Proteins are synthesized by the joining together of linear chains of amino acids. Different amino acids impart different chemical behaviors to the structure of proteins. Of the 20 common amino acids, some can be synthesized by our cells. Others, the essential amino acids, must be a part of our diet.
What Is an Amino Acid?
Amino acids all share a common chemical structure. An amino group (represented chemically as NH2) is joined to a carbon atom (the central or alpha carbon) which is then joined to another carbon atom, but this one is in the form of a carboxylic acid (chemical shorthand COOH). The amino group and the carboxylic acid group are critical for their involvement in the bonds that can be formed between amino acids when proteins are synthesized.
How are amino acids different from one another? They are different by virtue of the presence of different chemical groups attached to the alpha carbon atom (what are referred to commonly as side chains).
Amino Acids and Water or Fat
Dependent upon the side chain that is present on the alpha carbon, a particular amino acid can behave very differently from a chemical perspective. Because of the chemical nature of the different side chains used, there are groups of amino acids that do not mind being immersed in water (the hydrophilic amino acids), such as in the aqueous environment inside the cell, and there are those amino acids that do not care for being in water and would rather be hidden in fatty, oily suspensions (the hydrophobic amino acids), like those found within the confines of a cellular membrane.
Examples of hydrophilic (‘water-loving’) amino acids include lysine, arginine, aspartic acid and glutamic acid. Examples of hydrophobic (‘water-fearing’) amino acids include leucine, isoleucine, phenylalanine and valine.
Synthesizing Proteins from Amino Acids
Proteins are linear polymers of amino acids. The instructions that are encoded in our genes specify the order in which specific amino acids should be joined together to make a particular protein, such as insulin. The first amino acid in the chain donates part of its carboxylic acid group to become part of a chemical bond with the amino group of the next amino acid in the chain, and so on as the polymer is synthesized. When a chain is finished, the first amino acid still has an unused amino group so it is referred to as the amino terminal. Likewise, the last amino acid in the chain has an unused carboxylic acid group and therefore that end of the protein is referred to as the carboxyl terminal.
Amino acids are required in our diets every day. Human cells can synthesize 10 of the amino acids, for the other routinely used 10 amino acids, they must be acquired in our diet. These so-called essential amino acids are arginine, lysine, histidine, methionine, isoleucine, leucine, phenylalanine, valine, threonine and tryptophan. We need all of these amino acids in order to make not only the proteins our cells and body requires, but also for the synthesis of other compounds and even, in selected cases, for use as signals in the nervous system.
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