Mechanism glycoside hydrolases

In this article are discussed the results of those studies which have become available over the past 15 years and which permit some generalizations on the catalytic mechanism of glycoside hydrolases from widely differing sources and with different sugar and aglycon specificities. It will be seen that, with few exceptions, the data support a mechanism almost identical to that proposed by Phillips and his group for lysozyme. ... 

C. Moulis, G. Joucla, D. Harrison, E. Fabre, G. Potocki-Veronese, P. Monsan, and M. Remaud-Simeon, Understanding the polymerization mechanism of glycoside-hydrolase family 70 glucansucrases, /. Biol. Chem., 281 (2006) 31254-31267. 

In spite of the apparent extreme diversity of the / -(l- 4) glycans— from crystalline cellulose to components of bacterial cell walls—the similarity of the environments around their / -(1— 4) glycosidic linkage raises the question of a common catalytic site structure in the various / -(l- 4) glycoside hydrolases. This chapter will review the evidence for and against a general lysozyme type mechanism. 

Oxidation of two out of 13 tryptophan residues in a cellulase from Penicillium notatum resulted in a complete loss of enzymic activity (59). There was an interaction between cellobiose and tryptophan residues in the enzyme. Participation of histidine residues is also suspected in the catalytic mechanism since diazonium-l-H-tetrazole inactivated the enzyme. A xylanase from Trametes hirsuta was inactivated by N-bromosuc-cinimide and partially inactivated by N-acetylimidazole (60), indicating the possible involvement of tryptophan and tyrosine residues in the active site. As with many chemical modification experiments, it is not possible to state definitively that certain residues are involved in the active site since inactivation might be caused by conformational changes in the enzyme molecule produced by the change in properties of residues distant from the active site. However, from a summary of the available evidence it appears that, for many / -(l- 4) glycoside hydrolases, acidic and aromatic amino acid residues are involved in the catalytic site, probably at the active and binding sites, respectively. 

Ausili A, Cobucci-Ponzano B, Di Lauro B, D Avino R, Perugino G, Bertoli E, Scire A, Rossi M, Tanfani F, Moracci M. A comparative infrared spectroscopic study of glycoside hydrolases from extremophilic archaea revealed different molecular mechanisms of adaptation to high temperatures. Proteins 2007 67 991-1001. 

Fig. 9. Glycosyl hydrolysis - retaining mechanism. The charged acidic acid residue in the active site of a retaining glycoside hydrolase nucleophilically attacks the glycosidic bond while an opposing protonated residue donates the necessary proton. The covalently bound intermediate product is released by a second nucleophilic attack by a water molecule activated by electrostatic repulsion from the recently de-protonated residue...

Tn strict logic, inversion must arise from an odd number and retention from an even number of displacements. In biological practice, however, it is easier for natural selection to develop a protein which stabilises one or two transition states, than one which stabilises 3, 5, 7,. .. or 4, 6, 8,. .. transition states, as the case may be. The same argument makes it unlikely that simple glycoside hydrolases which work by a ring-opening mechanism will ever be found. 

Polysaccharide lyases have been grouped into a number of CAZy families and mechanistic and structural data are available for a number of them. As with the glycoside hydrolases, enzymes of the same CAZy family have the same protein fold and similar, if not identical, mechanisms. In June 2007 there were 18 polysaccharide lyase families. 

Enzymes classified into oxazoline intermediate type are typically chitinase and hyaluronidase. Chitinase from Bacillus sp. (EC 3.2.1.14) belonging to the glycoside hydrolase family 18 primarily catalyzes hydrolysis of (1— 4)-jff-A-acetylglucosaminide linkage in chitin (30,32,33). The hydrolysis mechanism is postulated to proceed through the substrate-assisted mechanism as follows (Figure 3). 

CGTase is a kind of glycoside hydrolases, and according to the deference in the conformation of the anomeric carbon atom of glycosidic bonds subjected to hydrolysis, there are two kinds of enzyme catalytic mechanisms One is conformation inversion type (Fig. 2.2(a)), and the other is conformation keeping... 

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