| Summary: | Type I collagen and gelatin have previously been shown to form complex and simple coacervates. We have consistently observed purified fish scale collagens (FSc) from several species to liquid-liquid phase separate (LLPS) and form coacervate-like droplets upon heating from 4˚C to room temperature. The observation of this phenomena led us to investigate the composition of fish scale collagen extracts. Collagen extract was purified from scales of several fish species and submitted to gel electrophoresis, amino acid analysis, liquid chromatography-mass spectrometry (LC-MS/MS), and circular dichroism (CD) for compositional analysis. We found that the purified extracts from each fish species contained proteins consistent with collagens in structure and composition, differing only slightly from mammalian collagens as well as other marine type I collagens. Typically, soluble marine type I collagens have lower thermal stability than the corresponding collagens from found in warm-blooded mammalian relatives. Optical rotation of FSc collagen molecules at increasing temperatures indicated that the collagen triple helix is denatured at about the same temperature at which coacervation takes place in concentrated FSc extract solutions under optical and confocal microscopy. We started a preliminary sequence analysis of FSc collagen from Salmo salar to provide a potential explanation for the observed LLPS behavior. We found that salmon collagen had a striking increase in GG repeats and a compositional preference for hydroxylated residues, serine and threonine, compared with bovine collagen. These substitutions replaced primarily proline, hydroxyproline and alanine residues in the X and Y positions of the bovine collagen sequence. We propose that compositional biases for these residues contribute to the predilection of forming the two-phase state, at least in part, by increasing collagen flexibility and stabilizing water-evicting interactions between gelatin molecules.
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