How Glycogen Is Produced and Degraded
When we digest a meal high in carbohydrates, we have a supply of glucose that exceeds our immediate needs. We store glucose as a polymer, glycogen, that is similar to the starches found in plants; glycogen differs from starch only in the degree of chain branching. In fact, glycogen is sometimes called “animal starch” because of this similarity. A look at the metabolism of glycogen will give us some insights into how glucose can be stored in this form and made available on demand. In the degradation of glycogen, several glucose residues can be released simultaneously, one from each end of a branch, rather than one at a time as would be the case in a linear polymer.
This feature is useful to an organism in meeting short-term demands for energy by increasing the glucose supply as quickly as possible (Figure 18.1). Mathematical modeling has shown that the structure of glycogen is optimized for its ability to store and deliver energy quickly and for the longest amount of time possible. The key to this optimization is the average chain length of the branches (13 residues). If the average chain length were much greater or much shorter, glycogen would not be as efficient a vehicle for energy storage and release on demand. Experimental results support the conclusions reached from the mathematical modeling.
Glycogen is found primarily in liver and muscle. The release of glycogen stored in the liver is triggered by low levels of glucose in blood. Liver glycogen breaks down to glucose-6-phosphate, which is hydrolyzed to give glucose. The release of glucose from the liver by this breakdown of glycogen replenishes the supply of glucose in the blood. In muscle, glucose-6-phosphate obtained from glycogen breakdown enters the glycolytic pathway directly rather than being hydrolyzed to glucose and then exported to the bloodstream.
Three reactions play roles in the conversion of glycogen to glucose-6-phosphate. In the first reaction, each glucose residue cleaved from glycogen reacts with phosphate to give glucose-1-phosphate. Note particularly that this cleavage reaction is one of phosphorolysis rather than hydrolysis.
In a second reaction, glucose-1-phosphate isomerizes to give glucose-6-phosphate.
Complete breakdown of glycogen also requires a debranching reaction to hydrolyze the glycosidic bonds of the glucose residues at branch points in the glycogen structure. The enzyme that catalyzes the first of these reactions is glycogen phosphorylase; the second reaction is catalyzed by phosphoglucomutase.
Glycogen + Pi OO3 => Glucose-1-phosphate + Remainder of glycogen
Glucose-1-phosphate => Glucose-6-phosphate
Glycogen phosphorylase cleaves the α(1 - > 4) linkages in glycogen. Complete breakdown requires debranching enzymes that degrade the α(1 - > 6) linkages. Note that no ATP is hydrolyzed in the first reaction. In the glycolytic pathway, we saw another example of phosphorylation of a substrate directly by phosphate without involvement of ATP: the phosphorylation of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. This is an alternative mode of entry to the glycolytic pathway that “saves” one molecule of ATP for each molecule of glucose because it bypasses the first step in glycolysis. When glycogen rather than glucose is the starting material for glycolysis, there is a net gain of three ATP molecules for each glucose monomer, rather than two ATP molecules, as when glucose itself is the starting point. Thus, glycogen is a more effective energy source than glucose. Of course, there is no “free lunch” in biochemistry and, as we shall see, it takes energy to put the glucoses together into glycogen.
The debranching of glycogen involves the transfer of a “limit branch” of three glucose residues to the end of another branch, where they are subse-quently removed by glycogen phosphorylase. The same glycogen debranching enzyme then hydrolyzes the α(1 - > 6) glycosidic bond of the last glucose resi-due remaining at the branch point (Figure 18.2).
When an organism needs energy quickly, glycogen breakdown is important. Muscle tissue can mobilize glycogen more easily than fat and can do so anaero-bically. With low-intensity exercise, such as jogging or long-distance running, fat is the preferred fuel, but as the intensity increases, muscle and liver glyco-gen becomes more important. Some athletes, particularly middle-distance run-ners and cyclists, try to build up their glycogen reserves before a race by eating large amounts of carbohydrates.
