Biochemistry of Lipid Metabolism

Study Notes

Non-Carbohydrate Precursors of Glucose

Amino acid catabolism is the main source of pyruvate and oxaloacetate for gluconeogenesis during fasting or carbohydrate starvation.
Muscle proteins break down to supply amino acids, which are transported to the liver and deaminated to gluconeogenesis inputs.
Glycerol, derived from hydrolysis of triacylglycerols in fat cells, is also a significant input to gluconeogenesis.

Bypass of Irreversible Steps in Glycolysis

In gluconeogenesis, three reactions are bypassed by a set of separate enzymes.
The bypass of pyruvate kinase involves two enzymes: pyruvate phosphoenoenolpyruvate and phosphoenolpyruvate carboxy kinase.
Carboxylation occurs in the mitochondria, where pyruvate is converted to oxaloacetate by pyruvate carboxylase, requiring biotin and ATP.
The oxaloacetate is then transported from the mitochondria to the cytosol using the malate-aspartate shuttle.

Conversion of Oxaloacetate to Glucose

Oxaloacetate is converted to 3-phosphoglycerate, which is then converted to glyceraldehyde-3-phosphate.
Two molecules of glyceraldehyde-3-phosphate combine to form fructose-1,6-bisphosphate, which is then converted to fructose-6-phosphate and glucose-6-phosphate.
Glucose-6-phosphate is isomerized to glucose-6-phosphate by phosphohexo isomerase.
Glucose-6-phosphate is hydrolyzed to free glucose by glucose-6-phosphatase, mainly in the liver.

Significance of Gluconeogenesis

Only the liver can replenish blood glucose through gluconeogenesis, maintaining blood glucose levels.
Gluconeogenesis maintains blood glucose levels during starvation, when glycogen stores are depleted.
The process requires energy, with 6 ATPs required to generate one glucose molecule.
Hormones like glucagon and glucocorticoids increase gluconeogenesis, while insulin inhibits it.

Glycogen Metabolism

Glycogen is a highly branched glucose polymer, stored in the liver and muscle.
Liver glycogen provides glucose during fasting, while muscle glycogen acts as a reserve fuel for muscle contraction.
Glycogen degradation (glycogenolysis) involves phosphorolysis, debranching, and glycogen phosphorylase.

Glycogen Synthesis and Degradation

Glycogen synthesis involves glycogenin, UDP glucose, and glycogen synthase.
Glycogen degradation involves glycogen phosphorylase, debranching enzymes, and glycogen synthase.
Regulation of glycogen metabolism involves carbohydrates, fatty acids, protein, and hormonal balance.

Description

This quiz covers the biochemistry of lipid metabolism, including the role of the liver, glycogenesis, lipogenesis, beta-oxidation, and the regulation of ketone production. It also touches on the density of lipoproteins and their function.