Proteins are large organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. The sequence of amino acids in a protein is defined by a gene and encoded in the genetic code. Although this genetic code specifies 20 "standard" amino acids, the residues in a protein are often chemically altered in post-translational modification: either before the protein can function in the cell, or as part of control mechanisms. Proteins can also work together to achieve a particular function, and they often associate to form stable complexes.
Proteins are broken down through digestion that begins in the stomach. Proteins are broken down by enzymes known as proteases into smaller polypeptides to provide amino acids for the organism, including the essential amino acids that the organism cannot biosynthesize itself. Aside from their role in protein synthesis, amino acids are also important nutritional sources of nitrogen.
Proteins, like carbohydrates, contain 4 kilocalories per gram as opposed to lipids which contain 9 kilocalories and alcohols which contain 7 kilocalories. Proteins can be converted into carbohydrates through a process called gluconeogenesis.
Dietary sources of protein include meats, eggs, grains, legumes, and dairy products such as milk and cheese. Of the 20 amino acids used by humans, the 10-12 nonessential amino acids can be synthesized by the body, and are not required in the diet. The 8-10 essential amino acids, however, cannot be created by the body and must come from dietary sources.
Most animal sources and certain vegetable sources have the complete complement of all 8-10 essential amino acids. However, it is not necessary to consume a single food source that contains all the essential amino acids, as long as all the essential amino acids are eventually present in the diet.
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