Surface and inside volumes in globular proteins

Abstract

MEASUREMENTS of the surface area accessible to solvent provide a convenient definition of the surface and the inside volumes in proteins of known X-ray structure. The study of the accessibility to solvent of amino acid residues in several proteins^1–3 has confirmed the early observation that polar residues are found mostly on the surface and non-polar residues mostly inside globular protein structures. But the accessibility shows systematic variations with the molecular weight, because of the change in surface to volume ratio. Experimental data⁴ indicate that the accessible surface area A (in Ų) of monomeric globular proteins follows the law^5,6: which implies that the mean accessible surface area per residues decreases like M^−1/3 (where M is molecular weight) with increasing M, from about 68 Ų for proteins of 6,000 molecular weight to 38 Ų for proteins of 35,000 molecular weight. Thus, the accessibility to solvent is not a characteristic of the amino acids.