How water holds us together

Water determines life and humans are three-quarters water, but an international research team has now discovered how water also determines the structure of the material that holds us together: collagen.

In a paper recently published in PNAS, the researchers, led by the University of Amsterdam, explain the role of water in the molecular self-assembly of collagen. They show that by replacing water with its ‘twin molecule’ - heavy water (D2O) - one can fine-tune the interaction between collagen molecules, influencing the process of collagen self-assembly.

The findings will help to better understand the tissue failures resulting from heritable collagen-related diseases, such as brittle bone disease.

"In studying these and other collagen diseases ... researchers ... have always missed an important part of the puzzle."

Dr Giulia Giubertoni, University of Amsterdam

Dr Giulia Giubertoni of the University's Van ’t Hoff Institute for Molecular Sciences (HIMS) says: "In studying these and other collagen diseases, many researchers, including myself, myself have always missed an important part of the puzzle, and the possibility that tissue failure might be partly due to water-collagen interaction was not taken very seriously. We now show that perturbing the water layer around the protein, even very slightly, has dramatic effects on collagen assembly."

The researchers also suggest that altered interactions between water and collagen are a contributing factor in various age-related diseases involving tissue dysfunction.

Collagen is to a large extent the stuff of which humans are made - around a third of all protein in our bodies is collagen, which ensures the mechanical integrity of all human connective tissue. For instance, our skin and arteries stretch without tearing and our bones can resist high stress without breaking.

Artist's impression of the structure of collagen, comprising single proteins that assemble into fibrils, which bundle into networks that form the scaffold of human tissues. Image: HIMS / Laura Canil, Giulia Giubertoni

Collagen is produced by our cells as single proteins that assemble into larger structures called fibrils. These fibrils further assemble into networks that form the scaffolds for our tissues.

Since collagen is formed in the aqueous environment of human cells, water plays a crucial role in its assembly. The interaction of water molecules with proteins results in collagen that is best suited for its function.

But what exactly is behind this collagen-optimising role of water? How does water do it? And will understanding this mechanism offer insights into conditions where something is wrong with collagen, such as brittle bone disease?

To investigate the role of water in collagen formation, Dr Giubertoni and colleagues decided to replace water (H2O) with its heavier ‘twin molecule’ D2O. In D2O the hydrogen (H) atoms are replaced with the isotope deuterium (D), which has an added neutron in its nucleus.

This makes D2O 'heavy water’, the closest replacement to ordinary water in nature.

However, in interaction with proteins, D2O is less potent than H2O. This is because bonds between D2O molecules are stronger than those between H2O molecules, which affects interactions with proteins such as collagen.

The researchers were keen to study the effect this would have on collagen assembly. Together with a multi-disciplinary collaborative research network, they were able to establish that the use of heavy water results in ten times faster collagen formation, and ultimately a less homogeneous, softer and less stable collagen-fibre network.

The explanation is that the reduced interaction of the heavy water with the collagen protein makes it easier for the protein to ‘shake off’ the D2O molecules and reorganise itself.

Water as mediator

This boosts the formation of the collagen network, but also results in a sloppier, less optimal collagen network. Water thus acts as a mediator between collagen molecules, slowing down the assembly to guarantee the functional properties of living tissues.

This discovery offers fresh perspectives on how water influences the characteristics of collagen, allowing for precise adjustments in the mechanical properties of living tissues. It also creates novel avenues for creating collagen-based materials where macroscopic properties can be controlled and fine-tuned by subtle variations in the composition of the solvent, rather than making significant changes to the chemical structure of the molecular building blocks.

A similar investigative approach might also be used in the future to elucidate the role of water in driving and guiding the assembly of other proteins capable of assembling in larger structures. Next Giubertoni will study how defects in collagen affect its interaction with water, and what role this plays in the failure of tissue in collagen diseases.