Chitin, Chitin Oligosaccharide, and Chitin Disaccharide Metabolism of Escherichia coli Revisited: Reassignment of the Roles of ChiA, ChbR, ChbF, and ChbG

Abstract

<i>Escherichia coli</i> is unable to grow on polymeric and oligomeric chitin, but grows on chitin disaccharide (GlcNAc-GlcNAc; <i>N,N</i>′-diacetylchitobiose) and chitin trisaccharide (GlcNAc-GlcNAc-GlcNAc; <i>N,N</i>′<i>,N</i>′′-triacetylchitotriose) via expression of the <i>chb</i> operon (<i>chbBCARFG</i>). The phosphotransferase system (PTS) transporter ChbBCA facilitates transport of both saccharides across the inner membrane and their concomitant phosphorylation at the non-reducing end, intracellularly yielding GlcNAc 6-phosphate-GlcNAc (GlcNAc6P-GlcNAc) and GlcNAc6P-GlcNAc-GlcNAc, respectively. We revisited the intracellular catabolism of the PTS products, thereby correcting the reported functions of the 6-phospho-glycosidase ChbF, the monodeacetylase ChbG, and the transcriptional regulator ChbR. Intracellular accumulation of glucosamine 6P-GlcNAc (GlcN6P-GlcNAc) and GlcN6P-GlcNAc-GlcNAc in a <i>chbF</i> mutant unraveled a role for ChbG as a monodeacetylase that removes the <i>N-</i>acetyl group at the non-reducing end. Consequently, GlcN6P- but not GlcNAc6P-containing saccharides likely function as coactivators of ChbR. Furthermore, ChbF removed the GlcN6P from the non-reducing terminus of the former saccharides, thereby degrading the inducers of the <i>chb</i> operon and facilitating growth on the saccharides. Consequently, ChbF was unable to hydrolyze GlcNAc6P-residues from the non-reducing end, contrary to previous assumptions but in agreement with structural modeling data and with the unusual catalytic mechanism of the family 4 of glycosidases, to which ChbF belongs. We also refuted the assumption that ChiA is a bifunctional endochitinase/lysozyme ChiA, and show that it is unable to degrade peptidoglycans but acts as a bona fide chitinase in vitro and in vivo, enabling growth of <i>E. coli</i> on chitin oligosaccharides when ectopically expressed. Overall, this study revises our understanding of the chitin, chitin oligosaccharide, and chitin disaccharide metabolism of <i>E. coli</i>.