Glycogenolysis and glycogenesis
Glycogenolysis is the biochemical breakdown of glycogen to glucose whereas glycogenesis is the opposite, the formation of glycogen from glucose. Glycogenolysis takes place in the cells of muscle and liver tissues in response to hormonal and neural signals. In particular, glycogenolysis plays an important role in the adrenaline-induced fight-or-flight response and the regulation of glucose levels in the blood. The reverse process, glycogenesis, the formation of glycogen from glucose, occurs in liver and muscle cells when glucose and ATP are present in relatively high amounts. In the synthesis of glycogen, one ATP is required for every glucose unit incorporated into the polymeric branched structure of glycogen. The glucose (in the form of glucose-6-phosphate) is synthesized directly from glucose or as the end product of gluconeogenesis.
Structure of glycogen
Figure 1. Glycogen structure (Click for enlarged view). Panel A. Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain around 30,000 glucose units. [Source: Mikael Häggström, . Panel B. Schematic of glycogen structure showing the glucose units in each chain linked together linearly by α(1→4 glycosidic bonds. Branches are linked to the chains from which they are branching off by α(1→6) glycosidic bonds between the first glucose of the new branch and a glucose on the stem chain.Glycogen is a multi-branched polysaccharide of glucose that serves as an energy store primarily in muscle and liver. It is stored in the form of granules in the cytoplasm of the cell and is the main storage form of glucose in the body. The concentration of glycogen in muscle is low (1-2% fresh weight) compared to the levels stored in the liver (up to 8% fresh weight). Glycogen is an energy reserve that can be quickly mobilized to meet a sudden need for glucose. The significance of the multi-branched structure is that multiple glucose units, rather than a single glucose can be mobilized from any glycogen molecule when glycogenolysis is initiated. The structure of glycogen is summarized in Figure 1.
Enzymes involved in glycogenolysis
The process of glycogenolysis involves the sequential removal of glucose monomers by phosphorolysis, a reaction catalysed by the phosphorylated (active) ‘a’ form of the enzyme glycogen phosphorylase. This enzyme cleaves the glycosidic bond linking a terminal glucose to a glycogen branch by substituting a phosphoryl group for the α[1→4] linkage producing glucose-1-phosphate and glycogen that contains one less glucose molecule.
A second enzyme, phosphoglucomutase, converts the glucose-1-phosphate to glucose-6-phosphate (see Figure 2). Glycogen involves two types of glycosidic linkage: the linear α[1→4] linkage and the branching α[1→6] linkages. During glycogenolysis glucose units are phosphorolysed from branches of glycogen until four residues before a glucose that is branched with an α[1→6] linkage.
A third enzyme, glycogen debranching enzyme transfers three of the remaining four glucose units to the end of another glycogen branch, exposing the α[1→6] branching point. This glycosidic bond is hydrolysed by a fourth enzyme α[1-6] glucosidase which eliminates the branch by removing the final glucose as a molecule of glucose, rather than glucose-1-phosphate. In muscle, but not liver cells, the glucose is subsequently phosphorylated to glucose-6-phosphate by a fifth enzyme hexokinase and enters the glycolytic pathway.
In liver cells the main purpose of the breakdown of glycogen is for the release of glucose into the bloodstream for uptake by other cells. The phosphate group of glucose-6-phosphate is removed by the enzyme glucose-6-phosphatase and the free glucose exits the cell via the membrane localized GLUT2 glucose transporter.
In muscle, glycogenolysis serves to provide an immediate source of glucose-6-phosphate for glycolysis to provide energy for muscle contraction but not for other body tissues. Muscle cells lack the enzyme glucose-6-phosphatase and thus cannot convert glucose-6-phosphate (which cannot be transported across the cell membrane) to glucose.
Regulation of glycogenolysis
The regulation of glycogenolysis involves phosphorylation cascades, as summarized in Figure 2. The conversion of inactive glycogen phosphorylase ‘b’ to active glycogen phosphorylase ‘a’ requires phosphorylation by the protein phosphorylase kinase, a Ser/The protein kinase which functions as an activated dimer. This kinase is itself activated by another Ser/Thr protein kinase - cyclic AMP-dependant protein kinase (also called protein kinase A, PKA), which in turn is activated by cyclic AMP. Cyclic AMP binding to the regulatory subunit of protein kinase A causes the catalytic subunit of PKA to dissociate from the regulatory subunit enabling it to phosphorylate other proteins including glycogen phosphorylase b.
