Enzymatic modification and characterization of xanthan

M.M. Kool

Research output: Thesisinternal PhD, WU

Abstract

In this thesis an enzymatic approach for the modification and characterization of xanthans was introduced. Complete backbone degradation of xanthan by cellulases was obtained independent on the molar composition of a xanthan sample. It was shown that only xanthan segments that occurred in a disordered xanthan conformation were susceptible to enzymatic backbone degradation. HILIC-ELSD-MS analysis revealed the presence of six different xanthan repeating units (RUs). All RUs consisted of the same pentasaccharide structure, with different acetyl and pyruvate substitution patterns. Interestingly the presence of an acetyl group at the O-6 position of the outer mannose unit was shown. Analysis of 5 xanthan samples showed that 5–19% of all acetyl groups present are positioned on the outer mannose. Furthermore, the relative abundance of the RUs present in xanthan samples can vary, even when their molar compositions are the same.

Analysis of the transitional behavior of xanthan based on the enzymatic release of the six types of RUs showed that the acetyl groups on the outer mannose, and not on the inner mannose, as was previously reported, are responsible for the stabilization of xanthans conformation. It was proposed that acetylation of the outer mannose also determines the functional properties of a xanthan solution. Furthermore, it was postulated that 1) The RUs that are either acetylated on the outer mannose units or solely acetylated on the inner mannose units are block wise distributed over the xanthan molecule. 2) Pyruvylated RUs and unsubstituted RUs are randomly distributed. 

Screening for xanthan modifying enzymes resulted in the discovery of the first two acetyl esterases being active towards xanthan. AXE3, a xylan acetyl esterase produced by Myceliophthora thermophila C1, showed to be specific for the removal of the acetyl groups at the inner mannose unit and was only active towards the disordered xanthan conformation. YesY, a pectin acetyl esterase produced by Bacillus subtilis strain 168, specifically removed the acetyl groups at the outer mannose units and its activity is not influenced by xanthans conformation.

Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Gruppen, Harry, Promotor
  • Schols, Henk, Co-promotor
Award date7 Feb 2014
Place of PublicationWageningen
Publisher
Print ISBNs9789461738653
Publication statusPublished - 2014

Keywords

  • xanthan
  • modification
  • enzymes
  • characterization

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