Glycolytic Energy System

Historically, the study of cellular energy catabolism has mostly commenced with carbohydrate catabolism. For muscle, this means the breakdown of glucose and its stored intracellular form; glycogen. More recently, the phosphagen system has now become the start of such study, and as you now know from my work, I prefer to start with other content. You started with neuromuscular physiology to know and appreciate the cause of skeletal muscle ATP demand. The initial energy system that restores cellular ATP is the phosphagen system, which you have now studied in detail. Now we look at muscle glycogenolysis and then glycolysis. Glycogenolysis, consisting of as many as 4 reactions, along with the cellular uptake of glucose from the blood, provides the glucose to fuel a sequence of 9 reactions that forms the pathway of glycolysis. You will learn that glycogenolysis is a rapid means to provide glucose (actually glucose-1-phosphate) which is then converted to the molecule that commences the glycolytic pathway; glucose-6-phosphate (G6P). G6P is a 6 carbon molecule, which is essentially a glucose molecule modified by the addition of a phosphate group (PO3-) on carbon 6. The final product of glycolysis is pyruvate, though there is growing evidence we should think about changing this to lactate, but alas, such acceptance has not occurred yet! The other notable feature of glycolysis is that there is proton (H+) release from this pathway, with the glyceraldehyde-3-phosphate dehydrogenase reaction causing the most H+ release of this pathway. No, such H+ release does not come from the production of metabolic acids. As I explain in the Topics in the section on Metabolic Acidosis, no metabolic acids are produced in energy catabolism in biological tissues. Though to grasp the biochemistry of metabolic acidosis, you must gain knowledge of each metabolic pathway, as each one is involved in cellular H+ balance.