Biomedical Engineering Reference
In-Depth Information
fp-1
fp-2
fp-3
fp-4
fp-5
Charged
Hydrophilic
DOPA
fp-6
Fig. 9.3 Schematic illustration of proteins in the disk of mussel byssal thread and the coating
layer. All proteins contain DOPA, with its content ranging from a few to 20 mol%. Each relative
length of illustration roughly corresponds to the molecular weight of the respective proteins.
Primary structures of larger proteins are composed of tandem repetitions. Although those of
smaller proteins are not composed of clear repetition, their complexities are actually lower. No
hydrophobic component is found
molecular weight of 50 kDa. A 11 kDa-protein is assumed to link two portions of
the disk, i.e. the coupling layer and the bulk. Another 90 kDa-protein hypothetically
links the bulk of the disk with the distal end of the thread portion. A protein in the
coating layer has a molecular weight of 110 kDa.
The primary structures of two surface coupling proteins [ 14 , 15 ] appear to be
simple and do not possess clear tandem repetitions. Both proteins have the highest
3, 4-dihydroxy phenylalanine (DOPA)-contents (10-30 mol%) and have additional
residues that are posttranslationally modified, namely 4-hydroxyarginine [ 16 ] and
O -phosphoserine. On the other hand, the bulk protein in the disk is composed of
11 tandem repetitions of epidermal growth factor (EGF)-like motif flanked by
negatively charged units in both ends of the primary structure [ 14 ]. The protein
has a relatively lower content of DOPA (3-5 mol%). The primary structure of the
linking 11 kDa-protein [ 12 ] is simpler and has no clear tandem repetitions.
The protein also has a lower content of DOPA (2 mol%), whereas a high Cys
content of 11 mol% was found. The linking 90 kDa-protein [ 13 ] is rich in His with
its content reaching to 22 mol%, and its primary structure is composed of two
domains, namely a His-rich decapeptide repeat and an Asp-rich undecapeptide
repeat. The protein also has a lower content of DOPA (2 mol%). The primary
structure of the coating protein [ 17 , 18 ] is composed of 80 tandem repetitions of a
decapeptide with a DOPA content of 10-15 mol%.
On the whole, proteins in the byssal thread, except for the bulk protein, have a
lower level of complexity in their primary structures because they have biased amino
acid compositions. Each protein therefore appears to have their specific amino acid
(s) with respect to the dominance or posttranslational modifications. Based on this, the
specific amino acid(s) has been considered important for the respective function(s)
rather than conformations or primary structures of the proteins. Among them, DOPA
has been well characterized and shown to be versatile in terms of its roles in surface
coupling and the potential to form cross-linkages.
 
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