In humans, there is a large amount of protein turnover. Approximately one pound of protein is degraded and re-synthesized each day. Most of the amino acids liberated by protein degradation are used again in the re-synthesis of new protein but about ten percent of the total amino acid content is lost as it is converted to other important molecules involved in nervous system function, pigments, various hormones, and a variety of other essential activities. Amino acids are also used as fuel and, when present in excess in the diet, can be converted to fat for storage of excess calories.
Because of the universal importance of protein molecules to living cells, both plant and animal tissues can provide dietary protein. During digestion, the long chains of amino acids that make up complex protein molecules are disassembled to produce the twenty different single amino acids. These are taken up by cells in the digestive system, mostly in the small intestine, and released to the blood where they are transported around the body. Amino acids from the diet are used in three ways. They are uniquely used in the synthesis of new protein and, in a well-fed body, cells are actively synthesizing the structural and enzymatic proteins required for healthy functioning. The synthesis of these proteins is closely regulated by the expression of particular genes. It is this selective regulation that determines which proteins are to be synthesized, and in a more global sense, the characteristics, abilities, and activities of each individual cell.
When present in excess, amino acids are used as fuel. The different carbon skeletons of the twenty different amino acids are each metabolised through a more or less unique series of reactions. Thus, the degradation of each amino acid occurs by means of a specific pathway. However, the end products of these pathways are the same as various intermediates in the breakdown of glucose. Thus, overall, amino acid degradation results in the production of acetyl-CoA or its precursors and several of the organic acids involved the tricarboxylic acid (TCA) cycle. This means that, like carbohydrate, the carbon atoms that make up the amino acids can be converted to CO2 with the production of energy needed to support the life of the cell and the organism.
Excess amino acids can also be converted to fat. Again the picture is similar to that for carbohydrate in that carbon structures derived from the amino acids can be converted to citrate, a TCA cycle compound, and the first intermediate in the pathway of fat synthesis. Because the liver is the major site of fat synthesis, excess amino acids are taken up by the liver, converted to fat, packaged, transported, and stored as fat in adipose tissue.
Both carbohydrate and fat can be stored by cells, and by the organism, for use at a later time. Glycogen represents the storage form for carbohydrate and is present in many types of cells, particularly in the liver. Triglycerides represent the storage form for fatty acids synthesized in the liver and stored in adipose tissue. There is, however, no storage form for amino acids. They are either converted into protein or they are converted into other compounds. As a consequence, during the fasting state the body begins to break down protein to obtain the amino acids for the synthesis of new protein molecules needed to maintain or change metabolic activities. Each individual kind of protein molecule has a particular rate of turnover. Some proteins are degraded rapidly, such that half of the total amount of the enzyme in a single cell is broken down every 15 minutes or so. Others are degraded more slowly, where the time it takes to degrade half is perhaps an hour, a few hours, or in some instances, several days or weeks. Also during fasting, the amino acids liberated by protein breakdown also assist in energy production. This occurs both at the level of the individual cell in which protein degradation occurs and in whole body metabolism. It occurs in the following way: sugars, particularly glucose, are sources of TCA cycle intermediates and essential for the production of energy as ATP. Oxaloacetate (OAA) is a critical intermediate in the TCA cycle and the first step in the cycle involves the combination of OAA and acetyl-CoA to form citrate. During the breakdown of amino acids, the carbon skeletons of many of the amino acids are converted to one of the intermediates of the TCA cycle. Because of its cyclic character, once these intermediates enter the cycle they are easily converted to OAA. The production of OAA from amino acids means that the cell no longer needs to use as much glucose to maintain adequate levels of OAA in the TCA cycle. This, in turn, means that blood glucose is used more sparingly.
In the fasting state, a significant portion of the amino acids produced by the breakdown of protein in peripheral tissues, such as muscle, is released to the blood. Because of its very rich blood supply, the liver has excellent access to these circulating amino acids. These free amino acids are used for two major purposes. The first purpose is, just as in peripheral tissue, for the support of the synthesis of proteins needed by the liver to maintain its own structures and processes. The second purpose is for the synthesis of additional glucose for use by other tissues by a process called gluconeogenesis that takes place in the liver. Glucose can be synthesized from several key intermediates in metabolism. One of these is malate, one of the components of the TCA cycle. Just as for OAA, all of the TCA cycle intermediates can be converted to malate. Since the carbon skeletons of many of the amino acids are converted into TCA cycle intermediates, they also serve as starting material for the synthesis of glucose. This newly synthesized glucose can be released to the blood for use by the central nervous system and by other tissues.
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