Protein catabolism (Krebs cycle) and Protein anabolism (protein synthesis)




• Deaminated acids are brought into the Krebs cycle to be oxidized to CO2 and H2O. • Before entering the Krebs cycle, the deaminated acids are converted into intermediate products (pyruvic acid, acetyl coenzyme A, carbonic acids). • In the Krebs cycle, amino acids are oxidized to form reduced coenzymes. • Electron transport chain uses energy in reduced coenzymes to generate ATP. • Through formation of acetyl coenzyme A and pyruvic acid molecules, amino acids are also used to form other energy molecules. • Glucose: formed during gluconeogenesis. • Triglycerides: formed during lipogenesis. • Ketone bodies: formed during ketogenesis. • The energy in these molecules can also be used to produce reduced coenzymes and generate ATP. • New proteins are produced through the formation of peptide bonds (covalent) between amino acids. • This formation occurs on the ribosomes in almost every cell in the body. • During translation, covalent bonds form between amino acids and result in the assembly of a polypeptide. Digested proteins are broken down into amino acids which are not stored, but are either oxidized to produce ATP or used to synthesize new proteins. Many proteins function as enzymes, some are involved in transportation, serving as antibodies, clotting blood, being hormones, or being part of muscle fibers. Protein catabolism (breaking down) yields amino acids which are converted to other amino acids, fatty acids, ketone bodies, or glucose. Cells oxidize amino acids to generate ATP via the Krebs cycle. Protein anabolism (synthesis) creates new proteins by bonding together amino acids on ribosomes. Key Molecules at Metabolic Crossroads Of the thousands of different chemicals in cells, glucose 6-phosphate, pyruvic acid and acetyl coenzyme A are extremely important in metabolism. Glucose 6-phosphate is involved in: Synthesis of glycogen Release of glucose into the bloodstream Synthesis of nucleic acids Glycolysis Pyruvic acid is involved in: Production of lactic acid Production of alanine Gluconeogenesis Acetyl coenzyme A is involved in: Helping 2-carbon acetyl groups enter the Krebs cycle Synthesis of lipids Regulation of metabolism depends on chemicals in the cells and signals from the nervous and endocrine systems. Some aspects of metabolism depend on time elapsed since the last meal. During the absorptive state, glucose is readily available. During the postabsorptive state, energy needs are met by fuels already in the body. During fasting and starvation, the body must make metabolic changes to survive. Fasting is going without food for several hours or a few days. Starvation is going without food or inadequate food intake for weeks or months. The most dramatic metabolic change occurring with fasting and starvation is an increase in production of ketone bodies as catabolism of fatty acids increases. They may be used for energy by all cells.



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