Human Biochemistry, Volume 1 - Murray R. 1993

Structure and Functions of Proteins and Enzymes
Amino Acids
Amino Acids

Amino Acids contain an amino group and a carboxyl group as their functional groups. In a-amino acids, both of these are attached to the same (a) carbon atom (Fig. 3.1).

Although about 300 amino acids exist in nature, only 20 of them are found in Proteins. Complete Hydrolysis¹ of proteins yields 20 L-a-amino acids (Table 3.2). The exact same 20 Amino acids are present in the protein molecules of all life forms—plants, animals, and microorganisms. The reason for this will become clear later when we discuss the universal nature of METABOLISM/28.html">The Genetic Code (Ch. 30). However, certain proteins contain derivatives of specific amino acids that are formed after the standard Amino acids have already been incorporated into the protein molecule (Table 3.4).

With the exception of Glycine, in which the R group is a hydrogen atom, all four groups attached to the a-carbon atom in amino acids are different. Due to the tetrahedral arrangement of these four distinct groups around the a-carbon atom, amino acids exhibit optical activity (The ability to rotate the plane of polarization of plane-polarized light). Some amino acids found in proteins are dextrorotatory (at pH 7.0), while others are levorotatory; nevertheless, they all share the absolute configuration of L-glyceraldehyde and are therefore classified as L-a-amino acids.

¹ Hydrolysis is the Cleavage of a covalent bond accompanied by the incorporation of the atoms of a Water molecule.

Class="center">

Fig. 3.1. Two ways of representing an a-amino acid.