The formation of glycogen from glucose is not the exact reversal of the breakdown of glycogen to glucose. The synthesis of glycogen requires energy, which is provided by the hydrolysis of a nucleoside triphosphate, UTP. In the first stage of glycogen synthesis, glucose-1-phosphate (obtained from glucose-6-phosphate by an isomerization reaction) reacts with UTP to produce uridine diphosphate glucose (also called UDP-glucose or UDPG) and pyrophosphate (PPi).
The enzyme that catalyzes this reaction is UDP-glucose pyrophosphorylase. The exchange of one phosphoric anhydride bond for another has a free-energy change close to zero. The release of energy comes about when the enzyme inorganic pyrophosphatase catalyzes the hydrolysis of pyrophosphate to two phosphates, a strongly exergonic reaction.
It is common in biochemistry to see the energy released by the hydrolysis of pyrophosphate combined with the free energy of hydrolysis of a nucleoside triphosphate. The coupling of these two exergonic reactions to a reaction that is not energetically favorable allows an otherwise endergonic reaction to take place. The supply of UTP is replenished by an exchange reaction with ATP, which is catalyzed by nucleoside phosphate kinase:
UDP + ATP < - - - > UTP + ADP
This exchange reaction makes the hydrolysis of any nucleoside triphosphate energetically equivalent to the hydrolysis of ATP.
The addition of UDPG to a growing chain of glycogen is the next step in glycogen synthesis. Each step involves formation of a new α(1 - > 4) glycosidic bond in a reaction catalyzed by the enzyme glycogen synthase (Figure 18.3). This enzyme cannot simply form a bond between two isolated glucose mol-ecules; it must add to an existing chain with α(1 - > 4) glycosidic linkages. The initiation of glycogen synthesis requires a primer for this reason. The hydroxyl group of a specific tyrosine of the protein glycogenin (37,300 Da) serves this purpose. In the first stage of glycogen synthesis, a glucose residue is linked to this tyrosine hydroxyl, and glucose residues are successively added to this first one. The glycogenin molecule itself acts as the catalyst for addition of glucoses until there are about eight of them linked together. At that point, glycogen synthase takes over.
Synthesis of glycogen requires the formation of α(1 - > 6) as well as α(1 - > 4) glycosidic linkages. A branching enzyme accomplishes this task. It does so by transferring a segment about 7 residues long from the end of a growing chain to a branch point where it catalyzes the formation of the required α(1 - > 6) glycosidic linkage (Figure 18.4).
Note that this enzyme has already catalyzed the breaking of an α(1 - > 4) glycosidic linkage in the process of transferring the oligosaccharide segment. Each transferred segment must come from a chain at least 11 residues long; each new branch point must be at least 4 residues away from the nearest existing branch point.
How does an organism ensure that glycogen synthesis and glycogen breakdown do not operate simultaneously? If this were to occur, the main result would be the hydrolysis of UTP, which would waste chemical energy stored in the phosphoric anhydride bonds. A major controlling factor lies in the behavior of glycogen phosphorylase. This enzyme is subject not only to allosteric control but also to another control feature: covalent modification. In that example, phosphorylation and dephosphorylation of an enzyme determined whether it was active, and a similar effect takes place here.
Figure 18.5 summarizes some of the salient control features that affect glyco-gen phosphorylase activity. The enzyme is a dimer that exists in two forms, the inactive T (taut) form and the active R (relaxed) form. In the T form (and only in the T form), it can be modified by phosphorylation of a specific serine resi-due on each of the two subunits. The esterification of the serines to phosphoric acid is catalyzed by the enzyme phosphorylase kinase; the dephosphorylation is catalyzed by phosphoprotein phosphatase. The phosphorylated form of glycogen phosphorylase is called phosphorylasea, and the dephosphorylated form is called phosphorylaseb. The switch from phosphorylase b to phosphorylase a is the major form of control over the activity of phosphorylase. The response time of the changes is on the order of seconds to minutes. Phosphorylase is also controlled more quickly in times of urgency by allosteric effectors, with a response time of milliseconds.
