In proteins, amino acids are linked by peptide bond, formed by the
condensation of the carboxyl group of one amino acid with the amine group
of another amino acid (see Figure 4.1). Proteins are linear polymers composed
of 20 “standard” amino acids. However, it should be noted that not every
protein contains all 20 types of amino acids, although most proteins contain
most if not all of the 20 types. The molecular weight of protein is roughly at
10,000 daltons and those having a molecular mass less than this are referred to
as peptides.