Amino acids, the building blocks of all protein molecules, are nitrogen-containing organic compounds that consist of at least one acidic carboxyl group (COOH) and one amino group (NH2). In alpha amino acids that are contained in the proteins found in cells, these two groups are both attached to a carbon atom, which also carries a hydrogen atom, plus a side chain known as the R group. The R group varies from one amino acid to another and gives each amino acid its distinctive properties. Although relatively simple compounds, amino acids can vary widely and to date more than 80 different amino acids have been found in living organisms. Of these 80 amino acids, 22 are considered the precursors of animal proteins.
Proteins are one of the most common types of molecules in living matter. There are countless members of this class of molecules. They have many functions from composing cell structure to enabling cell-to-cell communication. One thing that all proteins have in common is that they are composed of amino acids.
Amino acids are amphoteric organic acids that are able to biochemically react with both acids or bases.
Codons along the messenger RNA molecule (mRNA), synthesized from a DNA template, control the sequence of the insertion of amino acids into the protein chain during the process of translation.
The first few amino acids were discovered in the early 1800s. In 1806, French chemist, Louis-Nicolas Vauquelin, isolated a compound in asparagus that proved to be the amino acid, asparagine. In 1812, William Hyde Wollaston found a substance in urine that he identified as a cystic oxide, and was later named cystine. And in 1820, another French chemist, Henri Braconnot, discovered the first two natural amino acids, glycine and leucine. Several other compounds were discovered toward the end of the 19th century. In 1895, Sven Hedin isolated the compound arginine; in 1896, with the help of his colleague Albrecht Kossel, he discovered histidine. Three years later, in 1899, Edmund Dreschel identified another important amino acid, lysine. Although these scientists were able to determine that these were unique compounds, they were unsure of their exact significance. Scientists were also uncertain of the relationship between amino acids and protein molecules.
In 1899, the German chemist, Emil Fischer, began investigating both questions. Fischer synthesized many of the thirteen amino acids that were already known, and identified three more. More importantly, Fischer, showed how the various amino acids combined with each other inside the protein molecule. The amino group of one amino acid is linked to the acidic carboxyl group of the next by a peptide bond. Fischer suggested that the sequences and patterns formed by the various chains of amino acids helped establish the characteristics of different proteins.
Fischer also developed a method for linking amino acids together, as they were in natural proteins, to form polypeptides. In 1907, he managed to put together a synthetic protein molecule that contained eighteen amino acid units, a molecule so remarkably authentic that, as he demonstrated, digestive enzymes attacked it just as they would a natural protein.
Although much was now known about the structure of amino acids, their nutritional significance had yet to be determined. Since the early 1800s, scientists such as Gerardus Mulder, François Magendie and William Prout had established the nutritional importance of the proteins themselves. But even here, with few exceptions (Magendie, for instance, had proven that gelatin had almost no nutritional value) the various proteins were considered roughly identical. So, most felt, were their amino acid units.
By the turn of the twentieth century, however, the situation began to change. In 1901, the British biochemist, Frederick Gowland Hopkins, not only discovered the amino acid tryptophan but later also showed that it played an important role in the diet. In one of his feeding experiments, Hopkins demonstrated that the protein in corn, zein, a protein that contains no tryptophan, could not sustain life in laboratory rats if used as the sole protein. Only when the tryptophan-rich protein casein was added to the diet did the rats once again begin to thrive. Hopkins's experiment suggested that, if proteins were not nutritionally identical (which seemed increasingly evident), perhaps it was the amino acids they contained that made the difference.
At roughly the name time, two Americans, Thomas B. Osborne and Lafayette B. Mendel, reached similar conclusions. Between 1909 and 1928, the two biochemists were investigating the proteins in a great variety of plant seeds. They found that two amino acids in particular, tryptophan and lysine, were essential for normal growth in rats. Moreover, neither of the amino acids could be synthesized by the rats themselves, but had to be present in their diets.
