Enzyme - 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


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.


    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

Protein name Organism
Alpha-1,4 glucan phosphorylase L-2 isozyme, chloroplastic/amyloplastic Potato
Alpha-glucan phosphorylase 2, cytosolic Mouse-ear cress
Glycogen phosphorylase Fruit fly
Glycogen phosphorylase, brain form Human
Alpha-glucan phosphorylase 1 Mouse-ear cress