotransmitter and counter-regulatory hormone profiles during hypoglycemia: Role of L-lactate and octadecaneuropeptide.