In liver, glucose is an allosteric inhibitor of phosphorylase a. It binds to the substrate site and favors the transition to the T state. It also exposes the phosphorylated serines so that the phosphatase can hydrolyze them. This shifts the equilibrium to phosphorylase b. In muscle, the primary allosteric effectors are ATP, AMP, and glucose-6-phosphate (G6P). When the muscles use ATP to contract, AMP levels rise. This increase in AMP stimulates formation of the R state of phosphorylase b, which is active. When ATP is plentiful or glucose-6-phosphate builds up, these molecules act as allosteric inhibitors shifting the equilibrium back to the T form. These differences ensure that glycogen will be degraded when there is a need for energy, as is the case with high [AMP], low [G6P], and low [ATP]. When the reverse is true (low [AMP], high [G6P], and high [ATP]), the need for energy, and consequently for glycogen breakdown, is less. “Shutting down” glycogen phosphorylase activity is the appropriate response. The combination of covalent modification and allosteric control of the process allows for a degree of fine-tuning that would not be possible with either mechanism alone. Hormonal control also enters into the picture. When epinephrine is released from the adrenal gland in response to stress, it triggers a series of events, that suppress the activity of glycogen synthase and stimulate that of glycogen phosphorylase.
The activity of glycogen synthase is subject to the same type of covalent modification as glycogen phosphorylase. The difference is that the response is opposite. The inactive form of glycogen synthase is the phosphorylated form. The active form is unphosphorylated. The hormonal signals (glucagon or epi-nephrine) stimulate the phosphorylation of glycogen synthase via an enzyme called cAMP-dependent protein kinase. After the glycogen syn-thase is phosphorylated, it becomes inactive at the same time the hormonal sig-nal is activating phosphorylase. Glycogen synthase can also be phosphorylated by several other enzymes, including phosphorylase kinase and several enzymes called glycogen synthase kinases. Glycogen synthase is dephosphorylated by the same phosphoprotein phosphatase that removes the phosphate from phos-phorylase. The phosphorylation of glycogen synthase is also more complicated in that there are multiple phosphorylation sites. As many as nine different amino acid residues have been found to be phosphorylated. As the progressive level of phosphorylation increases, the activity of the enzyme decreases.
Glycogen synthase is also under allosteric control. It is inhibited by ATP. This inhibition can be overcome by glucose-6-phosphate, which is an activa-tor. However, the two forms of glycogen synthase respond very differently to glucose-6-phosphate. The phosphorylated (inactive) form is called glycogensynthase D (for “glucose-6-phosphate dependent”) because it is active onlyunder very high concentrations of glucose-6-phosphate. In fact, the level necessary to give significant activity would be beyond the physiological range. The nonphosphorylated form is called glycogen synthase I (for “glucose-6-phosphate independent”) because it is active even with low concentrations of glucose-6-phosphate. Thus, even though purified enzymes can be shown to respond to allosteric effectors, the true control over the activity of glycogen syn-thase is by its phosphorylation state, which, in turn, is controlled by hormonal states.
The fact that two target enzymes, glycogen phosphorylase and glycogen syn-thase, are modified in the same way by the same enzymes links the opposing processes of synthesis and breakdown of glycogen even more intimately.
Finally, the modifying enzymes are themselves subject to covalent modifica-tion and allosteric control. This feature complicates the process considerably but adds the possibility of an amplified response to small changes in conditions. A small change in the concentration of an allosteric effector of a modifying enzyme can cause a large change in the concentration of an active, modified target enzyme; this amplification response is due to the fact that the substrate for the modifying enzyme is itself an enzyme.
At this point, the situation has become very complex indeed, but it is a good example of how opposing pro-cesses of breakdown and synthesis can be controlled to the advantage of an organism.
Glycogen is the storage form of glucose in animals, including humans. Glycogen releases glucose when energy demands are high.
Glucose polymerizes to form glycogen when the organism has no immedi-ate need for the energy derived from glucose breakdown.
Glycogen metabolism is subject to several different control mechanisms, including covalent modification and allosteric effects.