1. Enzymes are regulated by three general mechanisms -
1. Breakdown of glycogen (a polysaccharide containing multiple units of glucose) is catalyzed by the enzyme glycogen phosphorylase (note that there are four forms - see below). In the reaction called a phosphorolysis, phosphate is used to break one glucose unit from glycogen, forming a glycogen shortened by one unit and a molecule of glucose-1-phosphate (G1P). The advantage of phosphorolysis is that a phosphate is attached to glucose without requiring ATP. G1P can readily be converted to G6P by phosphoglucomutase (and vice versa too) and used in glycolysis. Thus, G1P from glycogen can be used to generate cellular energy VERY fast.
2. Glycogen phosphorylase works from the ends of a glycogen molecule inwards. The more ends, the more G1P that can be released rapidly. The enzyme does not work well close to the 1,6 branches of glycogen, so another enzyme (called Debranching Enzyme) is used to move glucoses from a branch to a linear chain (in an alpha1,4 orientation) and make them available to the phosphorylase. Debranching moves all but one of the glucoses on the side chain. The last one, which is in a 1,6 configuration, is cleaved by the enzyme in a hydrolysis reaction. The product is free glucose and is the only free glucose produced from glycogen.
3. Glycogen synthesis is not exactly the same as the reversal of glycogen breakdown. Energy is required and this energy ultimately comes from UTP . In this reaction, glucose is "activated" by being attached to UDP to form UDP-glucose (UDPG). UDP-glucose is a high energy compound. The energy between the glucose and UDP is used to drive the reaction in point #6 below.
4. Glycogen synthase catalyzes the addition of a glucose to the end of a glycogen molecule from a UDP-glucose. Branching enzyme creates the alpha1,6 linkages of glycogen.
5. Breakdown and synthesis of glycogen are controlled in opposite ways. When glucose is needed quickly in an emergency, the hormone epinephrine (adrenalin) is released into the bloodstream. It binds to cells and stimulates the phosphorylation of enzymes (including glycogen phosphorylase and glycogen synthase). Phosphorylation has opposite effects on these two enzymes. Glycogen phosphorylase is activated by phosphorylation, but glycogen synthase is inhbited by phosphorylation. Thus, release of adrenalin stimulates breakdown of glycogen (to release glucose for energy) and inhibits the synthesis of glycogen.
6.The opposite effect of adrenalin is accomplished by the hormone insulin. Insulin is released by the pancreas when blood glucose levels rise. Insulin stimulates two things. First, it stimulates cells to take up glucose from the bloodstream, reducing blood glucose levels to normal. Second, insulin stimulates the dephosphorylation of enzymes. Dephosphorylation reverses the previous effects on glycogen phosphorylase and glycogen synthase. Thus, dephosphorylation inactivates glycogen phosphorylase (converts it to the b form) and activates glycogen synthase (favors synthesis of glycogen, using all of the glucose newly arrived in the cell).
7. The phosphorylation system set up by epinephrine uses cAMP to signal the phosphorylation. Thus when epinephrine binds to the cell, cAMP is produced. Normally an enzyme called phosphodiesterase breaks down cAMP after a bit so its levels do not stay too high and glycogen phosphorylase is not left in the phosphorylated state too long. If this were to happen, then glycogen would get broken down too far. It is interesting that caffeine inhibits phosphodiesterase and thus keeps glycogen phosphorylase phosphorylated (more active) longer and as a consequence favors release of more glucose, thus accounting partly for the 'buzz' of coffee.