P.-Y. Hong, M. Iakiviak, D. Dodd, M. Zhang, R.I. Mackie, I. Cann
Applied and Environmental Microbiology, 80, p. 2084-2093, (2014)
Xylan is an abundant plant cell wall polysaccharide and is a dominant
component of dietary fiber. Bacteria in the distal human
gastrointestinal tract produce xylanase enzymes to
initiate the degradation of this complex heteropolymer. These xylanases
typically derive from glycoside hydrolase (GH)
families 10 and 11; however, analysis of the genome sequence of the
human gut bacterium Bacteroides intestinalis DSM 17393 revealed the presence of two putative GH8 xylanases. In the current study, we demonstrate that the two genes encode
enzymes that differ in activity. The xyn8A gene encodes an endo-xylanase (Xyn8A) and rex8A encodes a reducing end xylose-releasing exo-oligoxylanase (Rex8A). Xyn8A hydrolyzed both xylopentaose (X5) and xylohexaose (X6) to a mixture of xylobiose (X2) and xylotriose (X3), while Rex8A hydrolyzed X3 through X6 to a mixture of xylose (X1) and X2. Moreover, rex8A is located downstream of a GH3 gene (xyl3A) that was demonstrated to exhibit β-xylosidase activity and would be able to further hydrolyze X2 to X1.
Mutational analyses of putative active site residues of both Xyn8A and
Rex8A confirm their importance in catalysis by these
enzymes. Recent genome sequences of gut bacteria
reveal an increase in GH8 Rex enzymes, especially amongst the
indicating that these genes contribute to xylan
utilization in the human gut.