Composition of Proteins
One key way in which proteins differ from carbohydrates and lipids is that proteins contain nitrogen atoms, while carbohydrates and lipids contain only carbon, hydrogen, and oxygen atoms. These nitrogen atoms give the name “amino,” meaning “nitrogen containing.” to the amino acids of which protein is made. Protein molecules resemble linked chains, with the links being amino acids joined by peptide bonds. A protein strand does not remain in a straight chain, however. The amino acids at different points along the strand are attracted to each other, and this pull causes some segments of the strand to coil, somewhat like a metal spring. Also, each spot along the coiled strand is attracted to, or repelled from, other spots along its length. This causes the entire coil to fold this way and that, forming a globular or fibrous structure.
Each protein has its own specific sequence of amino acids. The 22 amino acids that exist in nature are like an alphabet, forming the “letters” of the “words”—proteins—that make up the language of life itself. All amino acids have the same basic structure—a carbon with three groups attached to it: an amine group (-NH2), an acid group (-COOH), and a hydrogen atom (H). Attached to the carbon at the fourth bond is an R group or side chain (Chemistry Figure 2-23). It is this fourth attachment, the side chain, different for each amino acid, that gives the amino acid its unique identity and chemical nature (Chemistry Figure 2-24 on page 40). The simplest amino acid is glycine, with only a hydrogen for the R group. In other amino acids, the R group may consist of carbon chains or cyclic structures. Amino acids that are acidic contain more acid groups (-COON) than amine groups (-NH2), while alkaline amino acids contain more amine than acid groups.
The structure of an amino acid
H — C — R
The R represents different groups that can attach here. This group makes each amino acid different.