A 'sweet' new target

Technical University of Munich team develops a protein reagent that could identify biological sugar structures, which can be hijacked by viruses or tumor cells

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Viruses spread within a host by infecting an individual cell, replicating within it and then bursting out to infect more cells and multiply through the body. New research has shown that viruses specifically attach to the sugar structures of their target cells, and may also present with characteristic sugar structures on their own surfaces as well. To take advantage of this potential antiviral target, a research team at the Technical University of Munich (TUM), led by Arne Skerra, professor of Biological Chemistry, has developed a new kind of protein reagent. The reagent aids in identifying biological sugar structures, which in turn could be used to halt the spread of viruses by blocking sugar structures on host cells or pathogens.
Their work appeared in ChemBioChem in a paper titled “A Tetrahedral Boronic Acid Diester Formed by an Unnatural Amino Acid in the Ligand Pocket of an Engineered Lipocalin.”
According to Skerra, “The recognition of specific sugar molecules, or so-called carbohydrates, is of vital importance in many biological processes.” Regular cells and pathogens both carry sugar chains on the outer surface of the cell membrane or membrane proteins, and the body can use those to determine if cells are native or invaders.
One hurdle in the development of the new reagent was that water molecules and sugar molecules look similar, which can make identification difficult in the human body. In addition, proteins generally have a low affinity to sugars, “Hence, in order to mediate biological activities at low physiological concentrations, natural lectins use metal ions such as calcium, often together with the avidity effect that arises from multivalent target engagement,” the authors explained. As such, Skerra's lab needed to create an artificial binding protein capable of binding to biological sugar structures.
“Using the possibilities opened up by synthetic biology, we have employed an additional artificial amino acid,” said researcher Carina A. Sommer, who added that “We have succeeded in incorporating a boric acid group, which exerts intrinsic affinity to sugar molecules, into the amino acid chain of a protein. In doing this, we have created an entirely new class of binding protein for sugar molecules.”
“Our findings should not only support the future development of boronic‐acid‐based carbohydrate ligands with applications in biological chemistry but also enable further rational reshaping of the ligand pocket of the Borocalin [the artificial lectin], eventually to obtain a high‐affinity binding protein specific for medically relevant cell‐surface glycans,” the authors noted in their paper.
To test the Borocalin—“an artificial lectin [created] by incorporating p‐borono‐l‐phenylalanine (Bpa) into the ligand pocket of an engineered lipocalin,” according to the authors—the researchers expressed it in E. coli for X-ray crystallographic analysis.
“By using X-ray crystallography, we have succeeded in unraveling the crystal structure of a model complex of this artificial protein, which allowed us to validate our biomolecular concept,” stated scientist Dr. Andreas Eichinger.
“Our results should not only be used to support the future development of new carbohydrate ligands in biological chemistry, but should also pave the way for creating high-affinity agents for controlling or blocking medically relevant sugar structures on cell surfaces,” Skerra said in a press release.
It's thought that such agents could have use in slowing the spread of viruses or cancer cells.
SOURCE: Technical University of Munich press release

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