Adrenaline (epinephrine, which is released in response to a threat or stress - the ‘fight-or-flight’ response) and glucagon, (which is released by pancreatic alpha cells in response to low blood glucose levels), stimulate glycogenolysis by binding to their respective receptors (both of which are G-protein coupled receptors) which in turn activate the membrane localized protein adenyl cyclase. Adenyl cyclase converts ATP to cyclic AMP which activates PKA which in turn phosphorylates (and activates) glycogen phosphorylase as described above (Figure 2). Details of the structural changes associated with the activation of glycogen phosphorylase can be found
Figure 2. Enzyme reactions involved in glycogenolysis and glycogenesis and their regulation (Click for enlarged view and see text for details).elswhere.
At the same time as it stimulates glycogen breakdown, the adrenaline/glucagon-induced phosphorylation cascade inhibits glycogen synthesis through PKA’s phosphorylation of the enzyme glycogen synthase, the phosphorylated form of which is less active.
Insulin inhibits glycogenolysis by activating protein phosphatase 1 (PP1) and the enzyme phosphodiesterase. Both contribute to the inactivation of glycogen phosphorylase by reducing its phosphorylated state. Activated PP1 directly dephosphorylates glycogen phosphorylase a, reforming the inactive glycogen phosphorylase b, whereas phosphodiesterase converts cAMP to AMP, thus inactivating PKA and its ability to phosphorylate (activate) glycogen phosphorylase.
Enzymes involved in glycogenesis
A detailed description of the process can be found in any Biochemistry textbook. As summarized in Figure 2, glucose is converted into glucose-6-phosphate by the action of glucokinase (liver) or hexokinase (muscle). Glucose-6-phosphate is then converted into glucose-1-phosphate by the action of the enzyme phosphoglucomutase, passing through an obligatory intermediate step of glucose-1,6-bisphosphate.
Next the glucose-1-phosphate is converted into UDP-glucose by the action of uridyl transferase (also called UDP-glucose pyrophosphorylase). One molecule of UTP is used in this step and one molecule of pyrophosphate is formed, which is hydrolyzed by pyrophosphatase into 2 molecules of inorganic phosphate (Pi). UDP-glucose molecules are incorporated into the growing glycogen chain by the enzyme glycogen synthase, which must act on a pre-existing primer. Initially this is the small protein glycogenin, but once initiated the primer is the growing glycogen chain.
The mechanism for joining glucose units is that glycogen synthase binds to UDP-glucose, causing it to break down into an oxonium ion, which can readily add to the 4-hydroxyl group of a glucosyl residue on the 4 end of the glycogen chain. After every 10 to 14 glucose units a side branch with an additional chain of glucose units occurs. The side chain attaches at carbon atom 6 of a glucose unit, and the linkage is termed an alpha-1,6 glycosidic bond. To form this connection a separate enzyme known as a branching enzyme is used. Branching enzyme (systematic name: 1,4-alpha-D-glucan:1,4-alpha-D-glucan 6-alpha-D-(1,4-alpha-D-glucano)-transferase) attaches a string of seven glucose units.
Regulation of glycogenesis
As summarized in Figure 2, there is a reciprocal relationship between glycogen synthesis (glycogenesis) and glycogen breakdown (glycogenolysis) and factors that enhance one inhibit the other. One of the main forms of control is the varied phosphorylation of glycogen synthase and glycogen phosphorylase by protein kinase A (PKA). Phosphorylated glycogen synthase is inactive in contrast to glycogen phosphorylase which is activated following phosphorylation.
As discussed above, conditions such as low glucose levels or stress that promote the activation of PKA as a result of released adrenaline or glucagon binding to their G-protein coupled receptors, promote the process of energy generation through glycogen breakdown and inhibit the process of glycogen synthesis. Similarly calcium ions inhibit glycogen synthase indirectly through their activation of PKA. Finally glycogenesis is enhanced by elevated levels of ATP which act as an allosteric inhibitor of glycogen phosphorylase.
^ Krebs EG, Protein phosphorylation and cellular regulation, I. Nobel lecture December 1992. In: Nobel Lectures Physiology or Medicine 1991-1995, Editor Nils Ringertz, World Scientific Publishing Co., Singapore, 1997; pp 72-89. http://www.nobelprize.org/nobel_prizes/medicine/laureates/1992/krebs-lecture.html
^ Siu FY et al, 2013, Structure of the human glucagon class B G-protein-coupled receptor, Nature 499: 444-9. www.nature.com/nature/journal/vaop/ncurrent/full/nature12393.html
^ Williamson, J. R., Copper, R. H., and Hoek, J. B. (1981) Biochim. BioPhys. Acta 639. 243-295
^ Newgard CB, Hwang PK, Fletterick, RJ (1989). "The family of glycogen phosphorylases: structure and function". Critical Reviews Biochemistry and Molecular Biology 24 (1): 69–99. doi:10.3109/10409238909082552. PMID 2667896.