PhD ceremony: Mr. W.T. Ismaya, 14.45 uur, Doopsgezinde Kerk, Oude Boteringestraat 33, Groningen
Dissertation: Crystal structure of Agaricus bisporus tyrosinase
Promotor(s): prof. B.W. Dijkstra, prof. H.J. Wichers
Faculty: Mathematics and Natural Sciences
A tyrosinase from the mushroom Agaricus bisporus has successfully been crystallized by Wangsa Ismaya and its structure was elucidated by means of X-ray crystallography.
The structure of this copper-containing enzyme shows a tetramer of two heavy (H) and two light (L) subunits, which is the species occurring physiologically. The H subunit appeared to be the gene product of ppo3 and has the typical fold of type 3 binuclear copper proteins, although with several extensions of the peptide chain. The L subunit originates from orf239342. Its structure resembles that of proteins with agglutinating activity, but the carbohydrate binding residues are not conserved; therefore its function is not yet clear. The tyrosinase structure shows how calcium ions stabilize the tetramer. A thioether bond, which occurs between the Cu-A coordinating His85 and the adjacent Cys83, is present in the active site. This thioether bond restricts the rotational freedom of the His85 side chain. A structure of PPO3 with bound tropolone shows that this inhibitor forms a pre-Michaelis complex with the enzyme. It occupies a similar position to that of phenylthiourea in the structure of catechol oxidase, a tyrosinase homolog. Further details of the reaction mechanism remain unclear in the absence of data on the oxygen-bound (oxy-state) of the enzyme. Nevertheless, the presence of a thioether bond suggests that the current catalytic mechanism, which requires His85 to be flexible, should be updated. In addition, the structure provides now a solid base for the biochemical characterization of the L subunit.
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