Metabolism
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Fasting hypoglycaemia in an infant - and poor exercise tolerance in two brothers
Key points from this exercise:
Glycogen synthase in liver and muscle are different enzymes, coded for by different genes.
Lack of liver glycogen as a result of a mutation in liver glycogen synthase leads to:
profound fasting hypoglycaemia with very elevated ketone bodies, because there is no source of glucose to maintain the blood concentration
hyperglycaemia in the fed state because there is no capacity to take up glucose for glycogen synthesis in the liver and so buffer the plasma concentration
Lack of muscle glycogen as a result of a mutation in muscle glycogen synthase leads to:
poor exercise tolerance because of the need for glycogen metabolism for muscle activity
impaired cardiac function and cardiac hypertrophy
Lack of muscle glycogen synthesis does not impair glucose tolerance or the ability to maintain fasting blood glucose.
Muscle cannot act as a direct source of glucose in the fasting state because it lacks glucose 6-phosphatase.
Branch points are introduced into glycogen by transfer of a chain of 1-4-linked glucosyl units from the growing chain onto a neighbouring branch.
See also the exercise on A hypoglycaemic adolescent with an enlarged liver and gout