Glycogen metabolism and synthesis, learn about hundreds of medical conditions
November 6 2015 by
Ray Sahelian, M.D.

 

Glycogen is a polymer of glucose residues linked mainly by alpha(1-4) glycosidic linkages. There are a(16) linkages at branch points. Glucose is stored as glycogen predominantly in liver and muscle cells.

 

Metabolism

Glycogen Phosphorylase catalyzes phosphorolytic cleavage of the alpha(1-4) glycosidic linkages of glycogen, releasing glucose-1-phosphate as the reaction product. Pyridoxal phosphate (PLP), a derivative of vitamin B6, serves as prosthetic group for Glycogen Phosphorylase. Pyridoxal phosphate is held at the active site of Phosphorylase enzyme by a Schiff base linkage, formed by reaction of the aldehyde of PLP with the e-amino group of a lysine residue.

 

Nestle Nutr Inst Workshop Ser. 2013. Carnitine and fat oxidation. Fat and carbohydrate are the primary fuel sources for mitochondrial ATP production in human skeletal muscle during endurance exercise. However, fat exhibits a relatively low maximal rate of oxidation in vivo, which begins to decline at around 65% of maximal oxygen consumption (VO₂max) when muscle glycogen becomes the major fuel. It is thought that if the rate of fat oxidation during endurance exercise could be augmented, then muscle glycogen depletion could be delayed and endurance improved. The purpose of the present review is to outline the role of carnitine in skeletal muscle fat oxidation and how this is influenced by the role of carnitine in muscle carbohydrate oxidation. Specifically, it will propose a novel hypothesis outlining how muscle free carnitine availability is limiting to the rate of fat oxidation. The review will also highlight recent research demonstrating that increasing the muscle carnitine pool in humans can have a significant impact upon both fat and carbohydrate metabolism during endurance exercise which is dependent upon the intensity of exercise performed.

 

Muscle Glycogen function and role

Muscle glycogen is an important fuel for contracting skeletal muscle during prolonged strenuous exercise, and glycogen depletion has been implicated in muscle fatigue. It is also apparent that glycogen availability can exert important effects on a range of metabolic and cellular processes. These processes include carbohydrate, fat and protein metabolism during exercise, post-exercise glycogen resynthesis, excitation-contraction coupling, insulin action and gene transcription.

 

Glycogen Storage Disease

The glycogen storage diseases comprise several inherited diseases caused by abnormalities of enzymes that regulate the synthesis or degradation of glycogen.

 

McCardle Disease

Also known as glycogen storage disease type V and the most common type of glycogen storage disease. McArdle disease is an autosomal recessive disorder caused by mutations in the gene that encodes myophosphorylase, an enzyme that is essential for glycogenolysis. Exercise intolerance usually develops during childhood, along with pain, cramps, and fatigue in exercised muscle. These symptoms are more likely to be induced by brief, intense activities (such as weight lifting or sprinting) but can also occur after prolonged, low-intensity exercises (such as swimming or jogging). Severe muscle damage can lead to myoglobinuria (the release of myoglobin from muscle into the bloodstream) and renal failure. There is variation among patients with McArdle disease. Some patients become fatigued easily but have no clinically significant muscle cramps or pain, whereas others have with progressive atrophy and weakness of upper arm, and thigh muscles in late adulthood.