The glucose residues in the β-1,4-linked glucan chains in cellulose are arranged such that each residue is inverted with respect to its neighbor, giving rise to a twofold screw axis and a rather flat chain. If this arrangement of sugar residues is established during synthesis, it would entail either the rotation of the glucan chain or the cellulose synthase for addition of successive glucose residues to the growing end. A model suggesting that the active site of the enzyme can position two UDP-glucose molecules in an orientation such that the two glucose residues are positioned inverted to each other in the catalytic pocket was proposed by Saxena et al. (1995), and it was suggested that the glucose residues could be added sequentially or simultaneously to the growing end (Saxena et al., 1995). The growing end was later shown to be the nonreducing end of the β-1,4-linked glucan chain during cellulose synthesis (Koyama et al., 1997). Alternatively, the twofold symmetry in the glucan chain can be obtained from a single catalytic center, based on the reasoning that there is a fairly large degree of freedom of rotation about the β-glycosidic bond. According to this proposal, the glucose residue added in one orientation relaxes into the native orientation after polymerization (Delmer, 1999). Other proposals have suggested that two catalytic centers may be present in two subunits and be organized following dimerization or two different catalytic domains within the same catalytic site participate in the dual addition (Albersheim et al., 1997; Charnock et al., 2001). Cellulose synthase and other processive β-glycosyltransferases have so far resisted crystal structure determination although structure of a nonprocessive β-glycosyltransferase (SpsA from Bacillus subtilis) has been determined (Charnock and Davies, 1999). The SpsA protein lacks the conserved QXXRW motif found in the processive enzymes, and studies with site-directed mutants of cellulose synthase have indicated a role of this motif during the synthesis of cellulose (Saxena et al., 2001). The structure of the globular region of the A. xylinum cellulose synthase containing all the conserved aspartic acid residues and the QXXRW motif was predicted using the genetic algorithm, and it was estimated that the central elongated cavity can accommodate two UDP-glucose residues (Saxena et al., 2001). The alternating orientation of the N-acetylglucosamine (GlcNAc) residues within the chitin chain also led to the proposal that chitin synthases possess two active sites, and this possibility was tested using UDP-derived monomeric and dimeric inhibitors of chitin synthase activity in vitro (Yeager and Finney, 2004). Using these inhibitors, it was found that uridine-derived dimeric inhibitors exhibited a 10-fold greater inhibition of chitin synthase activity as compared to the monomeric control, consistent with the presence of two active sites in chitin synthases (Yeager and Finney, 2004).
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