Glycogen cycle

D5. di Mauro, S., Angelini, C., and Catani, C., Enzymes of the glycogen cycle and glycolysis in various human neuromuscular disorders. J. Neurol. Neurosurg. Psychiat. 30, 411-415 (1967). 

Fig. 6.—Interconversion of Glycogen and n-Glucose by way of Glycogen Cycle. (Key (1), hexokinase-adenosine 5-triphospliate (2), phosphoglucomutaae (3),UDPG-pyrophoaphorylase (4), glycogen-UDPglucosyl transferase and branching enzyme (5), phosphorylase and amylo-l,6-glucosidase (6), D-glucose 6-phosphatase.)...

Since glycogen-metabolizing tissues contain phosphoglucomutase, UDPG-pyrophosphorylase, ° glycogen-UDPglucosyl transferase, and phosphorylase, the presence of a glycogen cycle has been proposed (see Fig. 6). In rat tissues, the levels of the individual, enzymic activities are... 

The breakdown of glycogen to glucose. The glucose then enters glycolysis and the citric acid cycle, providing energy in the form of ATP. 

Figure 6. Glycogen content in the vastus lateralis muscle as a function of cycling time at 75-80% of maximal oxygen uptake (VO2 max). Data points are mean values from 10 subjects. For each subject, exercise was performed repeatedly in periods of 15 min separated by 15 min rest periods. At the point of exhaustion and muscle fatigue, muscle glycogen stores were depleted. From Bergstrom and Hultman (1967) with permission from the publisher.

Bergstrom et al. (1967) examined the relationship between initial muscle glycogen content and the capacity for prolonged submaximal exercise. Subjects cycled to exhaustion at 75% VO2 max on three occasions, each separated by three days. 

A normal mixed diet was given prior to the first ride, a CHO-poor diet prior to the second, and a CHO-rich diet before the third. The mixed, CHO-poor, and CHO-rich diets produced mean preexercise concentrations of 118,42, and 227 mmol/kg wet muscle, respectively. The corresponding exercise times were 126,59, and 189 min. An excellent correlation existed between preexercise glycogen content and cycling... 

Figure 10. The relationship between the initial glycogen content in vastus lateralis muscle and work time in six subjects who cycled to exhaustion at 75% VO2 max. Each subject cycled to exhaustion on three occasions. The first experiment was preceded by a mixed diet (a), the second by a carbohydrate-poor diet (o), and the third by a carbohydrate-rich diet ( ). The energy contents of the diets were identical. In all experiments depletion of the muscle glycogen store coincided with exhaustion and muscle fatigue. From Bergstrom et al. (1967) with permission from the publisher.

The ATP required as the constant energy source for the contraction-relaxation cycle of muscle can be generated (1) by glycolysis, using blood glucose or muscle glycogen, (2) by oxidative phosphorylation, (3) from creatine... 

Essig, D., Costill, D. L., and Van Handel, P. J., Effects of caffeine ingestion on utilization of muscle glycogen and lipid during leg ergometer cycling, International Journal of Sports Medicine, 1, 86, 1980. 

In addition to the common pathways, glycolysis and the TCA cycle, the liver is involved with the pentose phosphate pathway regulation of blood glucose concentration via glycogen turnover and gluconeogenesis interconversion of monosaccharides lipid syntheses lipoprotein formation ketogenesis bile acid and bile salt formation phase I and phase II reactions for detoxification of waste compounds haem synthesis and degradation synthesis of non-essential amino acids and urea synthesis. 

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