In the 1930s, another American biochemist, William Rose, added the finishing touch to the amino acid story. In 1935, Rose isolated threonine, the last nutritionally important amino acid to be discovered, and, over the next decade or so, determined which amino acids could be synthesized by humans and certain mammals, and which had to be supplied by the diet. Unless all these amino acids were attained through various protein foods, Rose explained, the body would not have the building blocks to form new protein molecules, and the growth and repair of body cells would be impaired.
The amino acids that receive the most research attention are the alpha-amino acids that genes are codes for, and that are used to construct proteins. These amino acids include glycine NH2CH2COOH, alanine CH3CH (NH2) COOH, valine (CH3)2CHCH (NH2)COOH, leucine (CH3)2CHCH2CH(NH2)COOH, isoleucine CH3CH2CH(CH3)CH(NH2)COOH, methionine CH3SCH2CH2CH(NH2)COOH, phenylalanine C6H5CH2CH(CH2)COOH, proline C4H8NCOOH, serine HOCH2CH(NH2)COOH, threonine CH3CH(OH)CH(NH2)COOH, cysteine HSCH2CH(NH2)COOH, asparagine, glutamine H2NC(O)(CH2)2CH(NH2)COOH, tyrosine C6H4OHCH2CHNH2COOH, tryptophan C8H6NCH2CHNH2COOH, aspartate COOHCH2CH(NH2)COOH, glutamate COOH(CH2)2CH(NH2)COOH, histidine HOOCCH(NH2)CH2C3H3H2, lysine NH2(CH2)4CH(NH2)COOH, and arginine (NH2)C(NH)HNCH2CH2CH2CH(NH2)COOH.
Proteins consist of long chains of amino acids connected by peptide linkages (-CO.NH-). A protein's primary structure refers to the sequence of amino acids in the molecule. The protein's secondary structure is the fixed arrangement of amino acids that results from interactions of amide linkages that are close to each other in the protein chain. The secondary structure is strongly influenced by the nature of the side chains, which tend to force the protein molecule into specific twists and kinks. Side chains also contribute to the protein's tertiary structure, i.e., the way the protein chain is twisted and folded. The twists and folds in the protein chain result from the attractive forces between amino acid side chains that are widely separated from each other within the chain. Some proteins are composed of two of more chains of amino acids. In these cases, each chain is referred to as a subunit. The subunits can be structurally the same, but in many cases differ. The protein's quaternary structure refers to the spatial arrangement of the subunits of the protein, and describes how the subunits pack together to create the overall structure of the protein.
Even small changes in the primary structure of a protein may have a large effect on that protein's properties. A single misplaced amino acid can alter the protein's function. This situation occurs in certain genetic diseases such as sickle cell anemia. In that disease, a single glutamic acid molecule has been replaced by a valine molecule in one of the chains of the hemoglobin molecule, the protein that carries oxygen in red blood cells and gives them their characteristic color. This seemingly small error causes the hemoglobin molecule to be misshapen and the red blood cells to be deformed. Such red blood cells cannot distribute oxygen properly, do not live as long as normal blood cells, and may cause blockages in small blood vessels.
Enzymes are large protein molecules that catalyze a broad spectrum of biochemical reactions. If even one amino acid in the enzyme is changed, the enzyme may lose its catalytic activity.
The amino acid sequence in a particular protein is determined by the protein's genetic code. The genetic code resides in specific lengths (called genes) of the polymer doxyribonucleic acid (DNA), which is made up of from 3000 to several million nucleotide units, including the nitrogeneous bases: adenine, guanine, cytosine, and thymine. Although there are only four nitrogenous bases in DNA, the order in which they appear transmits a great deal of information. Starting at one end of the gene, the genetic code is read three nucleotides at a time. Each triplet set of nucleotides corresponds to a specific amino acid.
Occasionally there an error, or mutation, may occur in the genetic code. This mutation may correspond to the substitution of one nucleotide for another or to the deletion of a nucleotide. In the case of a substitution, the result may be that the wrong amino acid is used to build the protein. Such a mistake, as demonstrated by sickle cell anemia, may have grave consequences. In the case of a deletion, the protein may be lose its functionality or may be completely missing.
