Reduction of thioredoxin significantly decreases its partial specific volume and adiabatic compressibility

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Abstract
The partial specific volume and adiabatic compressibility were determined at several temperatures for oxidized and reduced Escherichia coli thioredoxin. Oxidized thioredoxin had a partial specific volume of 0.785–0.809 mL/g at the observed upper limit for all proteins whereas the partial specific volume of reduced thioredoxin was 0.745–0.755 mL/g, a value in the range found for a majority of proteins. The adiabatic compressibility of oxidized thioredoxin was also much larger (9.8–18 × 10−12 cm2 dyne−1) than that of the reduced protein (3.8–7.3 × 10−12). Apart from the region immediately around the small disulfide loop, the structures of the oxidized (X-ray, crystal) and reduced protein (nuclear magnetic resonance, solution) are reported to be very similar. It would appear that alterations in the solvent layer in contact with the protein surface must play a major role in producing these large changes in the apparent specific volumes and compressibilities in this system. Some activities of thioredoxin require the reduced structure but are not electron transfer reactions. The large changes in physical parameters reported here suggest the possibility of a reversible metabolic control function for the SS bond.