Some electrochemical studies of the enzyme: Tyrosinase

Jinhuan Huangfu

The enzyme tyrosinase (E. C. 1.14.18.1) is of wide occurrence in nature. It is a copper-containing protein which is involved in the oxidation of mono-phenols to o-diphenols and subsequently to o-quinones. Further reaction yields melanins. Although it is a redox enzyme involving a pair of copper atoms which can be in either oxidation states (I) or (II), little is known about its electrochemistry. This thesis describes various aspects of this topic. In the first section of the thesis, an electrochemical study of hexacyanoferrate (II) as a substrate of tyrosinase is described. The rate of oxidation of hexacyanoferrate (II) to (III) is easily monitored using limiting currents at a rotating disc electrode. The effect of hexacyanoferrate (II) concentration, enzyme concentration, pH and certain inhibitors is described. This work was extended to the pentacyanoferrate (II) complexes in which the sixth ligand was ammonia, pyridine, dimethyl sulphoxide, histidine, lysine, methionine and nitrosyl. The purpose of this investigation was to determine whether the rate of catalysis of the chemically similar pentacyanoferrate (II) complexes could be linked to such properties as the half wave potential, rate of electron transfer, size etc. It was found that two of the complexes did not react because their half wave potentials were too high; the rate of oxidation of the others seemed to be primarily determined by size and half wave potential. While the oxidation of the pentacyanoferrate (II) complexes is a simple one electron process, the oxidation of the mono-phenol through to the o-quinone is not. For example, it is not known whether the o-diphenol leaves the enzyme during the oxidation reaction, or is oxidised through while remaining bonded. Using a rotating ring-disc technique, it has been established that the latter mechanism is the preferred one. The final section is devoted to a study of direct electron transfer of tyrosinase. Various modified electrodes were investigated without success. However, direct electron transfer was found to be possible at a dropping mercury electrode. Since the peaks observed during differential pulse polarography were not observed with the apo-enzyme, it is concluded that they are due to the redox behaviour of the copper atoms. Suggestions are made as to the origin of these peaks.

Some electrochemical studies of the enzyme: Tyrosinase

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