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Protein Metabolism
About three-quarters of the body solids are proteins. These proteins include structural proteins, enzymes, nucleoproteins, proteins that transport oxygen, proteins of the muscle that cause muscle contraction, and many other types that perform specific intracellular and extracellular functions throughout the body.
The basic chemical properties that explain the diverse functions of proteins are so extensive that they constitute a major portion of the entire discipline of biochemistry. For this reason, the current discussion is confined to a few specific aspects of protein metabolism that are important as background for other discussions in this text.
Basic Properties of Proteins
Amino Acids Are the Principal Constituents of Proteins
The principal constituents of proteins are amino acids. Twenty of these amino acids are present in the body proteins in significant quantities. Figure 70-1 , which shows the chemical formulas of these 20 amino acids, demonstrates 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
The amino acids of proteins are aggregated into long 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 carboxyl radical; these two ions combine to form a molecule of water. After the peptide linkage has been formed, an amino radical and a carboxyl radical are still at opposite ends of the new, longer molecule. Each of these radicals is capable of combining with additional amino acids to form a peptide chain . Some complicated protein molecules have many thousands of amino acids combined by peptide linkages, and even the smallest protein molecule usually has more than 20 amino acids combined by peptide linkages. The average is about 400 amino acids.
Other Linkages in Protein Molecules
Some protein molecules are composed of several peptide chains rather than a single chain, and these chains are bound to one another by other linkages, often by hydrogen bonding between the CO and NH radicals of the peptides, as follows:
Many peptide chains are coiled or folded, and the successive coils or folds are held in a tight spiral or in other shapes by similar hydrogen bonding and other forces.
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, which is an average of about 2 mg/dl for each of the 20 amino acids, although some are present in far greater amounts than are others. Because the amino acids are relatively strong acids, they exist in the blood principally in the ionized state, as a result of 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 distribution 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 different 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 whole protein molecules absorbed from the digestive tract into the blood. Soon after a meal, the amino acid concentration in a person’s blood rises, but the increase is usually only a few milligrams per deciliter, for two reasons: First, protein digestion and absorption are usually extended over 2 to 3 hours, which allows only small quantities of amino acids to be absorbed at a time. Second, after entering the blood, the additional amino acids are absorbed within 5 to 10 minutes by cells throughout the body, especially by the liver. Therefore, large concentrations of amino acids almost never accumulate in the blood and tissue fluids. Nevertheless, the turnover rate of the amino acids is so rapid that many grams of proteins can be carried from one part of the body to another in the form of amino acids each hour.
Active Transport of Amino Acids Into the Cells
The molecules of all the amino acids are much too large to diffuse readily through the pores of the cell membranes. Therefore, significant quantities of amino acids can move either inward or outward through the membranes only by facilitated transport or active transport using carrier mechanisms. The nature of some of the carrier mechanisms is not completely understood, but a few are discussed in Chapter 4 .
Renal Threshold for Amino Acids
In the kidneys, the different amino acids that are filtered by the glomerular capillaries can be reabsorbed through the proximal tubular epithelium by secondary active transport , which returns them to the blood. However, as is true of other active transport mechanisms in the renal tubules, there is an upper limit to the rate at which each type of amino acid can be transported. For this reason, when the concentration of a particular type of amino acid becomes too high in the plasma and glomerular filtrate, the excess that cannot be actively reabsorbed is lost into the urine.
Storage of Amino Acids as Proteins in the Cells
After entry into tissue cells, amino acids combine with one another by peptide linkages, under the direction of the cell’s messenger RNA and ribosomal system, to form cellular proteins. Therefore, the concentration of free amino acids inside most cells usually remains low, and storage of large quantities of free amino acids does not occur in the cells; instead, they are stored mainly in the form of actual proteins. However, many of these intracellular proteins can be rapidly decomposed again into amino acids under the influence of intracellular lysosomal digestive enzymes. These amino acids can then be transported back out of the cell into the blood. Special exceptions to this reversal process are the proteins in the chromosomes of the nucleus and the structural proteins such as collagen and muscle contractile proteins. These proteins do not participate significantly in this reverse digestion and transport back out of the cells.
Some tissues of the body participate in the storage of amino acids to a greater extent than do others. For example, the liver, which is a large organ and has special systems for processing amino acids, can store large quantities of rapidly exchangeable proteins, which is also true of the kidneys and the intestinal mucosa to a lesser extent.
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