Glucan chains, hydrogen bonding

Cellulose is composed of linear chains of 1,4-linked (3-d-G1c/j residues. The P-glucan chains associate by intra- and inter-chain hydrogen bonds and by hydro-phobic interactions to form microflbrils. These highly ordered and partially crystalline microfibrils provide much of the tensile strength of the primary wall. In... 

Fig. 22.—Antiparallel packing arrangement of the 2-fold helices of (1— 3)-a-D-glucan (21). (a) Stereo view of two unit cells approximately normal to the aoplane. The two chains in the back (open bonds) are antiparallel and so are the chains in the front (filled bonds). Each helix is stabilized by 2-OH 0-4 hydrogen bonds across the bridge oxygen atoms. Interchain hydrogen bonds are formed in sheets along the a direction, (b) An axial projection of the unit cell shows that the sheets in the front and back are also joined by hydrogen bonds.

Polymerization of the D-glucan chains occurs by way of a multi-subunit, enzyme complex embedded in the plasma membrane an almost simultaneous association, by means of hydrogen bonds, of the newly formed chains results in formation of partially crystalline microfibrils. This mechanism of polymerization and crystallization results in the creation of microfibrils whose chains are oriented parallel (cellulose I). In A. xylinum, the complex is apparently immobile, but, in cells in which cellulose is deposited as a cell-wall constituent, it seems probable that the force generated by polymerization of the relatively rigid microfibrils propels the complex through the fluid-mosaic membrane. The direction of motion may be guided through the influence of microtubules. 

Beta amylase is an exoenzyme that degrades amylose chains from the nonreducing end, to liberate maltose. The official notation of this enzyme is (1 — 4)-a-D-glucan maltohydiolase, EC 3.2.1.2. The changes as a function of time in the beta-amylolytic index (a percents of the convernon into maltose) for com starch is shown in Fig. 19. A small increase in the case of dextrinization at lower temperatures was interpreted as the result of cleavage of hydrogen bonds, which makes some r ons of hydrolyzed material more easily accessible to enzymes. ... 

Figure 4.35 Hydrogen bonding and conformation of an individual glucan chain in cellulose I allomorphs.

Figure 4.42 Cartoon of the view down the c axis of various cellulose polymorphs shaded rectangles represent glucan chains in the opposite sense and dotted lines hydrogen bonds.

Figure 4.68 Cartoon of the end view of a P-(1 3)-glucan triple helix, showing the interchain hydrogen bonds between the 2-OH groups. Each 2-OH probably forms one donor and one acceptor hydrogen bond, but the locations of the protons are not known. Ch = rest of the P-(l- -3)-glucan chain. Note how any substituent on 06 will stick out from the coiled rope of P-(l->3)-glucan chains.

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