Formation of the browning pigment melanin decoded

July 4, 2016, Universitaet Mainz
View into the catalytic center of a tyrosinase: The two amino acids Glu235 and Asn240 bind to a water molecule HOH112, which strips a proton (white) away from the substrate (p-tyrosol). The resulting phenolate can now bind to the copper ion (CuA), starting the tyrosinase reaction. Credit: © Institute of Molecular Biophysics, JGU

Melanin is a pigment which is present in almost all life forms and that determines hair and skin color in humans. It helps insects protect themselves against the effects of pathogenic microorganisms and it promotes tissue repair. The dark spots on fruits such as bananas can be attributed to the presence of melanin. However, the processes involved in the formation of this pigment were not yet fully understood. Researchers at the universities in Mainz and Kiel have now uncovered the molecular mechanism underlying melanin synthesis using a clever biotechnological procedure. With this, a major gap in our understanding of how this enzyme functions has been closed. At the core of the mechanism is the activity of the enzyme tyrosinase. This discovery opens the door to the development of numerous applications in the cosmetics and food industries as well as in environmental technology and medicine.

Tyrosinase initiates the melanin synthesis process. "We previously did not fully understand the role played by this enzyme. In fact, we knew more about the activities of catechol oxidase, a related but less potent enzyme that is also involved in the synthesis of melanin," explained Heinz Decker, Director of the Institute of Molecular Biophysics at Johannes Gutenberg University Mainz (JGU). Much research on the cause for the difference in the reactivity of tyrosinase and catechol oxidase has been conducted over the past few decades, but little success had been achieved to date.

Following up on clues from reported research undertaken by an Israeli team led by Dr. A. Fishman, Professor Heinz Decker and Even Solem of Mainz University and Professor Felix Tuczek of Kiel University decided to conduct experiments to discover the mechanism responsible for tyrosinase activity. They first isolated a catechol oxidase from Riesling wine leaves and converted it to a tyrosinase by means of a biotechnological process involving targeted mutation. They found that the difference in reactivity is attributable to two amino acids, a highly conserved glutamic acid and asparagine that are located near the catalytic center. They form such a strong bond with a specific water molecule within the protein matrix that the water molecule undergoes a charge displacement. This makes one side strongly negative, so that it strips a positive proton from a nearby monophenol. This then activates tyrosinase which converts the monophenols to chemically very reactive substances called quinones, which combine on their own to form melanin. However, in the absence of asparagine or a water molecule in the protein, only catechol oxidase is present and no tyrosinase.

This discovery is a major breakthrough in the understanding of the catalytic role played by tyrosinase in the synthesis of . This means that in the future it will be possible to make systematic improvements in the processes of stimulation, inhibition, and modification as well as in biotechnological methods employed in medicine, cosmetics production, and in environmental research, with the help of genetically-based approaches. "In addition, we have gained further insights into the functioning of copper in the body," concluded Decker. The results of the study have been published in the journal Angewandte Chemie International Edition.

Explore further: Tyrosinase inhibitors from terrestrial and marine resources

More information: Even Solem et al. Tyrosinase versus Catechol Oxidase: One Asparagine Makes the Difference, Angewandte Chemie International Edition (2016). DOI: 10.1002/anie.201508534

Related Stories

Tyrosinase inhibitors from terrestrial and marine resources

February 18, 2016

Tyrosinase is a multifunctional copper-containing enzyme widely distributed in microorganisms as well as plants and animals which has a primordial role in melanin biosynthesis thus impacting on skin color and pigmentation. ...

Yellow as the sunrise

March 21, 2016

What is it that walnut leaves, mushrooms and Coreopsis have in common? An enzyme that is also responsible for the browning reaction in bananas or apples is present in all of them in large amounts. For the first time, chemists ...

No more brown apples?

November 11, 2015

The longer an apple retains its beautiful colour, the better it is – especially for the food industry. Therefore, the industry works intensely on inhibiting the "browning" of fruits. Chemists of the University of Vienna ...

Banana peels can help identify the stages of melanoma

February 8, 2016

Human skin and banana peels have something in common: they produce the same enzyme when attacked. By studying fruit, researchers have come up with an accurate method for diagnosing the stages of this form of skin cancer.

Study finds that protein puts the brakes on melanin

May 27, 2016

A year and a half ago, researchers at Brown University found a molecular gas pedal for melanin production. Now they've found a brake. For scientists, the finding deepens not only the basic understanding of how eyes, skin ...

A treatment for one form of albinism?

September 26, 2011

Individuals with oculocutaneous albinism, type 1 (OCA1) have white hair, very pale skin, and light-colored irises because they have none, or very little, of the pigment melanin in their skin, hair, and eyes. Affected individuals ...

Recommended for you

Nanoscale Lamb wave-driven motors in nonliquid environments

March 19, 2019

Light driven movement is challenging in nonliquid environments as micro-sized objects can experience strong dry adhesion to contact surfaces and resist movement. In a recent study, Jinsheng Lu and co-workers at the College ...

OSIRIS-REx reveals asteroid Bennu has big surprises

March 19, 2019

A NASA spacecraft that will return a sample of a near-Earth asteroid named Bennu to Earth in 2023 made the first-ever close-up observations of particle plumes erupting from an asteroid's surface. Bennu also revealed itself ...

The powerful meteor that no one saw (except satellites)

March 19, 2019

At precisely 11:48 am on December 18, 2018, a large space rock heading straight for Earth at a speed of 19 miles per second exploded into a vast ball of fire as it entered the atmosphere, 15.9 miles above the Bering Sea.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.