Protein Intake in Diet and Metabolism
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Created by:
fraittrain on January 30, 2011
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 Collaboration of HCl & Pepsin in protein digestion | HCl unfolds or denatures protein and exposes the peptide bonds to the gastric proteolytic enzyme, pepsin. Pepsin acts specifically upon the peptide bonds between AAs containing an aromatic ring or carboxylic acid in their R group, breaking the proteins into shorter polypeptides. |
| Describe effect of pH change from stomach to small intestine on protein digestion | pH changes from acidic to basic & pepsin is inactivated. Digestive enzymes of small intestine such as trypsin, chymotrypsin and carboxypeptidase function at an alkaline pH and continue process of breaking down dietary protein into its AA building blocks. |
| Dietary AA absorption at the end of digestion | Free AAs absorbed across intestinal wall. This requires active transport and is energy dependent. The AAs absorbed into circulation are available to all types of body tissues undergoing protein synthesis. May be used for energy or for synthesis of other proteins or the nitrogen/amine group can be used to synthesize nucleic acids, other amino acids or other nitrogen compounds (non-protein nitrogens or NPNs). |
| Where does protein turnover/metabolism take place? | Many proteins are turned over in the cells where they exist by peptidases and proteases. Plasma proteins, however, are bound by specific receptors on the cells of the liver, internalized and degraded there. Albumin is the major source of amino acids to cells and is internalized by all cells in order to provide AAs. |
| Half-life of proteins | The time required to reduce the concentration by half if no new protein is produced. Aka Protein turnover/degradation rate. Enzymes have very short half-lives whereas muscle tissue and other structural proteins have long half-lives. |
| What can proteins degraded to AAs be used for? |  Energy (or gluconeogenesis) via direct TCA input or through acetyl CoA. Deamination of the AAs will produce amine/ammonium (NH3/NH4+) wastes. |
| What is the only source of the nitrogen (amines/amides) needed for synthesis of AAs, nucleic acids and other non-protein nitrogen compounds (porphyrins, glycosamines, etc)? | Amino acids are the only source. In order to utilize this nitrogen, amino acids must be deaminated. This entails various types of transamination reactions which move amine groups. |
| Describe the 3 Types of transamination/deamination reactions. |  Both deamination reactions remove amines, but Non-Oxidative Deamination reaction allows the carbon skeletons to be used for energy, while the amine group is transferred to oxaloacetate to form glutamate in the Oxidative Deamination reaction. Transamination reactions will synthesize amino acids (the 8 we can make) from TCA intermediates. |
| What is the danger in excess AAs? | Although excess amino acids can be used for other purposes, this releases the amine groups as waste products of these reactions in the form of ammonia/ammonium ion (NH3 or NH4+), which is toxic to the CNS. |
| What happens to the ammonia/ammonium waste products released as AAs are utilized for other purposes? | converted to urea in the liver via the urea cycle. Urea is filtered by the kidney and becomes a major constituent of urine, accounting for about half of the dissolved solids. NOTE: Urea is one of the major NPNs. |
| Is protein synthesis regulated by a master regulatory system? | Partially regulated by many hormones, but no "master" regulatory system. Triiodothyronine, cortisol, aldosterone, somatostatin and growth hormone (aka somatotropin) plays a significant role in protein production in tissues that they target by regulating the induction or repression of protein synthesis, but most proteins are regulated locally in specific tissue or within the individual cells. |
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