Medical brain notes 44

  Basic Properties Amino Acids The principal constituents of proteins are amino acids, 20 of which are present in the body proteins in significant quantities. Figure 69–1 shows the chemical formulas of these 20 amino acids, demonstrating that they all have two features in common: each amino acid has an acidic group (—COOH) and a nitrogen atom attached to the molecule, usually represented by the amino group (—NH 2 ). Peptide Linkages and Peptide Chains.  In proteins, the amino acids are aggregated intolong chains by means of  peptide linkages.  The chemical nature of this linkage is demonstrated by the following reaction:         Note in this reaction that the nitrogen of the amino radical of one amino acid bonds with the carbon of the carboxyl radical of the other amino acid. A hydrogen ion is released from the amino radical, and a hydroxyl ion is released from the car-boxyl radical; these two combine to form a molecule of water. Af...

  Transport and Storage of Amino Acids Blood Amino Acids The normal concentration of amino acids in the blood is between 35 and 65 mg/dl. This is an average of about 2 mg/dl for each of the 20 amino acids, although some are present in far greater amounts than others. Because the amino acids are relatively strong acids, they exist in the blood principally in the ionized state, resulting from the removal of one hydrogen atom from the NH 2  radical. They actually account for 2 to 3 milliequivalents of the negative ions in the blood. The precise distribu-tion of the different amino acids in the blood depends to some extent on the types of proteins eaten, but the concentrations of at least some individual amino acids are regulated by selective synthesis in the differ-ent cells. Fate of Amino Acids Absorbed from the Gastrointestinal Tract. The products of protein digestion and absorption in the gastrointestinal tract are almost entirely amino acids; only rarely are polypeptides or w...

  Functional Roles of the Plasma Proteins The major types of protein present in the plasma are  albumin, globulin,  and  fibrinogen. A major function of  albumin  is to provide  colloidosmotic pressure  in the plasma, which prevents plasmaloss from the capillaries. The  globulins  perform a number of  enzymatic func-tions  in the plasma, but equally important, they are prin-cipally responsible for the body’s both natural and acquired  immunity against invading organisms. Fibrinogen  polymerizes into long fibrin threadsduring blood coagulation, thereby  forming blood clots  that help repair leaks in the circulatory system. Formation of the Plasma Proteins.  Essentially all thealbumin and fibrinogen of the plasma proteins, as well as 50 to 80 per cent of the globulins, are formed in the liver. The remainder of the globulins are formed almost entirely in the lymphoid tissues. They are mainly the gamma glob...

  Essential and Nonessential Amino Acids Ten of the amino acids normally present in animal pro-teins can be synthesized in the cells, whereas the other 10 either cannot be synthesized or are synthesized in quantities too small to supply the body’s needs. This second group of amino acids that cannot be synthesized is called the  essential amino acids.  Use of the word “essential” does not mean that the other 10 “nonessen-tial” amino acids are not required for the formation of proteins, but only that the others are  not essential in thediet  because they can be synthesized in the body. Synthesis of the nonessential amino acids depends mainly on the formation of appropriate a-keto acids,  which are the precursors of the respective amino acids. For instance,  pyruvic acid,  which is formed in large quantities during the glycolytic breakdown of glucose, is the keto acid precursor of the amino acid  alanine.  Then, by the process of  tran...

  Use of Proteins for Energy Once the cells are filled to their limits with stored protein, any additional amino acids in the body fluids are degraded and used for energy or are stored mainly as fat or secondarily as glycogen. This degradation occurs almost entirely in the liver, and it begins with deamina-tion,  which is explained in the following section. Deamination.  Deamination means removal of the aminogroups from the amino acids. This occurs mainly by transamination, which means transfer of the amino group to some acceptor substance, which is the reverse of the transamination explained earlier in relation to the synthesis of amino acids. The greatest amount of deamination occurs by the following transamination schema: Note from this schema that the amino group from the amino acid is transferred to a-ketoglutaric acid, which then becomes glutamic acid. The glutamic acid can then transfer the amino group to still other substances or release it in the form of ammonia ...

  Obligatory Degradation of Proteins When a person eats no proteins, a certain proportion of body proteins is degraded into amino acids and then deaminated and oxidized. This involves 20 to 30 grams of protein each day, which is called the  obligatoryloss  of proteins. Therefore, to prevent net loss ofprotein from the body, one must ingest a minimum of 20 to 30 grams of protein each day; to be on the safe side, a minimum of 60 to 75 grams is usually recommended. The ratios of the different amino acids in the dietary protein must be about the same as the ratios in the body tissues if the entire dietary protein is to be fully usable to form new proteins in the tissues. If one particular type of essential amino acid is low in concentration, the others become unusable because cells synthesize either whole proteins or none at all in relation to protein synthesis. The unusable amino acids are deaminated and oxidized. A protein that has a ratio of amino acids different from that...

  Hormonal Regulation of Protein Metabolism Growth Hormone Increases the Synthesis of Cellular Proteins. Growth hormone causes the tissue proteins to increase. The precise mechanism by which this occurs is not known, but it is believed to result mainly from increased transport of amino acids through the cell membranes or acceleration of the DNA and RNA transcription and translation processes for protein synthesis. Insulin Is Necessary for Protein Synthesis.  Total lack ofinsulin reduces protein synthesis to almost zero. The mechanism by which this occurs is also unknown, but insulin does accelerate the transport of some amino acids into cells, which could be the stimulus to protein synthesis. Also, insulin increases the availability of glucose to the cells, so that the need for amino acids for energy is correspondingly reduced. Glucocorticoids  Increase  Breakdown  of  Most  Tissue  Proteins. The glucocorticoids secreted by the adrenal cortex...