Function of chitin oligosaccharides in plant and animal development

Degradation of chitosans with chitinase G from Streptomyces coelicolor A3(2): production of chito-oligosaccharides and insight into subsite specificities

We have studied the degradation of soluble heteropolymeric chitosans with a bacterial family 19 chitinase, ChiG from Streptomyces coelicolor A3(2), to obtain insight into the mode of action of ChiG, to determine subsite preferences for acetylated and deacetylated sugar units, and to evaluate the potential of ChiG for production of chito-oligosaccharides. Degradation of chitosans with varying degrees of acetylation was followed using NMR for the identity (acetylated/deacetylated) of new reducing and nonreducing ends as well as their nearest neighbors and using gel filtration to analyze the size distribution of the oligomeric products. Degradation of a 64% acetylated chitosan yielded a continuum of oligomers, showing that ChiG operates according to a nonprocessive, endo mode of action. The kinetics of the degradation showed an initial rapid phase dominated by cleavage of three consecutive acetylated units (A; occupying subsites -2, -1, and +1), and a slower kinetic phase reflecting the cleavage of the glycosidic linkage between a deacetylated unit (D, occupying subsite -1) and an A (occupying subsite +1). Characterization of isolated oligomer fractions obtained at the end of the initial rapid phase and at the end of the slower kinetic phase confirmed the preference for A binding in subsites -2, -1, and +1 and showed that oligomers with a deacetylated reducing end appeared only during the second kinetic phase. After maximum conversion of the chitosan, the dimers AD/AA and the trimer AAD were the dominating products. Degradation of chitosans with varying degrees of acetylation to maximum degree of scission produced a wide variety of oligomer mixtures, differing in chain length and composition of acetylated/deacetylated units. These results provide insight into the properties of bacterial family 19 chitinases and show how these enzymes may be used to convert chitosans to several types of chito-oligosaccharide mixtures.

Family 18 chitolectins: comparison of MGP40 and HUMGP39

Glycosidase and lectins both bind sugars, but only the glycosidases have catalytic activity. The glycosidases occur among over 100 evolved protein families and Family 18 is one of the two chitinases (EC 3, 2.1.14) families. Interestingly, lectins are also in this evolutionary group of Family 18 glycosidase proteins. The proteins belonging to the enzymatically inactive class are referred to as chitolectins and have a binding site that is highly similar to the catalytic Family 18 enzymes. We present a comparison of the recently obtained structures of two Family 18 chitolectins, MGP40 [A.K. Mohanty, G. Singh, M. Paramasivam, K. Saravanan, T. Jabeen, S. Sharma, S. Yadav, P. Kaur, P. Kumar, A. Srinivasan, T.P. Singh, Crystal structure of a novel regulatory 40kDa mammary gland protein (MGP-40) secreted during involution, J. Biol. Chem. 278 (2003) 14451-14460.] and HumGP39 [F. Fusetti, T. Pijning, K.H. Kalk, E. Bos, B.W. Dijkstra, Crystal structure and carbohydrate-binding properties of the human cartilage glycoprotein-39, J. Biol. Chem. 278 (2003) 37753-37760; D.R. Houston, D.R. Anneliese, C.K. Joanne, D.M.V. Aalten, Structure and ligand-induced conformational change of the 39kDa glycoprotein from human articular chondrocytes, J. Biol. Chem. 278 (2003) 30206-30212.] with a focus on the glycosidase active site. We compare the sequence and the structure of these two Family 18 protein classes. The difference between the active and inactive protein is a glutamic acid which acts as the essential acid/base residue for chitin cleavage and is replaced with leucine or glutamine in the chitolectins. Furthermore, a mechanism for the interaction between the chitolectin and oligosaccharides was proposed.

Development and application of a model for chitosan hydrolysis by a family 18 chitinase

Hydrolysis of partially deacetylated chitosans by ChitinaseB (ChiBeta) from Serratia marcescens results in mixtures of oligosaccharides typically between 2 and 20 sugar residues. The amounts of different oligomer fractions depend on the degree of acetylation of the starting chitosans. We have used experimentally determined distributions of hydrolysis products to develop a model for chitosan hydrolysis by ChiB. Important elements of the model include interaction parameters for acetylated/deacetylated units in each of the six subsites in the active cleft and degree of processivity (multiple attack). The hydrolysis reaction is described as a chemical reaction with an activation barrier that depends on the substrate sequence presented to the enzyme subsites. Using a Monte Carlo approach, the interaction parameters were refined by minimizing the difference between observed and predicted amounts of hydrolysis products obtained upon degradation of chitosan with a degree of acetylation of 65%. The final model can accurately predict complex patterns of oligosaccharides produced in the hydrolysis of chitosans with various degrees of acetylation, as well as patterns observed during reactions with chito-oligosaccharides. The behavior of a ChiB mutant with a mutation in subsite +2 (Gly188Asp), which reduces the affinity for an acetylated sugar, could be predicted correctly by introducing one single change in the model parameters (the interaction energy for an acetylated unit in the +2 subsite). The proposed model may be used to explore degradation products for different enzyme-substrates combinations and to optimize conditions for preparation of specific oligosaccharides. In addition, the model provides insight into subsite interaction parameters and the degree of processivity, which complements previous experimental studies on the mode of action of ChiB.

Chitin metabolism in insects: structure, function and regulation of chitin synthases and chitinases

Chitin is one of the most important biopolymers in nature. It is mainly produced by fungi, arthropods and nematodes. In insects, it functions as scaffold material, supporting the cuticles of the epidermis and trachea as well as the peritrophic matrices lining the gut epithelium. Insect growth and morphogenesis are strictly dependent on the capability to remodel chitin-containing structures. For this purpose, insects repeatedly produce chitin synthases and chitinolytic enzymes in different tissues. Coordination of chitin synthesis and its degradation requires strict control of the participating enzymes during development. In this review, we will summarize recent advances in understanding chitin synthesis and its degradation in insects.

Sequence analysis of chitooligosaccharides by matrix-assisted laser desorption ionization postsource decay mass spectrometry

Chitin/chitosan oligosaccharides composed of 2-acetamido-2-deoxy-D-glucopyranose (GlcNAc) and/or 2-amino-2-deoxy-D-glucopyranose (GlcN) were prepared by chemical degradation of chitin or chitosan and separated by gel permeation chromatography. Oligosaccharides obtained after enzymatic hydrolysis of chitosan [F(A) 0.19] with a fungal chitinase were derivatized by reductive amination with 2-aminoacridone and sequenced by matrix-assisted laser desorption ionization time-of-flight postsource decay (PSD) mass spectrometry (MS). The sequence of a trimer, D1A2, was established as D-A-A. The composition of a hexamer D3A3 was ca. 65% D-A-D-D-A-A and 35% D-D-A-D-A-A. The PSD MS of a nonamer D5A4-amac revealed four isobaric species D-X-Y-D-X-Y-D-A-A, where A is GlcNAc, D is GlcN, and X and Y (X not equal Y) are mutually either D or A. This structure motif was also observed in a dodecamer D7A5 which was composed of eight isobaric sequences of the general formula (D-X-Y)(3)-D-A-A.