Expansibility (Molecular Biology)

The expansion of a protein molecule with increasing temperature reflects its atomic packing and flexibility, as does its compressibility with increasing pressure. X-ray crystallography studies indicate that both the crystal lattice and protein structure expand with increasing temperature. For myoglobin, a volume expansion of 5% of the crystal lattice and 3% in the protein molecule occurred as the temperature increased from 80 to 300 K, which corresponds to a thermal expansion coefficient oftmp32C-187_thumbRibonuclease A exhibited a smaller expansion 0.9% for a temperature increase from 98 to 320 K (2). It is likely that a more compressible protein will also be more thermally expansible. It is believed that the observed expansions of proteins with temperature are due to motions of secondary structure and exposed surface loops and to a decrease in local atomic packing density (see van der Waals Surface, Volume). Thermal expansion of a protein is not uniform over the molecule, just as in the case of contraction by pressure.

It is known that the apparent and partial specific volumes of proteins in aqueous solution are linearly dependent on temperature in the range of 4 to 45°C. Positive values fortmp32C-188_thumb‘ of (2.5 to

tmp32C-189_thumbhave been reported, with the majority lying betweentmp32C-190_thumb tmp32C-191_thumbThese values are greater than the expansion coefficient found for the X-ray crystal structure.

If the thermal expansion of crystal structure is caused by the cavities, the positive values oftmp32C-192_thumb for folded proteins would be dominantly attributed to the hydration effect, ie, a volume increase due to dehydration of protein molecules on elevating the temperature (see Partial Specific (Or Molar) Volume). The temperature effect of v 2 is still pronounced for amino acids and small peptides,

probably due to a large dehydration effect:tmp32C-193_thumbfor glycine and tmp32C-194_thumbfor triglycine. In this sense, an unfolded protein is expected to have a larger dv 2/dTvalue than a folded one because the accessible surface area (amount of hydration) increases on denaturation. Hawley reported that dv 2′dT increases bytmp32C-195_thumb

tmp32C-196_thumbon pressure denaturation of ribonuclease A (pH 2.0) and chymotrypsinogen (pH 2.07), respectively (3). However, the temperature effects on v2 at high pressures and high concentration of denaturants, above where conformational changes or denaturation take place, are complex since the volume change of hydration Dvsol may involve the contributions of changes in water structure and preferential solvent interactions.

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