Marine bioinorganic materials: mussels pumping iron

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The oceans are filled with an amazing variety of biological materials including the glues and cements of mussels, barnacles, tube worms, algae, and starfish. Recent studies on mussel adhesive are providing increasing evidence for a unique mechanism of material generation involving iron-induced protein oxidation and cross-linking chemistry. Insights are also being gathered on many of the other marine creatures producing adhesives. Beyond understanding biology, this growing knowledge is inspiring application development. New classes of biomimetic polymers are poised to provide the next generation of surgical adhesives and orthopedic cements.

Section snippets

Introduction: chemistry at the beach

If you want to get an idea of all the amazing chemistry taking places in the seas, take a trip to the tidepools of a rocky coastline. You’ll see mussels, barnacles, sea urchins, starfish, anemones, seaweed, tube worms, and limpets, all clinging to the rocks (Figure 1). For the most part, all of these creatures are making glues or cements for staying in place. By doing so they cluster together for reproduction, signal to larvae the presence of sufficient food supply, prevent being knocked around

The mussel adhesive system

When you are there at the beach, take a close look at how blue mussels (Mytilus edulis) stick. You will see the byssal adhesive assembly (or ‘byssus’ or ‘beard’), much like that shown in Figure 2. Mussels affix themselves to surfaces by depositing a series (∼10–40) of small (∼2 mm diameter, ∼0.1 mm thick) adhesive plaques, each connected to the animal using a long thread. An impressive amount of force is needed to break the adhesive bonding. Mussels can even stick to Teflon (Figure 2). Recently

DOPA proteins make the adhesive

One interesting facet of this biomaterial is the presence of 3,4-dihydroxyphenylalanine (DOPA, Figure 3) [5]. This unusual amino acid is in the five adhesive proteins at levels up to 27% of total residue content [5]. Plaques are constructed from a hierarchy of some DOPA proteins contacting the surface and others forming an outer, protective coating [3, 6, 7]. Protein-bound DOPA is a result of post-translational oxidation of tyrosine. Generation of mussel glue is dependent on cross-linking of

Metals in the glue

Another fascinating aspect of this glue is that plaques are particularly rich in metals. Iron, zinc, and, to a lesser extent, copper and manganese are found in the adhesive at significantly higher levels than that of open seawater. Transition metals in seawater are essential to sustaining life, in general, but are typically found at low concentrations such as ∼10 parts per billion (ppb) for iron and zinc [8]. Mussel adhesive, by contrast, can contain Fe and Zn levels over 1 part per thousand

Oxidative protein cross-linking

Key to understanding how mussels produce their adhesive is insight on the reactions and products of DOPA protein cross-linking. The majority of this work has focused upon oxidative means of protein coupling. Relative to the standard 20 amino acids, DOPA is easily oxidized, thus coupling via semiquinone or quinone intermediates may be considered [5]. The DOPA-containing mussel adhesive precursor proteins cross-link upon reaction with the oxidant NaIO4 [11, 12, 13] or H2O2 [14]. Enzymatic

Metal–DOPA interactions in mussel adhesive

Potential roles for the involvement of metals in mussel protein cross-linking have also been considered. Both a mussel adhesive precursor protein [17] and hydrolyzed peptides thereof [18] have been shown to chelate iron through DOPA. Copper(II) can aggregate DOPA proteins and decrease viscoelasticity via interprotein chelation [11, 13]. When examined by atomic force microscopy, the adhesive energy of a mussel protein was increased upon addition of Fe3+ [19]. The affinity of iron for DOPA and

Bulk cross-linking of adhesive proteins

The spectroscopic results described above show the presence of iron-induced radical formation and cross-linking in mussel adhesive. Next, we wanted to move from a perspective of molecular reactivity up to a larger scale in which the influence of metals upon bulk mechanical properties could be probed. After extracting adhesive proteins from mussels and generating a viscous hydrogel, this protein matrix was reacted with a library of potential cross-linking agents including metal ions, oxidants,

Proposing a mechanism of adhesive formation

By combining the insights gained from both mechanical and chemical experiments we can propose a mechanism for how mussels may produce their fascinating adhesive (Figure 5). DOPA proteins are deposited onto a surface and could be mixed with Fe3+. When binding the catechol-like side chain of DOPA the Fe3+ ion brings together multiple protein chains, specifically into a tris Fe(DOPA-protein)3 configuration, to begin the cross-linking (Figure 5). By analogy to the catechol dioxygenase enzymes [32]

Pumping iron: transport to the adhesive

We may wonder how mussels are able to collect so much iron in their adhesive plaques. You can imagine that the strong chelating ability of DOPA [18] accumulates metal ions from the surrounding seawaters directly. However, isotopic labeling studies have shown that iron gets into the glue via the mussel's feeding system [33, 34]. Most iron in seawater is particulate, not dissolved. Mussels are filter feeders and get all their nutrients (e.g. phytoplankton) by pumping and straining nearly 40 l of

More fish in the sea: additional marine biological materials

Beyond mussels, there are many other amazing marine biological materials that are being studied (Figure 1). Barnacle cement is also composed of a cross-linked protein matrix, although the precursor proteins do not contain DOPA [37, 38]. We can imagine that noncovalent protein secondary structure [38, 39, 40] or oxidative cross-linking via cysteine thiol  disulfide chemistry [38, 40, 41] may be at play. Interestingly, cysteine residues are rather rare in mussel proteins. A recent study has shown

Applications and synthetic mimics

In addition to revealing the secrets of how marine biology makes materials, these characterization insights may provide a starting point for developing new man-made technologies. There is a great need to prevent adhesion of these organisms. With approximately 50 000 large commercial ships traversing the world's oceans, you can well imagine that fouling of hulls by barnacles, seaweeds, tube worms, and the like is quite undesirable from speed and fuel consumption standpoints. Current antifouling

Conclusions

Marine biology provides us with a wealth of research topics, as we try to understand how Nature synthesizes materials. Mussels present an interesting case in which a unique iron-induced cross-linking mechanism may be at the center of their adhesive formation. This knowledge is currently being used to develop the next generation of biomedical adhesives. With both characterization and biomimetic efforts ongoing, many exciting research avenues await.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Many thanks are due to all the undergraduate, graduate, and postdoctoral researchers, past and present, from our laboratory carrying out the work described above. Jeremy Burkett, Cristina Matos-Pérez, and James White were helpful in reviewing this paper. Our laboratory is also grateful to the Arnold and Mabel Beckman Foundation, the Alfred P. Sloan Foundation, the National Science Foundation, and especially the Office of Naval Research for the support of this research.

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