PROSITE documentation PDOC00511

Glycosyl hydrolases family 9 (GH9) active sites signatures

The microbial degradation of cellulose and xylans requires several types of enzymes such as endoglucanases (EC 3.2.1.4), cellobiohydrolases (EC 3.2.1.91) (exoglucanases), or xylanases (EC 3.2.1.8) [1,2]. Fungi and bacteria produces a spectrum of cellulolytic enzymes (cellulases) and xylanases which, on the basis of sequence similarities, can be classified into families. One of these families is known as the cellulase family E [3] or as the glycosyl hydrolases family 9 [4,E1]. The enzymes which are currently known to belong to this family are widely distributed among bacteria, fungi, amoebozoa, invertebrate metazoans, mosses, ferns, gymnosperms, and angiosperms:

• Butyrivibrio fibrisolvens cellodextrinase 1 (ced1).
• Cellulomonas fimi endoglucanases B (cenB) and C (cenC).
• Clostridium cellulolyticum endoglucanase G (celCCG).
• Clostridium cellulovorans endoglucanase C (engC).
• Clostridium stercoararium endoglucanase Z (avicelase I) (celZ).
• Clostridium thermocellum endoglucanases D (celD), F (celF) and I (celI).
• Fibrobacter succinogenes endoglucanase A (endA).
• Pseudomonas fluorescens endoglucanase A (celA).
• Streptomyces reticuli endoglucanase 1 (cel1).
• Thermomonospora fusca endoglucanase E-4 (celD).
• Dictyostelium discoideum spore germination specific endoglucanase 270-6. This slime mold enzyme may digest the spore cell wall during germination, to release the enclosed amoeba.
• Endoglucanases from unicellular green microalgae, such as the unicellular alga Chlamydomonas reinhardtii or the colonial algae Gonium pectorale and Volvox carteri. Microalgae can utilize cellulose for growth in the absence/ limitation of other C-sources by secreting endocellulases.
• Endoglucanases from plants such as Avocado or French bean. In plants this enzyme may be involved in the fruit ripening process.
• Invertebrate endoglucanases, secreted by salivary glands and the gut.

Three conserved regions in these enzymes are centered on conserved residues which have been shown [5,6,7] to be important for the catalytic activity. The first region contains the characteristic DAGD motif, where the C-terminal D acts as the catalytic base that extracts a proton from the nucleophilic water. The second region contains an active site histidine and the third one contains two catalytically important residues: an aspartate and a glutamate. The fully conserved nucleophilic D forms H-bonds with the residues of the active-site loop, comprising of regions I and II, to bring it in the proper alignement. The fully conserved E acts as an acid that protonates the leaving group and stabilizes the positively-charged oxocarbonium transition-state. We have used these three regions as signature patterns, with the first pattern corresponding to region I, the second to region II and the third to region III.