Seminars

Many cellular reactions involve both hydrophobic and hydrophilic molecules that reside within the chemically distinct environments defined by the phospholipid-based membranes and the aqueous lumens of cytoplasm and organelles.

Enzymes performing this type of reaction are required to access a lipophilic substrate located in the membranes and to catalyze its reaction with a polar, water-soluble compound. Here we focus on two enzymes involved in the early steps of the phosphatidylinositol mannosides (PIMs) biosynthetic pathway, unique glycolipids found in abundant quantities in the inner and outer membranes of the cell envelope of all Mycobacterium species. They are based on a phosphatidylinositol lipid anchor carrying one to six mannose residues and up to four acyl chains. PIMs are considered not only essential structural components of the cell envelope but also the structural basis of the lipoglycans lipomannan and lipoarabinomannan, important molecules implicated in host-pathogen interactions in the course of tuberculosis and leprosy. Of particular relevance, we demonstrate the occurrence of a conformational switch during the catalytic cycle of the retaining glycosyltransferase PimA, the enzyme that start the pathway, involving both β-strand–to–α-helix and α-helix–to–β-strand transitions. These structural changes seem to modulate catalysis and are promoted by interactions of the protein with anionic phospholipids in the membrane surface. Although scant structural information is currently available on protein catalysis at the lipid-water interface, our studies demonstrate that protein-membrane interactions might entail unanticipated structural changes in otherwise well conserved protein architectures, and suggests that similar changes may also play a functional role in other membrane enzymes. Further structural work, provided experimental evidence on the catalytic mechanism of retaining glycosyltransferases. Finally, we report the crystal structures of PatA, an essential membrane associated acyltransferase that transfers a palmitoyl moiety from palmitoyl–CoA to the 6-position of the mannose ring added by PimA, in the presence of its naturally occurring acyl donor palmitate and a nonhydrolyzable palmitoyl–CoA analog. The structures reveal an α/β architecture, with the acyl chain deeply buried into a hydrophobic pocket that runs perpendicular to a long groove where the active site is located. Enzyme catalysis is mediated by an unprecedented charge relay system, which markedly diverges from the canonical HX4D motif. Our studies establish the mechanistic basis of substrate/membrane recognition and catalysis for an important family of acyltransferases, providing exciting possibilities for inhibitor design.

SEMINAR’S DATE, TIME AND PLACE: Wednesday, March 14, 2018. 12:00. Assembly Hall

SPEAKER: Marcelo Guerín

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