Mutational analysis of copper binding by human tyrosinase

Area of Science:

• Biochemistry
• Enzymology
• Melanin Biosynthesis

Background:

• Tyrosinase (EC 1.14.18.1) is a copper-dependent enzyme crucial for melanin pigment production.
• Deficiency in tyrosinase activity leads to type I oculocutaneous albinism (OCA1).
• Previous studies suggested two copper-binding sites (CuA and CuB) in tyrosinase, but direct evidence in mammals was lacking.

Purpose of the Study:

• To demonstrate direct copper binding by mammalian tyrosinase.
• To investigate the role of the CuA and CuB sites in copper binding and catalytic activity.
• To explore the impact of human OCA1 mutations on tyrosinase copper binding and function.

Main Methods:

• Biochemical assays to confirm copper binding in mammalian tyrosinase.
• Site-directed mutagenesis to analyze the function of CuA and CuB sites.
• Analysis of human OCA1 mutations to assess their effect on copper binding and enzyme activity.

Main Results:

• Mammalian tyrosinase directly binds copper at both CuA and CuB sites.
• Both CuA and CuB sites are indispensable for tyrosinase catalytic activity.
• Copper binding at one site may cooperatively enhance binding at the other site.
• Specific histidine residues (His363, His367, His389) are likely involved in CuB coordination.
• Proper protein folding is necessary for tyrosinase copper uptake.
• Several OCA1 mutations were found to impair copper binding and reduce catalytic function.

Conclusions:

• Mammalian tyrosinase utilizes two essential copper-binding sites (CuA and CuB) for its enzymatic activity.
• Disruption of copper binding, often due to OCA1 mutations, impairs melanin synthesis.
• Understanding tyrosinase copper coordination provides insights into albinism pathogenesis and potential therapeutic targets.