Nutrition Liver Enzymes essay

Nutrition Liver Enzymes essay - Glucose and the Liver...

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Glucose and the Liver Glucose is the universal fuel for ATP production in all cells of the body, strictly regulated by homeostasis at a concentration between 4 and 6 mmol/L in the blood. This level may rise slightly after feeding but is returned to normal through maintenance by hormones. In a fasting state, however, the levels of blood glucose will naturally decrease with the absence of food intake. Thus, the body utilizes the liver to facilitate the breakdown and generation of other nutrients so it can provide the body with energy. This essay aims to describe the role of the liver in sustaining the body with systemic nutrients from which ATP can be produced. Glucagon, a twenty nine amino acid polypeptide ¹ , is the main hormone acting to control hypoglycaemia (low concentration of glucose in the blood) in times of starvation. When blood glucose levels are significantly decreased, this stimulates the alpha cells of the Islets of Langerhans, located in the endocrine domain of the pancreas, to synthesise and release glucagon. (See Figure.1) It acts by mobilizing liver glycogen in hepatocytes, in a process known as hepatic glycogenolysis. This is the breakdown of glucose from glycogen and is the most important factor in regulating blood glucose in early fasting; adrenaline has also been shown to simulate this procedure in muscles. Glucagon also inhibits fatty acid synthesis and similarly decreases the activity of acetyl-coA carboxylase, thus promoting beta oxidation of fatty acids. It also promotes gluconeogenesis – the generation of glucose from non- carbohydrate precursors. Blood glucose concentration can therefore be maintained. The mechanism involved in the release of glycogen stores is complex. Once glucagon has been released into the blood, it circulates till reaching hepatocytes. Here, glucagon is able to bind to an integral seven transmembrane G protein coupled receptor located on the plasma membrane causing a structural change 2 . The alpha subunit of the internal G protein is released and attaches to a transmembrane peptide enzyme, adenylate cyclase. This catalyses the conversion of an ATP molecule into cyclic AMP (cAMP) and pyrophosphate. cAMP is known as second messenger – it acts as signal transducer transferring the effects of glucagon to inside the cell. As glucagon is a peptide, it is unable to actually enter the cell itself, incapable of dissolving through the lipid bilayer; instead, binding to this receptor causes an internal kinase cascade while it remains outside. As the levels of cAMP increase, it binds to the two regulatory subunits of an inactive Protein Kinase A (PKA) molecule, causing the dissociation of this and its two catalytic subunits. Whilst the PKA regulatory subunits remain bound to cAMP, the released catalytic units are now active and go onto phosphorylate and activate phosphorylase kinase. This activated enzyme now phosphorylates glycogen phosphorylase, activating it to cleave the 1,4 glycosidic bonds present between
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Nutrition Liver Enzymes essay - Glucose and the Liver...

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