Janet Macaulay; Week 2 MED1011; Biochemistry
Catabolism of macromolecules feeds into glycolytic pathways; glycogen/starch > glucose 1 phosphate > glucose 6 phosphate > fructose 6 phosphate > fructose 1,6 biphosphate > glyceraldehyde 3 phosphate.
Triglycerides are broken down into glycerol and fatty acids. Fatty acids are oxidised to acetyl CoA, releasing NADHH and FADH2. The acetyl CoA formed will enter the citric acid cycle. NADH and FADH2 from fatty acid oxidation enter the ETC and produce ATP. Glycerol enters glycolysis.
Digestion of carbohydrates begins in the mouth, continues in small intestine, polysaccharides are broken down into monosaccharides. Glucose is used for energy production or storage as glycogen. When energy is required glycogen is mobilised to glucose-6-phosphate, in the liver is converted to glucose which is transported to the brain. In skeletal muscles G6P is metabolised to obtain ATP during activity. G6P cannot move across a cell membrane (BBB).
Glycogenolysis activated between meals to provide energy. Glycogen phosphorylase is the rate limiting step.
Alcohol provides 4-6% of ingested kilojoules from ethanol in beverages. Alcoholics can consume >50% of their daily energy intake as ethanol.
- Alcohol: 29 kJ/g; Carbohydrate 17 kJ/g, Fat: 37 kJ/g, Protein 17 kJ/g
Alcohol is absorbed all through the GIT, diffuses quickly across cell membranes, depends primarily on rate of stomach emptying, food (particularly fat) slows rate of stomach emptying, most alcohol metabolised in the liver.Ethanol is converted to acetaldehyde by alcohol dehydrogenase, then to acetate by acetaldehyde dehydrogenase, with both reactions producing NADH. Acetate goes through the citric acid cycle to produce CO2 and H2O. Acetaldehyde is also highly toxic with effects on the brain, liver and heart; and impairs intestinal absorption of some amino acids, vitamins and minerals. Fatty acid oxidation requires NAD, which alcohol metabolism uses up, so there is a buildup of fats. A fatty liver also leads to hepatitis and cirrhosis. High levels of lactate from pyruvate are generated to replenish NAD reserves. There is an alternate pathway of alcohol metabolism through the microsomal ethanol-oxidising system in the endoplasmic reticulum, and is induced by high concentrations of ethanol. It is an energy dependant pathway where NADPH is oxidised to NADP (ethanol to acetaldehyde).
Citrate from CAC has positive feedback to fatty acid synthesis from acetyl CoA. It also has a negative feedback effect to phosphofructokinase in glycolysis.
Biosynthesis of macromolecules will occur if there is an excess of NADH or ATP. Metabolism is a balance between catabolism and anabolism.