Structural Basis for the De-N-acetylation of Poly-β-1,6-N-acetyl-d-glucosamine in Gram-positive Bacteria

Abstract

Background: IcaB is a poly-␤-1,6-N-acetyl-D-glucosamine (PNAG) deacetylase required for polysaccharide intercellular adhesion-dependent biofilm formation by staphylococci. Results: The structure of Ammonifex degensii IcaB has been determined and its catalytic mechanism and localization characterized. Conclusion: IcaB is a membrane-associated PNAG deacetylase that uses an altered catalytic mechanism relative to other family 4 carbohydrate esterases. Significance: First structural characterization of a Gram-positive PNAG deacetylase.

The Protein BpsB Is a Poly-β-1,6-N-acetyl-d-glucosamine Deacetylase Required for Biofilm Formation in Bordetella bronchiseptica

“…3A). BpsB(35-307) MMS contains the same circular permutation of the conserved CE4 motifs present in PgaB and IcaB (30,35). However, two topological differences occur between BpsB(35-307) MMS and PgaB .…”

“…Conserved Residues in BpsB Are Required for PNAG Deacetylation-Previous studies on PgaB (30,31), IcaB (35,47), Vibrio cholerae chitin deacetylase (VcCDA) (59), and PgdA (60) suggest BpsB likely uses a metal-assisted general acid/base mechanism common to CE4 members. Based on a number of substrate-bound structures of VcCDA, the metal ion is required for substrate binding and coordinating the 3Ј-hydroxyl of the GlcNAc moiety and the carbonyl of the N-acetyl group, coordinating a water molecule involved in nucleophilic attack on the N-acetyl group, and assisting in the stabilization of the tetrahedral transition state intermediate (59).…”

“…Third, there are an additional 30 residues at the C terminus of BpsB that are predicted to adopt a random coil structure. Compared with IcaB, BpsB does not contain the hydrophobic residue loop L1 that is proposed to retain the enzyme to the outer leaflet of the membrane (35), and the sequence homology is predominantly based on the five canonical CE4 motifs.…”

“…3C). Our recent study of IcaB (35) suggested that His-49 is a multifunctional acid/base catalyst in the deacetylation mechanism. Interestingly, the electron density was poor for this residue in both BpsB molecules suggesting it may be inherently flexible.…”

“…The structure of BpsB shows a circularly permuted CE4 domain similar to PgaB (30,34) and IcaB (35) with several distinct structural differences. BpsB displays length-dependent PNAG deacetylation activity with the highest rates measured when Ni 2ϩ is present.…”