Non-linearity of the collagen triple helix in solution and implications for collagen function.

Kenneth T Walker,Barbara Brodsky,Anthony C Bishop,Stephen J Perkins,Ruodan Nan,Jayesh Gor,George I Makhatadze,David W Wright

doi:10.1042/bcj20170217

Abstract

Collagen adopts a characteristic supercoiled triple helical conformation which requires a repeating (Xaa-Yaa-Gly)n sequence. Despite the abundance of collagen, a combined experimental and atomistic modelling approach has not so far quantitated the degree of flexibility seen experimentally in the solution structures of collagen triple helices. To address this question, we report an experimental study on the flexibility of varying lengths of collagen triple helical peptides, composed of six, eight, ten and twelve repeats of the most stable Pro-Hyp-Gly (POG) units. In addition, one unblocked peptide, (POG)10unblocked, was compared with the blocked (POG)10 as a control for the significance of end effects. Complementary analytical ultracentrifugation and synchrotron small angle X-ray scattering data showed that the conformations of the longer triple helical peptides were not well explained by a linear structure derived from crystallography. To interpret these data, molecular dynamics simulations were used to generate 50 000 physically realistic collagen structures for each of the helices. These structures were fitted against their respective scattering data to reveal the best fitting structures from this large ensemble of possible helix structures. This curve fitting confirmed a small degree of non-linearity to exist in these best fit triple helices, with the degree of bending approximated as 4–17° from linearity. Our results open the way for further studies of other collagen triple helices with different sequences and stabilities in order to clarify the role of molecular rigidity and flexibility in collagen extracellular and immune function and disease.

Highlights

Collagen is the most abundant protein in the human body, providing required structural and mechanical properties, as well as cell signalling, to tissues such as tendon, skin and cartilage
In order to understand collagen flexibility better, we have here reported a multidisciplinary experimental study on varying lengths of collagen triple helices composed of the most stable Pro-Hyp-Gly units, the results of which were compared with atomistic molecular dynamics (MD) simulations
Complementary analytical ultracentrifugation (AUC) and small angle X-ray scattering (SAXS) data indicated that the shortest (POG)6 triple helix was linear, while the longer triple helices were increasingly less well explained by a linear structure

Summary

Introduction

Collagen is the most abundant protein in the human body, providing required structural and mechanical properties, as well as cell signalling, to tissues such as tendon, skin and cartilage. Collagen is found as a domain in a range of host-defense proteins including C1q, collectins, and macrophage scavenger receptors. The triple helix is composed of three polyproline II-like polypeptide chains staggered by 1 residue with respect to each other [1,2,3]. The close packing of the three chains can accommodate only the smallest residue Gly in the center, generating the requirement for Gly as every third residue in the triplet repeats of the collagen amino acid sequence, (Xaa-Yaa-Gly)n. The unique supercoiled triple helix conformation forms an elongated rod-like domain, which often associates to form fibrils or other supramolecular structures in many biologically important collagens and host defence proteins [4,5]

Methods

Results

Conclusion