Environmentally controlled curvature of single collagen proteins

TL;DR

This study uses atomic force microscopy and a new chain tracing algorithm to analyze collagen's mechanical properties, revealing that collagen's conformation is strongly influenced by salt and pH through induced curvature, not flexibility changes.

Contribution

It introduces a modified polymer model incorporating innate curvature to accurately describe collagen's conformational behavior in two dimensions.

Findings

Collagen types I, II, and III have similar molecular flexibilities.

Salt and pH modulate collagen conformation via curvature, not flexibility.

Collagen behaves as a semiflexible polymer regardless of source or environment.

Abstract

The predominant structural protein in vertebrates is collagen, which plays a key role in extracellular matrix and connective tissue mechanics. Despite its prevalence and physical importance in biology, the mechanical properties of molecular collagen are far from established. The flexibility of its triple helix is unresolved, with descriptions from different experimental techniques ranging from flexible to semirigid. Furthermore, it is unknown how collagen type (homo- vs. heterotrimeric) and source (tissue-derived vs. recombinant) influence flexibility. Using SmarTrace, a chain tracing algorithm we devised, we performed statistical analysis of collagen conformations collected with atomic force microscopy (AFM) to determine the protein's mechanical properties. Our results show that types I, II and III collagens - the key fibrillar varieties - exhibit molecular flexibilities that are very similar. However, collagen conformations are strongly modulated by salt, transitioning from compact to extended as KCl concentration increases, in both neutral and acidic pH. While analysis with a standard worm-like chain model suggests that the persistence length of collagen can attain almost any value within the literature range, closer inspection reveals that this modulation of collagen's conformational behavior is not due to changes in flexibility, but rather arises from the induction of curvature (either intrinsic or induced by interactions with the mica surface). By modifying standard polymer theory to include innate curvature, we show that collagen behaves as an equilibrated curved worm-like chain (cWLC) in two dimensions. Analysis within the cWLC model shows that collagen's curvature depends strongly on pH and salt, while its persistence length does not. Thus, we find that triple-helical collagen is well described as semiflexible, irrespective of source, type, pH and salt environment.