Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 178

protein soluble from the barnacle cement indicated that conformation of the
molecule as a building block and molecular interactions such as hydrophobic
interactions and hydrogen bonding are essential for curing. Molecular interactions
among the bulk proteins seem to be optimized in a manner similar to that found in
the process of amyloid-beta self-assembly [ 54 ]. Amyloid-beta is well-known for its
characteristics of forming ordered peptide self-assembly with beta-sheets. The
amyloid formation is different from a simple process of nonspecific aggregation
because the fibrils show ordered structures. This suggests the presence of a specific
pattern of molecular interactions, rather than nonspecific hydrophobic interactions,
that lead to such an ordered process. One of the bulk proteins of the barnacle cement
actually contains an amyloid-like sequence, thus a similar manner of protein-
protein interactions may partly occur in the bulk of the cement. Though a few
studies have produced speculation about the occurrence of intermolecular covalent
cross-linkage [ 55 ], no direct evidence for its involvement in the formation of the
bulk is yet known [ 56 ].

9.5.4 Interface of the Cement

Barnacles seem to employ two different types of proteins for surface functions:
Among all biological underwater adhesive proteins, the 20 kDa-protein is the only
one that has been shown to have a defined conformation [ 57 , 58 ]. The abundant
Cys-residues in the protein form intramolecular disulfide bonding, which appears to
be essential in keeping the conformation. The protein is adsorbed to specified
materials, and among these, calcified material is best. Calcified shell is at least
one of two foreign materials that the barnacle cement must attach to, thus the
protein is assumed to be a specific coupling agent for the most-encountered
calcareous material.
Since barnacles attach to diversified foreign materials, a function for surface
coupling to unspecified materials is required. This has been suggested to be fulfilled
by the 19 kDa-protein [ 53 ]. The protein actually adsorbs to various materials with
almost a monolayer amount [ 59 ]. Materials to be coupled include metals/metal
oxides and synthetic polymers. The protein might not exhibit a defined secondary
structure. Thus, the barnacle employs different types of surface coupling proteins,
one with a defined 3D-structure for coupling to a specific surface, and the other with
a more flexible structure for coupling to unspecified materials. Although the
function of the 68 kDa-protein is unknown, it is intriguing that it has an abundant
number of amino acids similar to the 19 kDa-protein. Since underwater attachment
is a multi-functional process [ 1 ], it may not be accomplished by a simple addition of
two functions, such as surface coupling and setting. It is possible that in-depth
characterization of each component might make the obscure multi-functional
underwater attachment process more clear.