Galactomannan catabolism conferred by a polysaccharide utilisation locus of Bacteroides ovatus : enzyme synergy and crystal structure of a β-mannanase

Viktoria Bågenholm, SUMITHA KRISHNASWAMYREDDY, Hanene Bouraoui, Johan Morrill, Evelina Kulcinskaja, Constance Bahr, OSKAR AURELIUS, Theresa Rogers, Yao Xiao, Derek Logan, Eric Martens, Nicole M Koropatkin, Henrik Stålbrand

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Abstract

A recently identified polysaccharide utilization locus (PUL) from Bacteroides ovatus ATCC 8483 is transcriptionally up-regulated during growth on galacto- and glucomannans. It encodes two glycoside hydrolase family 26 (GH26) β-mannanases, BoMan26A and BoMan26B, and a GH36 α-galactosidase, BoGal36A. The PUL also includes two glycan-binding proteins, confirmed by β-mannan affinity electrophoresis. When this PUL was deleted, B. ovatus was no longer able to grow on locust bean galactomannan. BoMan26A primarily formed mannobiose from mannan polysaccharides. BoMan26B had higher activity on galactomannan with a high degree of galactosyl substitution and was shown to be endo-acting generating a more diverse mixture of oligosaccharides, including mannobiose. Of the two β-mannanases, only BoMan26B hydrolyzed galactoglucomannan. A crystal structure of BoMan26A revealed a similar structure to the exo-mannobiohydrolase CjMan26C from Cellvibrio japonicus, with a conserved glycone region (-1 and -2 subsites), including a conserved loop closing the active site beyond subsite -2. Analysis of cellular location by immunolabeling and fluorescence microscopy suggests that BoMan26B is surface-exposed and associated with the outer membrane, although BoMan26A and BoGal36A are likely periplasmic. In light of the cellular location and the biochemical properties of the two characterized β-mannanases, we propose a schemeof sequential action by the glycoside hydrolasesencodedby the β-mannanPULandinvolved in the β-mannanutilization pathway in B. ovatus. The outer membrane-associated BoMan26B initially acts on the polysaccharide galactomannan, producing comparably large oligosaccharide fragments. Galactomanno-oligosaccharides are further processed in the periplasm, degalactosylated by BoGal36A, and subsequently hydrolyzed into mainly mannobiose by the β-mannanase BoMan26A.
Translated title of the contributionGalactomannan catabolism conferred by a polysaccharide utilisation locus of Bacteroides ovatus : enzyme synergy and crystal structure of a β-mannanase
Original languageEnglish
Pages (from-to)229-243
Number of pages15
JournalJournal of Biological Chemistry
Volume292
Issue number1
Early online date2016 Nov 21
DOIs
Publication statusPublished - 2017 Jan 6

Subject classification (UKÄ)

  • Biochemistry and Molecular Biology
  • Structural Biology
  • Other Chemistry Topics

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