Activity, structural and stability changes of mushroom tyrosinase by sodium dodecyl sulfate

Abstract

Cresolase and catecholase activities of mushroom tyrosinase (MT) were optimized in the presence of 0.35 and 0.5 mM sodium dodecyl sulfate (SDS), respectively, at pH 6.8 and 20 °C. While far-UV circular dichroism (CD) studies ruled out the possibilities of any changes in the secondary structure of the enzyme, near-UV CD results indicated conformational changes in the MT structure. In addition, intrinsic fluorescence spectroscopy studies referred to a special unfolding of MT in the presence of SDS. This unfolding opens out tryptophan and phenylalanine residues and, at the same time, shrinks tyrosine residues or places them in a more asymmetric environment. Near-UV CD studies on the inactivated MT by heat showed the importance of the tyrosine residues arrangement for the enzyme activities, as well. Although this partial unfolding of the MT structure causes its activation, it makes it more vulnerable too. Based on the Pace theory, the Gibbs free energy of both MT and MT activated by SDS, was calculated from UV spectrophotometry experiments using n-dodecyl trimethylammonium bromide as denaturing agent. The results of this study confirmed that activation of MT by SDS was accompanied by destabilization of the enzyme by 5 kJ/mol.