The influence of urea on the structure of proteins in reversed micelles

Static fluorescence measurements from the protein Tryptophan (Trp) residues and circular dichroism (CD) spectroscopy were used to investigate changes on the tertiary and secondary structures of the protein bovine serum albumin (BSA) and the dimeric enzyme hexokinase (HK) type PII from yeast, entrapped in reversed micelles (RMs). The latter were obtained from the amphiphilic AOT (sodium bis-2-ethylhexyl sulfosuccinate) in n-hexane, at several water to surfactant ratios, W. BSA and HK were found to be anchored at the RM interface in close contact with the surfactant layer, regardless of the size of the waterpool. For BSA, such interaction promotes partial protein unfolding, according to CD data that showed a decrease in the content of helical structure from 66% in a buffer solution to 48% in the micellar moiety. When urea was present in the micelle, further loss in helical structure occurred, thus indicating that the combined effect (micellar environment and urea) altered the BSA conformation to a greater extent than did RM or urea alone. Interestingly, Trps probed the same environment in the micelle, regardless of the presence of urea, but the fluorescence was quenched to a higher extent with urea. Thus, the fluorofore emission must have been affected either by the direct interaction of urea or by indirect exchange of water structure caused by urea interacting with water and the micellar interface. Both mechanisms might be of relevance in the solvation properties nearby the Trps. For HK, an association between the enzyme and the micelle interface was indicated in the CD spectra, which exhibited a randomized structure upon interaction, whatever the RM droplet size. The urea addition to the micelle water pool did not cause further impact on the HK conformation. In addition, the influence of urea at the RM interface was not sensed by the exposed tryptophans of the enzyme, unlike the results for BSA.