How amino acids drive mussel power

The distance between lysine and dopa decides how bivalves hold their own against even the mightiest of waves

Nipuni Dineesha

The ability to stick different things together is useful anywhere.

It is a bit harder, though for man-made glues to work under water. And yet, underwater creatures – corals, mussels, oysters, barnacles et al – have been using their variety of adhesives to stick to surfaces for eons.

In fact, mussels can adhere so strongly to a rock that if you try to separate them, it is more likely that the rock will fall apart before the mussel comes off. This allows them to stave off strong tides, fierce waves, and storms.

Why mussels matter

Mussels are bivalves, mollusks of a group with a symmetrical, elongated body, and they often have “beards,” called byssa, strings with a high percentage of collagen wrapped with strengthening proteins. They are produced by byssal glands in the mussel’s foot.

Collagen is also what we find in our skin, bone and connective tissues. Each byssal thread is extra stretchable at one end, and fixed at the other. The flat part that attaches to the rock is where the adhesive does its work.

A small forest of trees have been sacrificed to study the secret of the mussel’s bonding power. After all, wouldn’t it be nice to have a great superglue that works in every circumstance, including underwater?

That is what researchers from the Pohang University of Science and Technology (POSTECH) and the Kangwon National University, Republic of Korea, wanted to know. In a recently published article, a team led by Hyung Joon Cha, head of the chemical engineering department at POSTECH, addressed just that.

The plaques on bivalves

Mincheol Shin, a PhD fellow at the Magic Lab and the first author of the article, explained to Truly Curious why they decided to study mussels.

“Mussel attachment is special because it can form the same sturdy attachment on rocks, the metal surfaces of ships, or Styrofoam,” Shin said. This is possible because the sticky stuff that holds mussels in place, the plaque, is enriched with adhesive proteins.

Hence, the goal of the research team was to study these proteins and reproduce mussel’s adhesion properties. With the key components of the mussels’ ‘super glue’ identified, scientists would be able to create new powerful adhesives to be used in medical applications and tissue engineering, a field dedicated to the study and development of biological tissues.

“Adhesives are really friendly materials to humans. Almost all things use adhesives to connect and become complex,” Shin said. This applies to a variety of biomedical fields.

A major problem when selecting adhesives for medical applications is biocompatibility, meaning the ability to do its job without harming living tissues. Since mussel adhesive proteins are biomolecules of natural origin, scientists believe they may work better than synthetic materials in humans.

Dopa and lysine

Proteins are essentially long chains of small building blocks called amino acids that fold into 3-D shapes. Scientists can see the order of amino acids strung together in the protein to replicate it in the laboratory.

According to Shin, many studies have shown that an amino acid called 3,4-dihydroxyphenylalanine, also known as dopa, is responsible for mussel adhesion.

Dopa is not a common amino acid in living cells, but it is found in the adhesive proteins the mussels use to plug themselves to rocks. When it was first found in the mussel adhesive, scientists thought dopa was what made the mussels hang on. But many studies obstinately rejected that idea.

“Usually, nature uses a systematic approach like a union of various type of molecules, like lysine or arginine or other amino acids, alongside dopa,” Shin explained. So the team started hunting for any other amino acid that could be working in tandem with dopa.

They knew that previous studies had identified that lysine, another amino acid, was also a major component of the mussels’ adhesive proteins. Lysine and dopa often tend to hang around in pairs. Scientists over the world, including Cha’s team, wanted to see if these amino acids were working together in the mussel adhesive, too. The researchers found that, they indeed do cooperate.

Opposite ions that stick together

The next step was to understand what exactly was going on.

“We tried to find what is happening when a dopa and lysine pair is together in the neighborhood, and when [they are] separate. It is related to forming cation–π interactions, which are a very important [type of] non-covalent interaction that is commonly found in nature,” Shin explained.

Cation–π interactions are very important in nature. They are what hold together most proteins. These stabilizing interactions come from some strong attraction – when the positive and negative charges of different molecules come together.

In the case of the mussel adhesive, the positive charge is on lysine, an amino acid needed by all animals but naturally produced by none, while the negative one is found on catechol, which is part of the dopa molecule.

Testing the concept

To study how things worked in mussels’ adhesive proteins, Cha’s team designed and tested several peptides based on their structure. A peptide is two or more amino acids linked in a chain. Among other variations, they made peptides had either lysine and dopa in pairs, or separated by other amino acids.

They measured the interactions underwater, doing fine measurements of distances and forces between the two surfaces using a surface forces apparatus.

Adhesion and cohesion

Now adhesion and cohesion both relate to stickiness, but mean slightly different things.

Surface adhesion involves the interaction between the surface and the adhesive (the glue), while cohesion is about the interaction between similar molecules (in this case, the adhesive molecules themselves).

Shin et al showed that, in mussels, adhesion and cohesion are affected by the distance of lysine and dopa in the protein.

When lysine and dopa are at a distance, cohesion increases. On the other hand, when they get close, surface adhesion increases.

“The observations suggest, as we expected, [that] when lysine and dopa are in neighborhood they have negative synergy in cation–π interaction,” Shin explained.

This is probably why mussels’ adhesive proteins do not always have lysine and dopa in pairs. Using a combination of portions with lysine-dopa pairs and portions where the two amino acids are separated by other residues, mussels achieve the balance between surface adhesion and cohesion that is required for their powerful underwater adhesion.

Rocks usually have a negative charge, one that should attract lysine, which is positively charged. Opposites attract, remember – at least in magnets and charged atoms? But the rocks are usually covered with positively charged salts in the sea. The lysine with the dopa close at hand, repels the positively charged crud on the rocks, pushing it out of the way, and getting to the plain rock beneath. That’s what sticky dopa, which is close at hand, can stick to.

According to Shin, “With these findings, we can now design adhesive molecules, [changing] the molecular positions of lysine and dopa to obtain various adhesive qualities. If you need more surface adhesion, then you can put these molecules together; if you need more cohesion, you need to separate these molecules.”

Jumping those hurdles

Shin et al used nuclear magnetic resonance (NMR) and Raman spectroscopy to measure how lysine and dopa may work together.

In both techniques, energy excites molecules within a sample. In NMR, the energy comes from radio waves; in Raman spectroscopy from other light, such as one provided by a laser. The result is a spectrum that experts get meaningful chemical and structural information from.

The team wanted to use more complicated options, but had to settle for what was easily possible.

“Actually, I wanted to use solid-state NMR, or molecular dynamics or DFT [density functional theory] studies. We did not [get] the opportunity to access any of those. So I chose the alternative, which is Raman spectroscopy,” Shin said. Molecular dynamics and DFT are computational methods used to study molecules and other entities.

Cha’s team is now trying to produce mussel adhesive protein in the bacteria Escherichia coli. That involves putting the fragment of DNA with the information to produce the correct protein. DNA, of course, is the molecule that contains the genetic information of organisms. If the bacteria indeed produces adhesive protein, we, too, could get to use an adhesive that can work underwater.

Nipuni Dineesha is earning her master’s degree in the Department of Advanced Science and Technology Convergence at Kyungpook National University, South Korea.

Editor Juliana Campos is a researcher with a PhD in biochemistry from the University of Aveiro, Portugal