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X-ray crystallography unveils how intestinal mucus protects cells against copper toxicity

X-ray crystallography and biochemical studies revealed that intestinal mucus binds copper, an essential but potentially dangerous dietary trace metal. The two-tiered copper binding site discovered in mucus prevents copper from consuming antioxidants such as vitamin C and producing reactive oxygen species.

Hundreds of square metres of epithelial tissue in the human body are exposed to the environment and are thus at risk of infection or chemical and physical damage. To protect tissues while enabling the exchange of gases and nutrients, specialised cells in the epithelium secrete mucus, a sticky substance consisting of a glycoprotein scaffold filled with water, ions and numerous associated proteins, many involved in innate immunity. Due to the complexity and structural heterogeneity of mucus, it has been challenging to determine the biochemical mechanisms of its protective functions.

Mucus is composed of very long glycoprotein chains known as gel-forming mucins. An important feature of mucins is their ability to become linked by disulfide bonds near their ends to form even longer polymers, which make up the mucus hydrogel (Figure 14a). Previous analysis of disulfide-linked regions of the glycoprotein by cryo- electron microscopy (cryo-EM) gave high-resolution insight into a major portion of a gel-forming mucin [1].

A surprising finding during inspection of the intestinal mucin structure was a cluster of histidine and methionine side chains, which looked like it might be a transition metal-binding site, though no metal density was seen

Fig 14: Intestinal mucin binds copper and protects cells. a) Schematic of the intestinal mucus hydrogel. b) Two-tiered copper binding site in the human intestinal mucin. X-ray crystallography revealed that Cu(II) binds in a histidine-rich site (green side chains) and Cu(I) binds at a nearby cluster of methionines (magenta side chains). c) The addition of high concentrations of copper disrupted the monolayer of cultured colon cells, as indicated by magenta arrowheads. Supplying the recombinant mucin copper-binding domain rescued the cells. Green indicates junctions between cells, blue indicates nuclei.