Food Chemistry and Fish in Nutrition ( 2+1)

6.4.1. Gluconeogenesis

Gluconeogenesis is the synthesis of glucose from noncarbohydrate and then conversion to glycogen. The major substrates for gluconeogenesis are the glucogenic amino acids, lactate, glycerol, and propionate. Liver and kidney are the major tissues involved, since they contain a full complement of the necessary enzymes.

Gluconeogenesis meets the needs of the body for glucose when carbohydrate is not available in sufficient amounts from the diet. A continual supply of glucose is necessary as a source of energy, especially for the nervous system and the erythrocytes. Failure of gluconeogensis is usually fatal. Below a critical blood glucose concentration, there is brain dysfunction, which can lead to coma and death. Glucose is also required in adipose tissue as a source of glyceride-glycerol, and it probably plays a role in maintaining the level of intermediates of the citric acid cycle in many tissues. Even under conditions where fat may be supplying most of the caloric requirement of the organism, there is always a certain basal requirement for glucose.

Glucose is the only fuel that will supply energy to skeletal muscle under anaerobic conditions. In addition, gluconeogenic mechanisms are used to clear the products of the metabolism of other tissues from the blood, e.g. lactate, produced by muscle and erythrocytes, and glycerols, which is continuously produced by adipose tissue.

Gluconeogenesis involves glycolysis, the citric acid cycle, plus some special reactions

The energy barriers obstruct a simple reversal of glycolysis

i)between pyruvate and phosphoenolpyruvate,

ii)between furctose 1,6 bisphosphate and furctose 6-phosphate,

iii)between glucose 6-phosphate and glucose, and

iv)between glucose 1-phosphate and glycogen.

These reactions are all nonequilibrium, releasing much free energy as heat and therefore physiologically irreversible. These reactions are circumvented by the following special reactions.

1. the conversion of pyruvate into phosphoenolpyruvate

2.Conversion of fructose 1,6-bisphosphate into fructose 6- phosphate