Abstract
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 xylan-degrading 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 endoxylanase (Xyn8A), and
rex8A
encodes a reducing-end xylose-releasing exo-oligoxylanase (Rex8A). Xyn8A hydrolyzed both xylopentaose (X
5
) and xylohexaose (X
6
) to a mixture of xylobiose (X
2
) and xylotriose (X
3
), while Rex8A hydrolyzed X
3
through X
6
to a mixture of xylose (X
1
) and X
2
. 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 X
2
to X
1
. 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 among the
Bacteroidetes
, indicating that these genes contribute to xylan utilization in the human gut.