Peptides and proteins are polymeric compounds composed of amino acid units. It is difficult to overemphasize the role of proteins in living matter. They are the principal components of muscles, skin, hair, and blood. Proteins are constituents of antibodies and hemoglobin, having protective and transport functions. Many antibiotics and hormones are proteins, and all enzymes are proteins only. Therefore it is no wonder that the term proteins originates from the Greek protos -the first.
In this chapter, we will mainly discuss the structure, chemical and some biochemical properties of amino acids. We will next consider briefly the structure and properties of peptides, and, finally, the main features of a protein structure.
15.1. б-AMINO ACIDS
We studied amino acids to some extent in Chapter 11. From various types of these heterofunctional compounds, б-amino acids are the most important in biological processes being the building blocks of proteins. Recall that б-amino acids are carboxylic acids with an amino group attached to the б-carbon atom; they may be represented by the general formula RCH(NH2)COOH. Only б-amino acids will be the target of our consideration so the symbol б will further be omitted.
15.1.1. Structure and Classification
Hydrolysis of most animal proteins produces about twenty different amino acids listed in Table 15.1. With the exception of glycine (R = H in the general formula), all amino acids have four different groups attached to the б-carbon. This carbon is chiral, and two enantiomeric forms of each amino acid are therefore possible. Most natural amino acids belong to the same stereoisomeric family, namely, to the L series. D-Amino acids are also known in nature, but only as constituents of certain antibiotics and of proteins of bacterial cell walls. The generalized formulas for enantiomers are given in the Fischer projections.