Chitooligosaccharides (CHOS) are homo- or heterooligomers of N-acetylglucosamine and D-glucosamine. CHOS can be produced using chitin or chitosan as a starting material, using enzymatic conversions, chemical methods or combinations thereof. Production of well-defined CHOS-mixtures, or even pure CHOS, is of great interest since these oligosaccharides are thought to have several interesting bioactivities. Understanding the mechanisms underlying these bioactivities is of major importance. However, so far in-depth knowledge on the mode-of-action of CHOS is scarce, one major reason being that most published studies are done with badly characterized heterogeneous mixtures of CHOS. Production of CHOS that are well-defined in terms of length, degree of N-acetylation, and sequence is not straightforward. Here we provide an overview of techniques that may be used to produce and characterize reasonably well-defined CHOS fractions. We also present possible medical applications of CHOS, including tumor growth inhibition and inhibition of TH2-induced inflammation in asthma, as well as use as a bone-strengthener in osteoporosis, a vector for gene delivery, an antibacterial agent, an antifungal agent, an anti-malaria agent, or a hemostatic agent in wound-dressings. By using well-defined CHOS-mixtures it will become possible to obtain a better understanding of the mechanisms underlying these bioactivities.

Serratia marcescens produces three chitinases, ChiA, ChiB and ChiC which together enable the bacterium to efficiently degrade the insoluble chitin polymer. We present an overview of the structural properties of these enzymes, as well as an analysis of their activities towards artificial chromogenic chito-oligosaccharide-based substrates, chito-oligosaccharides, chitin and chitosan. We also present comparative inhibition data for the pseudotrisaccharide allosamidin (an analogue of the reaction intermediate) and the cyclic pentapeptide argadin. The results show that the enzymes differ in terms of their subsite architecture and their efficiency towards chitinous substrates. The idea that the three chitinases play different roles during chitin degradation was confirmed by the synergistic effects that were observed for certain combinations of the enzymes. Studies of the degradation of the soluble heteropolymer chitosan provided insight into processivity. Taken together, the available data for Serratia chitinases show that the chitinolytic machinery of this bacterium consists of two processive exoenzymes that degrade the chitin chains in opposite directions (ChiA and ChiB) and a non-processive endo-enzyme, ChiC.

We have studied the degradation of well-characterized soluble heteropolymeric chitosans by a novel family 46 chitosanase, ScCsn46A from Streptomyces coelicolor A3(2), to obtain insight into the enzyme's mode of action and to determine its potential for production of different chitooligosaccharides. The degradation of both a fully deacetylated chitosan and a 32% acetylated chitosan showed a continuum of oligomeric products and a rapid disappearance of the polymeric fraction, which is diagnostic for a nonprocessive endomode of action. The kinetics of the degradation of the 32% acetylated chitosan demonstrated an initial rapid phase and a slower second phase, in addition to a third and even slower kinetic phase. The first phase reflects the cleavage of the glycosidic linkage between two deacetylated units (D-D), the primary products being fully deacetylated dimers, trimers, and tetramers, as well as longer oligomers with increasing degrees of acetylation. In the subsequent slower kinetic phases, oligomers with a higher degree of acetylated units (A) appear, including oligomers with A's at the reducing or nonreducing end, which indicate that there are no absolute preferences for D in subsites -1 and +1. After maximum degradation of the chitosan, the dimers DA and DD were the dominant products. The degradation of chitosans with varying degrees of acetylation to a maximum degree of scission showed that ScCsn46A could degrade all chitosan substrates extensively, although to decreasing degrees of scission with increasing F(A). The potential use of ScCsn46A to prepare fully deacetylated oligomers and more highly acetylated oligomers from chitosan substrates with varying degrees of acetylation is discussed.

