Diffusion of Chitin Oligosaccharides Across the OccD-like Chitoporin in Escherichia coli

Abstract

Chitin, a biopolymer of β-1,4-glycosidic linked GlcNAc residues, is an abundant source of carbon and nitrogen for microorganisms. Chitin degradation by microorganism is achieved using a complex pathway including various enzymes and transporters. Diffusion of chitin oligosaccharides across the outer membrane of Gram-negative bacteria usually occurs through chitoporin channels. In this study we described the characterization of a novel chitoporin (so-called EcChiP) which helps to uptake chitin oligosaccharides in nonchitinolytic E. coli. Single channel recoding in Black Lipid Membrane (BLM) reconstitution technique demonstrated that EcChiP form monomeric channel with specificity towards chitooligosaccharides. The molecular mass obtained by size exclusion chromatography for the purified EcChiP was 60,000 Da, which is in good agreement with theoretical molecular weight for monomeric channel. Together with bulk permeation study by liposome swelling assays, we demonstrate that EcChiP is a sugar-specific transporter, with pronounced specificity towards long-chain chitooligosaccharides such as chitohexaose, chitopentaose and chitotetraose. Thermodynamic assessment by isothermal titration microcalorimetry (ITC) suggested that chitohexaose-EcChiP channel interactions are driven by an endothermic process, yielding a binding constant (K) value of 2.5 x105 M-1. Analysis of protein fluorescence enhancement suggested that the binding process was hydrophobic. Non-linear curve fitting of the chitohexaose titration curve from protein fluorescence enhancement yielded the binding constant (K) of 2.9x105 M-1, a value that agreed well with the values obtained from ITC experiments and from single channel recordings. For the first time, these data provide insights into chitooligosaccharide uptake by OccD-like chitoporin in non-chitinolytic bacteria.