Heterotrimeric collagen helix with high specificity of assembly results in a rapid rate of folding

Abstract

The most abundant natural collagens form heterotrimeric triple helices. Synthetic mimics of collagen heterotrimers have been found to fold slowly, even compared to the already slow rates of homotrimeric helices. These prolonged folding rates are not understood. Here we compare the stabilities, specificities and folding rates of three heterotrimeric collagen mimics designed through a computationally assisted approach. The crystal structure of one ABC-type heterotrimer verified a well-controlled composition and register and elucidated the geometry of pairwise cation–π and axial and lateral salt bridges in the assembly. This collagen heterotrimer folds much faster (hours versus days) than comparable, well-designed systems. Circular dichroism and NMR data suggest the folding is frustrated by unproductive, competing heterotrimer species and these species must unwind before refolding into the thermodynamically favoured assembly. The heterotrimeric collagen folding rate is inhibited by the introduction of preformed competing triple-helical assemblies, which suggests that slow heterotrimer folding kinetics are dominated by the frustration of the energy landscape caused by competing triple helices. The mechanism of collagen heterotrimer folding is difficult to recapitulate synthetically. Now an ABC collagen mimetic heterotrimer has been designed that employs pairwise amino acid interactions, validated by X-ray crystallography, to promote composition- and register-specific assembly. The high specificity of its assembly leads to an increased rate of folding compared with similar collagen heterotrimers.