by Louis Rosenfeld

Created on: August 24, 2009

Amino acids, the building blocks of proteins, are composed of a central carbon atom to which is attached an amino group (-NH2) and an acid group (carboxylic acid: -COOH); hence their name. All carbon atoms have four bonds. Besides the amino group and acid group, the central carbon atom has a hydrogen atom, and a fourth group (called the "R" group), which is a variable organic group that imparts the function of that amino acid. There are twenty different amino acids.

With four different groups, the three-dimensional arrangement of the atoms on the central carbon becomes important. There are two possible arrangements of the groups around the central carbon which are non-superimposable mirror images of each other, called D- or L- amino acids. These enantiomers can bend polarized light in opposite directions, so they are called optical isomers. In nature, all amino acids that form proteins are in the L- arrangement, except for the simplest amino acid, glycine, which has hydrogen as its R group (thus it does not have four different groups on the central carbon).

The carboxylic acid group of one amino acid is joined by ribosomes to the amino group of the next amino acid to form a peptide bond (called an amide bond by organic chemists). The reaction is as follows:

H2N-CHR1-COOH + H2N-CHR2-COOH -> H2N-CHR1-CONH-CHR2-COOH + H2O

Note that water is eliminated, so this kind of reaction is called dehydration synthesis. This process can continue many hundreds of times to form a protein. Proteins, or polypeptides, can vary in length from about 20 amino acids to thousands of amino acids long. Shorter lengths of amino acids are called peptides.

The types of R groups of the amino acids plus the three dimensional structure of the protein allow a protein to function as an enzyme, an antibody, a hormone, a receptor, or a transporter. The R groups can be classified as follows:

Nonpolar hydrocarbons: glycine, alanine, valine, leucine, isoleucine, proline, tryptophan, and phenylalanine. These amino acids can associate with nonpolar membrane phospholipids, or with other nonpolar amino acids, for example.

Sulfur-containing groups: methionine, cysteine, and cystine (the dimer of cysteine). These amino acids can form chemical cross-links between different amino acid chains.

Polar organic groups: serine, threonine, tyrosine, asparagine, and glutamine. These amino acids are water-soluble.

Acidic groups (containing carboxylic acid groups): aspartic

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