BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 5. STRUCTURE, PROPERTIES, AND FUNCTIONS OF PROTEINS

5.1. Structure and Properties of Amino Acids Comprising Proteins. Peptide Bonds

5.1.1. General Structural Features of Amino Acids

A general structural feature of Amino Acids is the presence of amino and carboxyl groups attached to a single α-carbon atom. R is The amino acid side chain (radical), which in the simplest case is represented by a hydrogen atom (Glycine), but can have a more complex Structure (in Other Amino Acids):

Class="center">

In aqueous solutions at neutral pH values, α-amino acids exist as zwitterions (dipolar ions).

Unlike the other 19 amino acids, Proline is an imino acid whose side chain is bonded to both the α-carbon atom and the amino group, conferring a cyclic structure upon the molecule.

19 out of the 20 amino acids contain an asymmetric carbon atom at the α-position, bonded to four different groups. Consequently, these amino acids naturally occur in two distinct isomeric forms, L and D. The exception is glycine, which lacks an asymmetric α-carbon atom because its side chain consists solely of a hydrogen atom. Only L-isomers of Amino acids are incorporated into Proteins.

Pure L- or D-stereoisomers can spontaneously and non-enzymatically convert into an equimolar mixture of L- and D-isomers over time. This process is known as racemization. The racemization of each L-amino acid proceeds at a specific rate at a given Temperature. This property can be utilized to determine the age of humans or animals. For instance, hard dental enamel contains the protein dentin, in which L-aspartate converts into the D-isomer at human body temperature at a rate of 0.01% per year. Since dentin contains exclusively the L-isomer during the period of tooth formation, the age of an individual can be calculated based on the D-aspartate content. All 20 amino acids in The Human Body differ in their structure, size, and PHYSICOCHEMICAL PROPERTIES OF the side chains attached to the α-carbon atom.