How enzymes build sugar trees

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The ALG6 enzyme is made up of a structurally conserved (green) and a structurall

The ALG6 enzyme is made up of a structurally conserved (green) and a structurally variable (red) module. The antigen-binding fragment (purple, cyan) that attaches to the enzyme made it possible to study the latter using cryo-EM. (Image: ETH Zurich)

Researchers have used cryo-electron microscopy to elucidate for the first time the structure and function of a very small enzyme embedded in cell membranes. This enzyme builds complex sugar trees that are subsequently attached to other membrane proteins. The findings could accelerate the development of new, protein-based medications.

A record in cryo-electron microscopy

The researchers also see their results as a breakthrough in determining the molecular structures of proteins using cryo-electron microscopy (cryo-EM). In 2017, the Swiss researcher Jacques Dubochet received the Nobel Prize in Chemistry for his contribution to this groundbreaking technology, which has since become the method of choice for the structural elucidation of large molecular complexes.

Determining the structures of small proteins at high resolution, especially those embedded in membranes, had not been possible using cryo-EM because measurements taken of particles below a certain mass do not permit precise structural calculations.

Working together with a research group from the University of Chicago, the ETH team have now found a solution to this problem. In collaboration with the Chicago-based researchers they produced a synthetic antibody that binds to ALG6. This antibody increased the mass of the ALG6 enzyme such that its structure could be determined in high resolution using cryo-EM.

"With our approach, we currently hold the world record for the highest structural resolution obtained for a membrane-bound complex of this size," Locher says with a hint of pride. He explains that these advances with cryo-EM will enable many other scientists to elucidate the structures of small membrane proteins: "Our approach paves the way for the scientific community to make rapid progress in the study of membrane proteins associated with a wide range of diseases."

A chemo-enzymatic toolbox

As if that were not enough, the ETH researchers, in collaboration with chemists from the University of Bern, have now developed methods for synthesising highly complex lipid-linked oligosaccharides in the lab - something that had not been possible with conventional synthesis methods in organic chemistry.

As a result, the researchers have now gained new insight into the essential cellular pathway of LLO biosynthesis, helping them explain how cells build complex glycans. "This marks a milestone in glycobiology that could form the basis for many glycobiologists’ future research and for the production of glycoproteins," Locher says.


Bloch JS et al. Structure and mechanism of the ER-based glucosyltransferase ALG6. published online 26th Feb 2020. doi: 10.1038/s41586-020-2044-z

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Sticking sugar to protein (ETH News 08.02.2018)