The adenosine monophosphate (AMP) serves as the most sensitive indicator of a cell's energetic status, as its concentration can fluctuate significantly compared to adenosine diphosphate (ADP) and adenosine triphosphate (ATP). This sensitivity makes AMP a crucial marker for assessing cellular energy levels.
Glycogen phosphorylase is the key enzyme responsible for breaking down glycogen by catalyzing the cleavage of α-1,4 glycosidic bonds, releasing glucose as glucose-1-phosphate. This process requires the assistance of a debranching enzyme, which removes glucose units at branch points. As a phosphorylase, glycogen phosphorylase cleaves a phosphate group, resulting in glucose-1-phosphate, which must then be converted to glucose-6-phosphate for further metabolism.
Additionally, glycogen branching enzyme plays a vital role in glycogen synthesis by catalyzing the formation of α-1,6 bonds. It transfers 6 to 10 glucose subunits to the sixth position of a glucose molecule on another chain, creating the branched structure of glycogen. Glycogenin acts as the primer for glycogen synthesis, with initial glucose units attached to tyrosine residues on glycogenin itself.
Glycogen phosphorylase can be allosterically inhibited by glucose. The active form of glycogen phosphorylase, known as phosphorylase A, is phosphorylated and contains two phosphate groups. When glucose binds to this enzyme, it exposes the phosphate groups, allowing another enzyme to deactivate glycogen phosphorylase by removing these phosphate groups.
Phosphofructokinase-2 (PFK-2) is another important enzyme that is inhibited by ATP. PFK-2 catalyzes the formation of fructose-2,6-bisphosphate, which activates phosphofructokinase-1 (PFK-1) and promotes glycolysis. Glycolysis directly produces ATP and indirectly contributes to ATP production through the citric acid cycle and electron transport chain. Understanding the feedback mechanisms, such as ATP acting as an inhibitor for PFK-2, is essential for grasping the regulation of metabolic pathways.