ABSTRACT
The amino acid
composition of α and β structural class of proteins from five species, Escherichia coli, Thermotoga maritima, Thermus thermophilus, yeast, and
humans were investigated. Amino acid residues of proteins were classified into
interior or surface residues based on the relative accessible surface area. The
hydrophobic Leu, Ala, Val, and Ile residues were rich in interior residues, and
hydrophilic Glu, Lys, Asp, and Arg were rich in surface residues both in α and β proteins. The amino acid composition of α proteins was different from that of β proteins in five species, and the difference was derived from the
different contents of their interior residues between α and β proteins. α-helix content of α proteins was rich in interior residues than surface ones. Similarly, β-sheet content of β proteins was rich in interior residues than surface ones. The content of
Leu residues was very high, approximately 20%, in interior residues of α proteins. This result suggested that the Leu residue plays an important
role in the folding of α proteins.