a b s t r a c tThe contributions of the -3 subsite and a putative +3 subsite to substrate positioning in ChiA from Serratia marcescens have been investigated by comparing how ChiA and its -3 subsite mutant W167A interact with soluble substrates. The data show that Trp -GlcNAc stacking in the -3 subsite rigidifies the protein backbone supporting the formation of the intermolecular interaction network that is necessary for the recognition and positioning of the N-acetyl groups before the -1 subsite. The +3 subsite exhibits considerable substrate affinity that may promote endo-activity in ChiA and/or assist in expelling dimeric products from the +1 and +2 subsites during processive hydrolysis.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 ABSTRACT: Chitotriosidase (HCHT) is one of two family 18 chitinases produced by 45 humans, the other being acidic mammalian chitinase (AMCase). The enzyme is thought to be 46 part of the human defense mechanism against fungal parasites, but its precise role and the 47 details of its enzymatic properties have not yet been fully unraveled. We have studied 48properties of HCHT by analyzing how the enzyme acts on high molecular-weight chitosans, 49 soluble co-polymers of β-1,4-linked N-acetylglucosamine (GlcNAc, A) and glucosamine 50 (GlcN, D). Using methods for in-depth studies of the chitinolytic machinery of bacterial 51 family 18 enzymes, we show that HCHT degrades chitosan primarily via an endo-processive 52 mechanism, as would be expected on the structural features of its substrate-binding cleft. The 53 preferences of HCHT subsites for acetylated versus non-acetylated sugars were assessed by 54 sequence analysis of obtained oligomeric products showing a very strong, absolute, and a 55 relative weak preference for an acetylated unit in the -2, -1, +1 subsite, respectively. The 56 latter information is important for the design of inhibitors that are specific for the human 57 chitinases and also provide insight into what kind of products may be formed in vivo upon 58 administration of chitosan-containing medicines or food products. 59 KEYWORDS: Human chitinase; chitosan; chitin; processivity; chitotriosidase. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 Chitin, an insoluble linear polysaccharide consisting of repeated units of β-1,4-N-66 linked acetylglucosamine [(GlcNAc) n ], is common as a structural polymer in crustaceans, 67 arthropods, fungi, and parasitic nematodes. The metabolism of chitin in nature is controlled 68 by enzymatic systems that produce and break down chitin, primarily chitin synthases and 69 chitinases, respectively. Chitinases are thought to play important roles in anti-parasite 70 responses in several life forms, including humans (1-4). Even though chitin and chitin 71 synthases have not been found in humans, we produce two active chitinases that are 72 categorized as family 18 chitinases based on sequence-based classification of glycoside 73 hydrolases (5). These two enzymes are called acidic mammalian chitinase (AMCase) (6) and 74 human chitotriosidase (HCHT) (7) and both are believed to play roles in anti-parasite 75 responses (8, 9). While AMCase is found in the stomach (6), in tears (10), sinus mucosa (11), 76 and lungs (12), HCHT is primarily expressed in activated human macrophages (13). 77HCHT is up-regulated in a series of diseases and medical conditions such as 78Ga...

a b s t r a c tThermodynamic parameters for binding of N-acetylglucosamine (GlcNAc) oligomers to a family 18 chitinase, ChiB of Serratia marcescens, have been determined using isothermal titration calorimetry. Binding studies with oligomers of different lengths showed that binding to subsites À2 and +1 is driven by a favorable enthalpy change, while binding to the two other most important subsites, +2 and +3, is driven by entropy with unfavorable enthalpy. These remarkable unfavorable enthalpy changes are most likely due to favorable enzyme-substrate interactions being offset by unfavorable enthalpic effects of the conformational changes that accompany substrate-binding.

Enzymatic depolymerization of polysaccharides is a key technology in the biorefining of biomass. The enzymatic conversion of the abundant insoluble polysaccharides cellulose and chitin is of particular interest and complexity, because of the bi-phasic nature of the process, the seemingly complicated tasks faced by the enzymes, and the importance of these conversions for the future biorefinery. Here we review recent work on family 18 chitinases that sheds light on important aspects of the catalytic action of these depolymerizing enzymes, including the structural basis of processivity and its direction, the energies involved in substrate-binding and displacement.

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