Tyrosinases mediate the ortho-hydroxylation and two-electron oxidation of monophenols to ortho-quinones. Catechol oxidases only catalyze the oxidation of diphenols. Although it is of significant interest, the origin of the functional discrimination between tyrosinases and catechol oxidases has been unclear. Recently, it has been postulated that a glutamate and an asparagine bind and activate a conserved water molecule towards deprotonation of monophenols. Here we demonstrate for the first time that a polyphenoloxidase, which exhibits only diphenolase activity, can be transformed to a tyrosinase by mutation to introduce an asparagine. The asparagine and a conserved glutamate are necessary to properly orient the conserved water in order to abstract a proton from the monophenol. These results provide direct evidence for the crucial importance of a proton shuttle for tyrosinase activity of type 3 copper proteins, allowing a consistent understanding of their different chemical reactivities.

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This method for iron estimation deserves special attention. 1. The specificity for iron is high. The influence ofcopper is approximately 2% only in comparison to methods using ferrozine (up to 15%). Hemoglobin-iron does not disturb the measurement ofplasma or serum iron. 2. The reaction is performed in citrate buffer at a pH from 2.1 to 2.5. This pH is optimal for the liberation of iron from transferrin. At this pH with serum, no precipitation occurs. Thus f a direct photometry is possible without previous protein precipitation. Since this pH is also optimal for colour development the complete reaction can be accomplished in one step and automation ofanalysis can easily be performed. 3. This fest can be performed at Iow cost. Compared to commercial tests, the price can be reduced between V A andV, ö .