aEssential amino acids.

acids in aspartic acid and glutamic acid, or the basic amines of lysine, histidine or argi-nine. Amines, including the one connected to the central carbon, ionize by accepting an additional hydrogen ion at pH levels below a characteristic pK value. Because all amino acids have both an acidic and a basic functional group, all will have an ionic charge at either sufficiently high or low pH values. For some, it is possible at certain pH values for both groups to be ionized:


NH2 NH3+

These behaviors are important in forming attractions between different parts of the protein molecule, affecting its shape. Table 3.6 also gives the isoelectric point (pI), which is the pH at which the number of positive charges on the amino acid in solution equals the number of negative charges.

The fact that all amino acids contain nitrogen explains the importance of that element to all organisms. Nitrogen limitations can cause growth problems in biological wastewater treatment and in natural ecosystems. Humans also have a nutritional requirement for proteins. We can form many of the amino acids from other compounds. However, we cannot synthesize the following eight, which are called essential amino acids: isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These must be provided in the diet daily, since free amino acids are not stored in the body. The absence of any one causes protein synthesis to stop. Most animal proteins contain all eight, but many plant proteins do not, or do not have them in balanced amounts. For this reason vegetarians need to be aware that they must eat vegetables in combinations that eliminate deficiencies. For example, rice is deficient in lysine but has sufficient methionine, whereas beans are low in methionine but contain adequate lysine. However, beans and rice together in a diet supply a complete protein supply.

Amino acids form polypeptides by covalently bonding the carbon from the carboxylic acid group to the nitrogen of the amine, with loss of a water molecule. The result is called a peptide bond:

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