Heat capacity changes upon burial of polar and nonpolar groups in proteins

Loladze, VV Ermolenko, DN Makhatadze, GI

Vv. Loladze et al., Heat capacity changes upon burial of polar and nonpolar groups in proteins, PROTEIN SCI, 10(7), 2001, pp. 1343-1352

60

INGLESE

Article

Biochemistry & Biophysics

PROTEIN SCIENCE

0961-8368 → ACNP

10

7

2001

1343 - 1352

ISI

0961-8368(200107)10:7<1343:HCCUBO>2.0.ZU;2-Q

In this paper we address the question of whether the burial of polar and no npolar groups in the protein locale is indeed accompanied by the heat capac ity changes, DeltaC(p), that have an opposite sign, negative for nonpolar g roups acid positive for polar groups. To accomplish this, we introduced ami no acid substitutions at four fully buried positions of the ubiquitin molec ule (Val5, Val17, Leu67, and Gln41). We substituted Val at positions 5 and 17 and Leu at position 67 with a polar residue, Asn. As a control, Ala was introduced at the same three positions. We also replaced the buried polar G ln41 with Val and Leu, nonpolar residues that have similar size and shape a s Gin. As a control, Asn was introduced at Gln41 as well. The effects of th ese amino acid substitutions on the stability, and in particular, on the he at capacity change upon unfolding were measured using differential scanning calorimetry. The effect of the amino acid substitutions on the structure w as also evaluated by comparing the H-1-N-15 HSQC spectra of the ubiquitin v ariants. It was found that the Ala substitutions did not have a considerabl e effect on the heat capacity change upon unfolding. However, the substitut ions of aliphatic side chains (Val or Leu) with a polar residue (Asn) lead to a significant (> 30%) decrease in the heat capacity change upon unfoldin g. The decrease in heat capacity changes does not appear to be the result o f significant structural perturbations as seen from the HSQC spectra of the variants. The substitution of a buried polar residue (Gln41) to a nonpolar residue (Leu or Val) leads to a significant (> 25%) increase in heat capac ity change upon unfolding. These results indicate that indeed the heat capa city change of burial of polar and nonpolar groups has an opposite sign. Ho wever, the observed changes in DeltaC(p) are several times larger than thos e predicted, based on the changes in water accessible surface area upon sub stitution.