 In this video I will discuss the ways in which carbohydrate metabolic pathways, glycolysis, and the citric acid cycle interrelate with protein and lipid metabolic pathways. Gluconeogenesis is the synthesis of glucose from non-carbohydrates. We need to maintain glucose levels in the blood in order to support aerobic cellular respiration of tissues throughout the body in order to maintain ATP levels in cells to support life. And so if there's not sufficient carbohydrate intake from the diet, one of the ways to produce glucose is to start with non-carbohydrates such as lactic acid. We previously discussed lactic acid fermentation following glycolysis in anaerobic cellular respiration. Well, lactic acid is a waste product of a cell that's carried out anaerobic cellular respiration. However, lactic acid can serve as a starting point for a cell to produce new glucose in gluconeogenesis. So lactic acid can be converted back to pyruvate and this will produce an N-A-T-H molecule. Then pyruvate will enter into gluconeogenesis as it is converted to phosphoenolpyruvate. And then from phosphoenolpyruvate, the energy releasing phase of glycolysis can essentially be run in reverse, generating 1,3-bisphosphoglycerate. Then glyceraldehyde-3-phosphate dehydrogenase can catalyze the reverse reaction from glycolysis, converting 1,3-bisphosphoglycerate into glyceraldehyde-3-phosphate and consuming N-A-D-H in the process. Then glyceraldehyde-3-phosphate can be joined together into a fructose 6-phosphate that is then isomerized to glucose 6-phosphate and the glucose 6-phosphate can be the reagent for the hexokinase enzyme to run its reaction in reverse, phosphorylating ADP to produce an ATP and a glucose as the product. Protein catabolism is the process where proteins are broken down to release amino acids. There's a variety of proteolytic enzymes found within cells that can catalyze the hydrolysis of peptide bonds in polypeptides to release amino acids. One of those organelles is the lysosome. Another of those organelles is called the proteosome. The proteosome will specifically recognize misfolded or worn-out proteins that have been labeled with a tag known as ubiquitin. A protein with a ubiquitin tag has been marked for protein catabolism and the proteosome will then catalyze hydrolysis, breaking down the polypeptide to release amino acids. After proteases have catalyzed the hydrolysis of the peptide bonds, releasing amino acids from polypeptides, those amino acids could be used in translation to synthesize new polypeptides or those amino acids could be broken down further to generate ATP or be used for gluconeogenesis. Deamination is the removal of the nitrogen-containing amino group from an amino acid. Deamination is required before an amino acid can be used in gluconeogenesis or before an amino acid can be broken down further in order to generate ATP. The amino group when removed from an amino acid during deamination is converted to ammonia and ammonia if it accumulated at high levels in our blood is a relatively toxic chemical that can cause denaturation of proteins. Therefore in the liver ammonia will be converted to urea that is a less toxic nitrogenous waste that can then be transported in the blood and urea will be removed from the blood by the kidneys and excreted from the body in the urine. Lipolysis is the hydrolysis of triglycerides producing glycerol and fatty acids. Glycerol can be converted to glyceraldehyde phosphate and enter glycolysis or gluconeogenesis whereas the fatty acids will enter beta oxidation a metabolic pathway in the mitochondrial matrix where fatty acids are converted to acetyl coenzyme a acetyl coenzyme a can then enter the citric acid cycle. Following lipolysis beta oxidation will occur in the mitochondrial matrix and the first step in beta oxidation is a mechanism to transport the fatty acid into the mitochondrial matrix. Fatty acids will react with carnitine to form the fatty acid carnitine shuttle that will be able to transport fatty acids into the mitochondrial matrix then the fatty acids are released from carnitine then as beta oxidation proceeds fatty acids are oxidized and electrons are transferred to fad plus forming the high energy electron carrier fadh2. In this process two carbon units from the fatty acid are attached to coenzyme a forming the product acetyl coa and acetyl coa can then enter the citric acid or Krebs cycle to react with oxaloacetate forming citrate. Ketogenesis is the production of the ketone bodies acetoacetate and beta hydroxybutyrate from acetyl coenzyme a. Ketogenesis occurs if a cell is generating more acetyl coa from beta oxidation of fatty acids than can be used by the citric acid cycle. The excess acetyl coa is converted into ketone bodies acetoacetate and beta hydroxybutyrate. Although beta hydroxybutyrate is not a ketone in the technical sense that organic chemists use that term physicians clinicians and physiologists still commonly use the term ketone body to refer to beta hydroxybutyrate although acetoacetate is technically a ketone beta hydroxybutyrate would be technically a carboxylic acid. However these ketone bodies acetoacetate and beta hydroxybutyrate can be transported out of cells and provide an energy source to other cells that can use them. So ketone oxidation is the process of converting the ketone bodies beta hydroxybutyrate and acetoacetate back to acetyl coa then acetyl coa can enter into the citric acid cycle in order to generate ATP. To summarize the connections between lipid protein and glucose metabolism we've seen that gluconeogenesis can produce new glucose molecules from the carbon skeletons of amino acids that are generated in deamination from glycerol that's generated by lipolysis of triglycerides and also from lactic acid generated by anaerobic cellular respiration. And so while the amino acids can be converted to glucose, glucose cannot be converted to amino acids, nitrogen is required in order to produce those amino acids and our amino acids are one of the essential nutrients that we must obtain from the diet but amino acids as well as carbohydrates like glucose can both be used for lipogenesis used to synthesize new fat. Fat on the other hand cannot be converted into amino acids or glucose. The fatty acids from lipolysis will be broken down through beta oxidation to generate acetyl coenzyme A and acetyl coenzyme A can either be used to synthesize ketone bodies or enter the citric acid cycle but acetyl coenzyme A cannot enter gluconeogenesis and cannot be used to generate amino acids.