Glycogen phosphorylase is an enzyme that regulates the use of stored glucose as an energy source. It generates glucose for metabolism when sugar levels in the blood are low. There are multiple forms found in humans, primarily in the muscles and liver, and some in the brain. Since this enzyme controls the first step of glucose utilization, it is complex and highly regulated. The body’s glucose stores are in the form of glycogen, which is a long, branching chain of thousands of glucose units stored as granules.
Since glucose is the primary carbohydrate used as fuel by the body, its storage and use is tightly regulated. Glycogen phosphorylase carries out the first step in the breakdown of the glucose storage polymer glycogen to produce individual units of glucose. This enzyme adds a molecule of inorganic phosphate to a glucose unit on the end of a chain of glycogen, converting it to glucose-1-phosphate and freeing it from the glycogen. The glucose-1-phosphate molecule is subsequently transformed to glucose-6-phosphate by another enzyme and enters into the glucose cycle, ending up being used in aerobic respiration. This breakdown of glycogen to produce glucose is known is glycogenolysis.
This pathway only operates under certain conditions. When blood sugars are low, the hormone glucagon is secreted by the pancreas to end up in the liver. Through a complex series of signals, it then causes a phosphate group to be added to glycogen phosphorylase. This enzyme is then capable of adding its inorganic phosphate group to glucose to commence the degradation of liver glycogen.
The majority of the glycogen phosphorylase enzyme, a dimer composed of two subunits, is bound to the glycogen granule. It remains in an inactive form until an inorganic phosphate molecule is added to a serine amino acid group on the enzyme. This transforms it to an active state, and it can begin phosphorylating glucose.
Glycogen phosphorylase can be regulated in alternate manners, depending on the tissue group. Muscle tissue uses a lot of energy in the form of the high-energy three phosphate compound adenosine triphosphate (ATP). When this is broken down, it forms a low-energy molecule with a single phosphate group known as adenosine monophosphate (AMP). An entirely different mode of regulation in muscle tissue is the binding of AMP to the glycogen degrading enzyme. This causes the structure of the enzyme to shift, and it can then degrade glycogen.
Granules of glycogen are an efficient way of storing a lot of chemical energy in a small amount of space. Many glucose chains have branches, but glycogen phosphorylase can only act on terminal units of glucose on linear chains. A special debranching enzyme is required before the enzyme can fully degrade the glycogen.