Molecular ‘staples’ resolve puzzle of how collagen stays together

 


We’re held together by collagen. Structured like a twisted rope, the fibrous macromolecule accounts for 15% to 20% of the protein in our bodies, and it plays an essential role in mechanically supporting our cells and tissues. But surprisingly, collagen is inherently unstable at body temperature. Now, results recently detailed at the American Physics Society’s (APS’s) Global Summit finally reveal how collagen holds its form: clusters of molecular “staples” formed by sulfur-bearing amino acids.

Identifying this key reason collagen holds together marks an “important step toward understanding the biological puzzle” of the protein, says Jing Xu, a biophysicist at the University of California, Merced who wasn’t involved with the study. The finding may also offer new tools for bioengineering and regenerative medicine.

Collagen is a threadlike protein made up of three intertwined chains of amino acids, twisted together into a triple-helix structure. Fibers made of these helices then assemble into networks, which in turn form robust scaffolds that hold cells in place—all while remaining malleable enough to respond to changes in the environment. In fact, this malleability helps collagen strands transmit information that regulates cell behavior, by letting the fibrous network convert mechanical stimuli into biochemical signals.

The conundrum is that whereas most proteins hold their folded form well above body temperature, collagen—folded and twisted into its triple-helix form—does not. In 2002, researchers showed that in humans, collagen’s unfolding or “melting” point hovers just below 37°C, our average body temperature. Bizarrely, the same is true for a vast array of other organisms with different body temperatures: Collagen’s melting point scales with an animal’s temperature, with fish collagen unfolding at much lower temperatures than human collagen does.

Past studies had examined which amino acids make up the long and complex sequence of collagen fibers, but these studies couldn’t reveal how the protein’s underlying triple helix folds and unfolds, because of the difficulty of observing such small-scale processes in such a bulky structure. Moreover, researchers couldn’t learn more from repeatedly untwisting and twisting collagen, as its untwisting takes hours to reverse, an agonizingly long time in an experimental setting.

To figure out how collagen gets its twist, Forde and Alaa Al-Shaer—then a Ph.D. student in Forde’s lab—used a method called atomic force microscopy to image at an atomic level hundreds of collagen triple helixes as the structures fell apart at different temperatures. Al-Shaer and Forde found that over the course of 1 hour, collagen gradually lost its structure at 37oC, shortening in length as it collapsed into an unstructured blob. And subsequent images also let the researchers trace exactly which parts of the collagen protein helped the bigger strands gradually relengthen and twist back together when the molecules were allowed to cool.

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