Flagellin glycosylation with pseudaminic acid in Campylobacter and Helicobacter: prospects for development of novel therapeutics

Abstract

Many pathogenic bacteria require flagella-mediated motility to colonise and persist in their hosts. Helicobacter pylori and Campylobacter jejuni are flagellated epsilonproteobacteria associated with several human pathologies, including gastritis, acute diarrhea, gastric carcinoma and neurological disorders. In both species, glycosylation of flagellin with an unusual sugar pseudaminic acid (Pse) plays a crucial role in the biosynthesis of functional flagella, and thereby in bacterial motility and pathogenesis. Pse is found only in pathogenic bacteria. Its biosynthesis via six consecutive enzymatic steps has been extensively studied in H. pylori and C. jejuni. This review highlights the importance of flagella glycosylation and details structural insights into the enzymes in the Pse pathway obtained via a combination of biochemical, crystallographic, and mutagenesis studies of the enzyme-substrate and -inhibitor complexes. It is anticipated that understanding the underlying structural and molecular basis of the catalytic mechanisms of the Pse-synthesising enzymes will pave the way for the development of novel antimicrobials.

Keywords: Bacterial flagellum; Bacterial motility; Bacterial virulence; Biosynthetic pathway; Crystal structure; O-linked glycosylation; Small molecule inhibitor.

Fig. 1

Structures of pseudaminic acid and legionaminic acid

Fig. 2

The Pse biosynthesis pathway in H. pylori and C. jejuni [129]

Fig. 3

Cartoon representation of the structure of H. pylori PseB in complex with nicotinamide adenine dinucleotide phosphate (NADP⁺) and uridine-diphosphate-N-acetylglucosamine (PDB ID: 2GN6 [132]). The part of the N-terminal domain that has the Rossmann fold is coloured cyan, and the additional four β-strands are coloured wheat. The NADP molecule is drawn as black sticks, the substrate is drawn using a ball-and-stick representation

Fig. 4

Cartoon representation of the structure of the H. pylori PseC dimer (PDB ID: 2FNU [136]). The N- and C-terminal domains of one of the two monomers are coloured green and magenta, respectively. The β-hairpin involved in domain swapping is coloured cyan; the bound pyridoxamine-5′-phosphate (PMP) cofactor is drawn as black sticks; the product UDP-4-amino-4,6-dideoxy-β-l-AltNAc (UD1) is shown using a ball-and-stick representation. The star (*) indicates domains from the second monomer of the PseC homodimer

Fig. 5

Cartoon representation of the structure of H. pylori PseH in complex with AcCoA (PDB ID: 4RI1 [129]). The motifs that are conserved across all GNAT enzymes are coloured as follows: motif C—green, motif D—blue, motif A—red, motif B—magenta. Non-conserved N-terminal and C-terminal regions are coloured wheat. The AcCoA cofactor is drawn in black using a stick representation

Fig. 6

Cartoon representation of the structure of C. jejuni PseG in complex with uridine-5′-diphosphate (UDP) (PDB ID: 3HBN) [142]. The N-terminal domain (red) is connected with the C-terminal domain (green) by an α-helix (magenta coloured). The UPD molecule is drawn as black sticks

Fig. 7

Cartoon representation of the structure of N. meningitidis NeuB in complex with substrate analog N-acetylmannosaminitol (rManNAc), phosphoenolpyruvate (PEP) and Mn²⁺ (PDB ID: 1XUZ [147]). The linker region and antifreeze protein-like (AFPL) domain are coloured magenta and cyan, respectively. The PEP cofactor is drawn as black sticks, whereas the rManNAc is shown using a ball-and-stick representation; the Mn²⁺ ion is shown as a blue sphere

Fig. 8

Cartoon representation of the structure of the N. meningitidis CNS homodimer in complex with cytidine-5′-diphosphate (CDP) (PDB ID: 1EYR [151]). The dimerisation hairpin (HP) domain is coloured magenta. The substrate analog CDP is shown using a stick representation