Amino acids are also the core construction materials for neurotransmitters and hormones. Neurotransmitters are chemicals that allow nerve cells to communicate with one another and to convey information through the nervous system. Hormones also serve a communication purpose. These chemicals are produced by glands and trigger metabolic processes throughout the body. Plants also produce hormones.
Important neurotransmitters that are created from amino acids include serotonin and gamma-aminobutyric acid. Serotonin (C10H12N2O) is manufactured from tryptophan, and gamma-aminobutyric acid (H2N(CH2)3COOH) is made from glutamic acid. Hormones that require amino acids for starting materials include thyroxine (hormone produced by the thyroid gland), and auxin (a hormone produced by plants). Thyroxine is made from tyrosine, and auxin is constructed from tryptophan.
A class of chemicals important for both neurotransmitter and hormone construction are the catecholamines. The amino acids tyrosine and phenylalanine are the building materials for catecholamines, which are used as source material for both neurotransmitters and for hormones.
Amino acids also play a central role in the immune system. Allergic reactions involve the release of histamine, a chemical that triggers inflammation and swelling. Histamine is a close chemical cousin to the amino acid histidine, from which it is manufactured.
Melatonin, the chemical that helps regulate sleep cycles, and melanin, the one that determines the color of the skin, are both based on amino acids. Although the names are similar, the activities and component parts of these compounds are quite different. Melatonin uses tryptophan as its main building block, and melanin is formed from tyrosine. An individual's melanin production depends both on genetic and environmental factors.
Proteins in the diet contain amino acids that are used within the body to construct new proteins. Although the body also has the ability to manufacture certain amino acids, other amino acids cannot be manufactured in the body and must be gained through diet. Such amino acids are called the essential dietary amino acids, and include arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Foods such as meat, fish, and poultry contain all of the essential dietary amino acids. Foods such as fruits, vegetables, grains, and beans contain protein, but they may lack one or more of the essential dietary amino acids. However, they do not all lack the same essential dietary amino acid. For example, corn lacks lysine and tryptophan, but these amino acids can be found in soy beans. Therefore, vegetarians can meet their dietary needs for amino acids as long by eating a variety of foods.
Amino acids are not stockpiled in the body, so it is necessary to obtain a constant supply through diet. A well-balanced diet delivers more protein than most people need. In fact, amino acid and protein supplements are unnecessary for most people, including athletes and other very active individuals. If more amino acids are consumed than the body needs, they will be converted to fat, or metabolized and excreted in the urine.
However, it is vital that all essential amino acids be present in the diet if an organism is to remain healthy. Nearly all proteins in the body require all of the essential amino acids in their synthesis. If even one amino acid is missing, the protein cannot be constructed. In cases in which there is an on-going deficiency of one or more essential amino acids, an individual may develop a condition known as kwashiorkor. Which is characterized by severe weight loss, stunted growth, and swelling in the body's tissues. The situation is made even more grave because the intestines lose their ability to extract nutrients from whatever food is consumed. Children are more strongly affected by kwashiorkor than adults because they are still growing and their protein requirements are higher. Kwashiorkor often accompanies conditions of famine and starvation.
Phenylketonuria (PKU) is an inherited metabolic disorder in which an enzyme (phenylalanine hydroxylase) that is crucial to the appropriate processing of the amino acid phenylalanine, is absent or deficient. Normally, phenylalanine is converted to tyrosine in the body. When phenylalanine cannot be broken down, it accumulates in excess quantities throughout the body, causing mental retardation and other neurological complications. Treatment is usually started during babyhood; delaying such treatment results in a significantly lowered intelligence quotient (IQ) by age one. Because tyrosine is involved in the production of melanin (pigment), people with PKU usually have lighter skin and hair than other family members.
PKU is an autosomal recessive disorder, and is caused by mutations in both alleles of the gene responsible for phenylalanine hydroxylase. Understanding of the exact mechanisms of the neurological complications associated with PKU are, however, little understood, and knowledge of the precise genetic mutations responsible for PKU have yet to yield significant advances in treatment or prevention of PKU. Because it is vital to begin diet treatment immediately, most nations in the developed world require all that all infants be tested for the disease within the first week of life.
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