Tyrosinase is a copper-containing enzyme found in animals, plants, fungi, and bacteria. Classified as a polyphenol oxidase, it acts as a catalyst for oxidation reactions involving phenolic compounds. Its active site contains two copper ions, which are essential for utilizing molecular oxygen to modify its substrates. Tyrosinase performs two sequential chemical reactions that drive the synthesis of pigments in many organisms, influencing processes like skin color and the browning of cut fruits.
The Central Role in Melanin Production
In humans and other mammals, the primary function of tyrosinase is to control the production of melanin, the pigment responsible for the color of skin, hair, and eyes. This process, known as melanogenesis, takes place exclusively within specialized cells called melanocytes, specifically inside membrane-bound compartments known as melanosomes. Tyrosinase is considered the rate-limiting enzyme of this pathway, meaning the speed of the entire process is determined by how quickly this enzyme works.
The enzyme initiates the reaction sequence by converting the amino acid L-tyrosine into L-3,4-dihydroxyphenylalanine, commonly called L-DOPA. This step is an ortho-hydroxylation reaction, where a hydroxyl group is added to the tyrosine molecule. Tyrosinase then catalyzes the second step, oxidizing L-DOPA into dopaquinone, a highly reactive compound.
Dopaquinone is an unstable intermediate that spontaneously undergoes a series of non-enzymatic and enzyme-catalyzed reactions to form the final melanin polymers. The pathway can branch to produce two main types of melanin pigment. One branch leads to the creation of eumelanin, a dark brown or black insoluble polymer that provides significant protection against ultraviolet (UV) radiation.
The other branch results in pheomelanin, a reddish-yellow, sulfur-containing pigment that contributes to lighter skin tones and red hair. The relative amounts and types of melanin produced depend on the activity of tyrosinase and other related proteins within the melanocyte.
Tyrosinase and Pigmentation Disorders
Hypopigmentation (Albinism)
When tyrosinase activity is genetically reduced or completely absent, it results in a group of inherited disorders known as Oculocutaneous Albinism (OCA). The most severe form, OCA Type 1A (OCA1A), is caused by mutations in the tyrosinase gene that render the enzyme completely inactive. Individuals with OCA1A produce virtually no melanin, leading to white hair, pale skin, and translucent irises. Another subtype, OCA1B, involves a mutation that only reduces the enzyme’s activity, allowing for some pigment production to occur over time.
Hyperpigmentation
Conversely, overactivity or misregulation of tyrosinase can lead to hyperpigmentation, characterized by the excessive accumulation of melanin in the skin. Conditions like melasma, age spots (solar lentigo), and freckles are common examples. In these cases, the enzyme is functioning, but its activity is unevenly or excessively stimulated, often by factors like UV exposure or hormonal changes.
The Role in Food Browning and Preservation
Outside of human biology, tyrosinase is known for its role in enzymatic browning in agriculture and the food industry. In this context, particularly in plants and fungi, the enzyme is often referred to as Polyphenol Oxidase (PPO). PPO is responsible for the darkening that occurs when fruits (apples, bananas, avocados) or vegetables (potatoes, mushrooms) are cut or bruised and exposed to air.
The reaction begins when the internal structure of the plant tissue is damaged, releasing the PPO enzyme, which then comes into contact with oxygen and phenolic compounds. The enzyme oxidizes these colorless phenolic substrates into reactive o-quinones. These o-quinones quickly polymerize to form the brown, black, or reddish pigments, causing significant economic losses by reducing the quality and shelf life of fresh produce.
Modulating Tyrosinase Activity
The activity of tyrosinase makes it a target for modulation in both the cosmetic and food preservation industries. Inhibiting the enzyme’s action is the primary strategy for treating hyperpigmentation disorders like melasma and age spots. Tyrosinase inhibitors are applied topically to reduce the rate of melanin synthesis in the skin.
These compounds work through several mechanisms, including directly interfering with the enzyme’s catalytic function or competing with its natural substrate, tyrosine. Common agents such as Kojic acid, a fungal metabolite, and Alpha-arbutin, a derivative of hydroquinone, are known to chelate the copper ions in the enzyme’s active site, thereby blocking its activity. Vitamin C derivatives are also used as inhibitors, often by reducing the intermediate dopaquinone back into L-DOPA before it can form melanin.
Controlling tyrosinase activity is equally important in the food industry to prevent enzymatic browning. Simple, traditional methods involve modifying the environment surrounding the cut food to slow down the enzyme. Acidifying agents like lemon juice or vinegar are effective because tyrosinase activity is significantly reduced at lower pH levels. Additionally, heat treatments, such as blanching, denature the enzyme, permanently destroying its structure and stopping the browning reaction entirely.