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Sci Rep. 2018 Sep 14;8(1):13809. doi: 10.1038/s41598-018-31937-x.

Proline provides site-specific flexibility for in vivo collagen.

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Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.
Leibniz-Forschungsinstitut für Molekulare Pharmakologie (FMP) im Forschungsverbund Berlin e.V., Campus Berlin-Buch, Robert-Rössle-Str 10, 13125, Berlin, Germany.
Northwestern University, 633 Clark St, Evanston, IL, 60208, USA.
Department of Biochemistry, University of Cambridge, Downing Site, Cambridge, CB2 1QW, UK.
University of Calgary, 2500 University Dr. NW, Calgary, Alberta, T2N 1N4, Canada.
Institute of Infection and Immunity, School of Medicine, Cardiff University, Cardiff, CF14 4XN, UK.
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.


Fibrillar collagens have mechanical and biological roles, providing tissues with both tensile strength and cell binding sites which allow molecular interactions with cell-surface receptors such as integrins. A key question is: how do collagens allow tissue flexibility whilst maintaining well-defined ligand binding sites? Here we show that proline residues in collagen glycine-proline-hydroxyproline (Gly-Pro-Hyp) triplets provide local conformational flexibility, which in turn confers well-defined, low energy molecular compression-extension and bending, by employing two-dimensional 13C-13C correlation NMR spectroscopy on 13C-labelled intact ex vivo bone and in vitro osteoblast extracellular matrix. We also find that the positions of Gly-Pro-Hyp triplets are highly conserved between animal species, and are spatially clustered in the currently-accepted model of molecular ordering in collagen type I fibrils. We propose that the Gly-Pro-Hyp triplets in fibrillar collagens provide fibril "expansion joints" to maintain molecular ordering within the fibril, thereby preserving the structural integrity of ligand binding sites.

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