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). These two groups are 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, 22 are considered the precursors of animal proteins.
The first few amino acids were discovered in the early 1800s. In 1806, the 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 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. Though 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 renowned German chemist, Emil Fischer, began investigating both questions. He synthesized many of the thirteen amino acids that were already known, and identified three more. He also showed how the various amino acids combined with each other inside the protein molecule--explaining, for instance, that the amino group of one amino acid is linked to the acidic carboxyl group of the next by a peptide bond. He suggested, as well, that the sequences and patterns formed by the various chains of amino acids helped establish the characteristics of different proteins.
In addition, Fischer 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 quite a lot 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--were reaching similar conclusions. Between 1909 and 1928, the two biochemists were investigating the proteins in a great variety of plant seeds. They found (among other things) 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. In humans, the dietary essential amino acids proved to be isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine, and, in growing children, histidine. 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.
Nutritionists have since determined that all of these essential amino acids can be obtained from meats, eggs, milk, cheese and other foods derived from animals. Plant proteins, however, generally lack several of these amino acids and, unless supplemented, can lead to protein deficiency problems.
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