Three Conformationally Distinct Domains in the Amino‐Terminal Segment of Type III Procollagen and Its Rapid Triple Helix ⇄ Coil Transition

Abstract

Peptide Col 1–3 which comprises the whole amino‐terminal precursor‐specific region of bovine type III procollagen was isolated after digestion of type III procollagen with bacterial collagenase at 37 °C. Further treatment of Col 1–3 with collagenase at 55 °C released two non‐collagenous segments, Col 1 and Col 2. Ultracentrifugal and circular dichroism (CD) studies indicated that Col 1–3 consists of three chain constituents associated to each other by both disulfide bridges and non‐covalent bonds. The three interchain disulfide bridges are located in fragment Col 2, i.e. near the carboxyl end of the precursor‐specific region. The CD spectrum indicates a random coil structure for peptide Col 2. Peptide Col 1 possesses five intrachain disulfide bridges and shows a CD spectrum indicating aperiodic structural elements and β structure. The non‐covalent association of the peptide chains is due to a triple helix formed from a collagenous segment (Col 3) located between the Col 1 and Col 2 domains of the peptide. This triple helix has a melting temperature of 53 °C which decreases by about 20 °C when the peptide is reduced and S‐carboxymethylated. The thermal transition of Col 1–3 is accounted for by a non‐staggering zipper model without nucleation difficulty, using a standard enthalpy of triple helix formation of δHθ=− 35 kJ/mol tripeptide unit or by an all or none mechanism with δHθ=− 12.5 kJ/mol tripeptide units. Refolding of the helix from denatured Col 1–3 occurred in 100% yield with a rate constant of 8 × 10−3 S−1 at 20 °C. This is the highest rate constant yet observed for triple helix formation in vitro. Prior reduction of disulfide bonds prevented rapid helix formation. Copyright © 1978, Wiley Blackwell. All rights reserved