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Proteins are compounds composed of carbon, hydrogen, oxygen, and nitrogen, which are arranged as strands of amino acids. They play an essential role in the cellular maintenance, growth, and functioning of the human body. Serving as the basic structural molecule of all the tissues in the body, protein makes up nearly 17 percent of the total body weight. To understand protein's role and function in the human body, it is important to understand its basic structure and composition.
Amino acids are the fundamental building blocks of protein. Long chains of amino acids, called polypeptides, make up the multicomponent, large complexes of protein. The arrangement of amino acids along the chain determines the structure and chemical properties of the protein. Amino acids consist of the following elements: carbon, hydrogen, oxygen, nitrogen, and, sometimes, sulfur. The general structure of amino acids consists of a carbon center and its four substituents, which consists of an amino group (NH2), an organic acid (carboxyl) group (COOH), a hydrogen atom (H), and a fourth group, referred to as the R-group, that determines the structural identity and chemical properties of the amino acid. The first three groups are common to all amino acids. The basic amino acid structure is R-CH(NH2)-COOH.
There are twenty different forms of amino acids that the human body utilizes. These forms are distinguished by the fourth variable substituent, the R-group, which can be a chain of different lengths or a carbon-ring structure. For example, if hydrogen represents the R-group, the amino acid is known as glycine, a polar but uncharged amino acid, while methyl (CH3) group is known as alanine, a nonpolar amino acid. Thus, the chemical components of the R-group essentially determine the identity, structure, and function of the amino acid.
The structural and chemical relatedness of the R-groups allows classification of the twenty amino acids into chemical groups. Amino acids can be classified according to optical activity (the ability to polarize light), acidity and basicity, polarity and nonpolarity, or hydrophilicity (water-loving) and hydrophobicity (water-fearing). These categories offer clues to the function and reactivity of the amino acids in proteins. The biochemical properties of amino acids determine the role and function of protein in the human body.
Of the twenty amino acids, eleven are considered nonessential (or dispensable), meaning that the body is able to adequately synthesize them, and nine are essential (or indispensable), meaning that the body is unable to adequately synthesize them to meet the needs of the cell. They must therefore be supplied through the diet. Foods that have protein contain both nonessential and essential amino acids, the latter of which the body can use to synthesize some of the nonessential amino acids. A healthful diet, therefore, should
|Name||Abbreviation||Linear structure formula (atom composition and bonding)|
|SOURCE: Institute for Chemistry|
|Histidine||his||NH-CH=N-CH=C-CH2-CH(NH2)-COOH |____________| (nitrogen bonded to carbon)|
consist of a sufficient and balanced supply of both essential and nonessential amino acids in order to ensure high levels of protein production.
Author Info: Jeffrey Radecki, Susan Kim, The Gale Group Inc., Macmillan Reference USA, New York, Gale Nutrition and Well-Being A to Z, 2004
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