Exploiting Tyrosinase Activity in Aqueous and Nonaqueous Media

Abstract

Methods of utilising tyrosinase (polyphenol oxidase), E.C. 1.14.18.1, to prepare o-diphenols and o-quinones from phenols are described and discussed. Characteristics of enzyme activity under low-water conditions in organic solvents are reviewed including the use of salt hydrate pairs as water buffers. Recent progress in the use of the enzyme in synthetic chemistry, polymer modification, bioremediation and biosensors is highlighted.The last decade has seen a rapid increase in our knowledge of the molecular biology of tyrosinase. The tyrosinase gene has been sequenced from both plant and animal sources allowing comparisons of size and sequence homology. It has also provided extra evidence for the location of the enzyme in leaves -namely that it is in the lumen of the chloroplast. The next logical development of over-expression of the gene has been accomplished (/) heralding the prospect of ready availability of the enzyme and also the opportunity to undertake site-directed mutagenesis. It is therefore an appropriate time to consider how we might use the enzyme in biotechnology. Role in NatureIn nature, the enzyme plays an essential part in melanogenesis in animals and sclerotization in insects (2, 3). Although it occurs widely in plants its role in intact tissue is not always obvious (4)\ it is, however, involved in adventitious browning caused when cell structure is lost allowing substrate and enzyme to mix. Figure 1 provides a simplified view of these processes, emphasizing that the enzyme is involved at an early stage of a complicated sequence. It utilises oxygen to hydroxylate phenols and to oxidise o-diphenols, in each case the product being the corresponding o-quinones. These undergo addition reactions with nucleophiles such as the amino and sulfhydryl groups of amino acids (XH in Figure 1). In melanogenesis leading to the polymer eumelanin, tyrosine provides both the phenolic 0097-6156/95/0600-0159$12.00/0