Question

if given all enzymes, chemical names and chemical structures, a student should be able to construct the entire glycolytic pathway.

Understand the regulation and process of glycogen breakdown (glycogenolysis).

Understand the central role of glucose-6-phosphate as an important junction in metabolism.

Answer 1

Glycogen Breakdown

Glycogenolysis is generally catalyzed by two main enzymes unique to this pathway: glycogen phosphorylase and the second is debranching enzyme. The former normally helps in regulating the rate of glucose release from the glycogen. Glycogen phosphorylase helps in catalyzing the release of glucose-1-phosphate from the terminal residue of a non-reducing end of a glycogen branch by means of phosphorolysis. A molecule of inorganic phosphate comes and attacks the C₁ side of an α(1→4) glycosidic bond,therefore leaving a hydroxyl group on C₄ that remains in the glycogen polymer. The energy stored in the α(1→4) glycosidic bond during the condensation reaction in glycogen synthesis is sufficient to permit the formation of a glucose–phosphate bond without using ATP.

Glucose-1-phosphate is then next converted by phosphoglucomutase to the glucose-6-phosphate. The latter may then enter the glycolytic pathway, but only if glucose-6-phosphatase is present, free glucose can be formed . thus the role of glucose 6 phosphate is very important.

Glucose 6-Phosphate

Since glucose 6-phosphate is also a main product of gluconeogenesis, it serves as an important substrate for glucose-6-phosphatase in liver. The action of this enzyme helps in  release of free glucose into the bloodstream.

Conversion of glucose 6-phosphate to glucose 1-phosphate with the help of  phosphoglucomutase provides for the interchange between glycogen, galactose, and uronic acid metabolism (see Chapters 8 and 9). First of all, glucose 1-phosphate is activated to the uridine diphosphate precursor, which then contributes to the glycogen polymerization, to then galactose metabolism, or to the glucuronic acid formation.

If glucose 6-phosphate is oxidized by the  glucose 6-phosphate dehydrogenase, it will enter the pentose phosphate pathway.