Abstract
During the past decade, detailed studies using well-defined ‘second generation’ chitosans have amply proved that both their material properties and their biological activities are dependent on their molecular structure, in particular on their degree of polymerisation (DP) and their fraction of acetylation (FA). Recent evidence suggests that the pattern of acetylation (PA), i.e., the sequence of acetylated and non-acetylated residues along the linear polymer, is equally important, but chitosan polymers with defined, non-random PA are not yet available. One way in which the PA will influence the bioactivities of chitosan polymers is their enzymatic degradation by sequence-dependent chitosan hydrolases present in the target tissues. The PA of the polymer substrates in conjunction with the subsite preferences of the hydrolases determine the type of oligomeric products and the kinetics of their production and further degradation. Thus, the bioactivities of chitosan polymers will at least in part be carried by the chitosan oligomers produced from them, possibly through their interaction with pattern recognition receptors in target cells. In contrast to polymers, partially acetylated chitosan oligosaccharides (paCOS) can be fully characterised concerning their DP, FA, and PA, and chitin deacetylases (CDAs) with different and known regio-selectivities are currently emerging as efficient tools to produce fully defined paCOS in quantities sufficient to probe their bioactivities. In this review, we describe the current state of the art on how CDAs can be used in forward and reverse mode to produce all of the possible paCOS dimers, trimers, and tetramers, most of the pentamers and many of the hexamers. In addition, we describe the biotechnological production of the required fully acetylated and fully deacetylated oligomer substrates, as well as the purification and characterisation of the paCOS products.
Highlights
During the past decade, detailed studies using well-defined ‘second generation’ chitosans have amply proved that both their material properties and their biological activities are dependent on their molecular structure, in particular on their degree of polymerisation (DP) and their fraction of acetylation (FA)
Kauss et al [3] reported that the elicitor activity of chitosan polymers with sufficiently high DP decreased with increasing FA, i.e., high-DP low-FA chitosans were most active in inducing resistance reactions in Catharanthus roseus cells
Vander et al [4] reported that the elicitor activity of chitosan polymers with sufficiently high DP first increased, decreased with increasing FA; very low-FA chitosans and very high-FA chitosans were elicitor-inactive, only high-DP medium-FA chitosans were active in inducing resistance reactions in wheat leaves
Summary
The term ‘chitosan’ describes a large and versatile family of oligo- and polysaccharides with different structures and multiple functions. Two landmark papers by Kauss et al [3] and Vander et al [4] represent crucial steps in this process of understanding structure-function relationships of partially acetylated chitosan polymers Both papers clearly indicated the crucial role of FA and, less marked, of DP for the biological activity of different chitosans, as exemplified in their elicitor activities towards plant cells. The enzymatic production of defined paCOS using CDAs depends on the availability of suitably pure substrates, namely fully acetylated chitin oligomers and fully deacetylated glucosamine oligomers of defined DP Different methods to this end have been described, and such oligomers are commercially available, though at rather high prices only, due to the rather expensive production and purification steps involved. ‘Cγo’,nwsehqiuleenthtley,utnhiet unneixttnteoxtt-htoe rthedeuncoinn-greednudcuinngiteinsdnaumnietdis‘ψn’a,mpreedce‘βd’e,dfoblylo‘wχ’e.dFibnyal‘lγy’,, twohinildeitchaeteutnhiet fnuelxl tsetoqutheencreedouf cainpgaCenOdS,utnhiet uisnnitasmareedli‘sψte’,dpfrreocmedtehde bnyon‘χ-r’e. dFuincianllgy,totothinedreicdautecitnhgeefnudll, see.gq.u, ‘eAnDceAoAf Aa ’pfaoCr Oa βS,-mthoenuon-ditesaacreetylliastteedd pfreonmtatmheerniconp-arCedOuSc.inAgs tao cthonesreeqduuecninceg, e‘AndD,’ei.ng.d, i‘cAatDesAAthAe’αfo-arcaetβyl-amteodno(o-dreωac-deteyalcaetteydlapteedn)tadmimereicr pGalcCNOASc. -AGslcaNc,ownsheiqleu‘eAn1ceD,1‘A’ sDu’minmdaircialtyesdethneotαe-saAceDtyalantdedth(eorαω-d-edaecaecteytlyaltaetded(o)rdωim-aecreGtyllcaNteAd)c-dGimlcNer, wGlhciNle-G‘Alc1NDA1’cs,ui.me.m, itarcialyn deietnhoetreisnAdiDcaatendontheeoαf t-hdeemace(twyiltahteodut(oinrdωic-aatcientgylwathedic)hdoimnee)roGr lacNm-iGxtlucNreAocf, bi.eo.t,hitocfatnheemith. er indicate one of them (without indicating which one) or a mixture of both of them
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