Glycogen and gluconeogenesis

Several enzymes are common to both the glycolytic and gluconeogenic pathways, but four enzymes catalyse steps that only occur in gluconeogenesis pyruvate carboxylase (reaction 3.6), phosphoenol- 

They catalyse rate-limiting steps in the pathway and are important control points. Glucogenic intermediates of the tricarboxylic acid cycle and amino acids that are transaminated or deaminated to tricarboxylic acid intermediates do not require pyruvate carboxylase (Fig. 3.1). Pyruvate and metabolites such as lactate, alanine, serine, glycine, cysteine that are converted into pyruvate require all four enzymes (Fig. 3.1). Gluconeogenesis is often linked to glycogen synthesis catalysed by glycogen synthase (see Section 3.5). 

Glycogen body 440 (at hatch) 0.01 Reaches maximum of 5-10% of total glycogen at E15-E21, but becomes a very small fraction in the adult 

Source From de Gennaro (1982) Asotra (1986) Yamano et al. (1988). Approximate values. 

The metabolic steps in gluconeogenesis occur in two intracellular compartments (Fig. 3.2) the cytosol and the mitochondrial matrix. The enzymes of the tricarboxylic acid cycle reside in the mitochondrial matrix, apart from succinate dehydrogenase which is present in the inner mitochondrial membrane, whereas most of the enzymes of the gluconeogenic pathway are present in the cytosol. Transaminases, such as alanine aminotransferase and aspartate aminotransferase, are present both in mitochondria and cytosol of the domestic fowl liver (Sarkar, 1977). One of the control enzymes in gluconeogenesis, PEPCK, has a different intracellular distribution in avian liver compared with mammalian liver (Table 3.3). PEPCK in both pigeon and domestic fowl liver is present almost exclusively ( 99%) in mitochondria (Soling et al.. 1973), whereas in most mammals that have been studied, it is present mainly in the cytosol, and only present, if at all, in smaller amounts in 

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Summary of some interconversions and synthetic reactions in which amino sugars participate. Substrates for the pathway can be derived from glucose, glycogen, and gluconeogenesis. 

Glucagon and the liver. The action of glucagon on the liver is shown above. The major overall effect is an increase in blood glucose, from glycogen and gluconeogenesis, and an increase in ketone bodies (see text for more detail). Major allosteric and covalent effects are shown, with the final form of covalently converted enzymes shown in purple, and reactions increased by glucagon shown with purple arrows and those slowed shown with dashed lines (see text for... 

Corticosteroids traditionally are divided into mineralocorticoids and glucocorticoids according to their relative potencies in Na retention and effects on carbohydrate metabolism i.e., hepatic deposition of glycogen and gluconeogenesis) In general, potencies of steroids to sustain life in adrenalectomized animals closely parallel their mineralocorticoid activity, while potencies as antiinflammatory agents closely parallel their effects on glucose metabolism. The effects on Na" retention and the carbohydrate/anti-inflammatory actions are not closely related and reflect selective actions at distinct receptors. 

Liver s main job is to keep up the levels of blood glucose. To do this, it breaks down glycogen and turns on gluconeogenesis. The liver takes lactate and alanine from the circulation and through gluconeogenesis converts it into glucose. The ammonia from the alanine is pushed... 

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. 

Carbohydrate Increase fasting levels of liver glycogen Increase gluconeogenesis and glycogenesis... 

Amino acids are used to synthesize liver and plasma proteins, or their carbon skeletons are converted to glucose and glycogen by gluconeogenesis the ammonia formed by deamination is converted to urea. 

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