Polysaccharides

Unbranched chain, α(1→4) glycosidic bonds

Amylose^[2] is an example of a polysccharide which can contain up to several thousand glucoses connected by α(1→4) glycosidic bonds. The initial view () shows an eleven unit segment of amylose with yellow halos marking some of the oxygens which form the 1→4 glycosidic bonds. Rotate to view the glucopyranosyl units on edge to see that the bonds are α linkages. (Remember: With the glucose providing C-1 on the left, the glucose providing C-4 on the right and C-6 of the glucoses projecting to the back of the screen both bonds of the oxygen of the α linkage project down.) From this perspective you are looking down the axis of a helix that is formed as a result of the angle that is form between the glucopyranosyl residues when they are connected by the α(1→4) bonds. This characteristic of the α(1→4) bond was seen when studying maltose. ends of the polymer yellow and green. with 20 glucose units; . In addition to this native helical structure amylose can accommodate other helical forms^[3]

Branched chain, α(1→4) glycosidic bonds

^[4] is also a large glucose polymer that has α(1→4) glycosidic bonds connecting the glucose units, but it also contains α(1→ 6) glycosidic bonds. In this scene the main branch is colored yellow, the side branch is green and the oxygen atoms of the α(1→4) bonds are red. Rotate to view the glucopyranosyl units on edge and verify that the bonds are α linkages. , branching point, colored yellow with the oxygen atom connecting C-6 of unit four to the C-1 of the side chain colored green. The is colored CPK, C-1 of this unit is able to open to the aldehyde and function as a reducing agent, but all the other termini of an amylopectin molecule are non-reducing because the C-1 of these terminal units are involved in glycosidic bond and can not form the aldehyde. In this scene the non-reducing termini are colore green.

of a 20-unit chain (colored CPK) having a 5-unit chain (green) branching at unit five and a 10-unit chain (yellow) branching at unit 10. The red atoms in the yellow and green branching chains are the oxygen atoms of the α(1→4) glycosidic bonds. Structure . Notice that the helical structure is more open than the unbranched amylose and the previous structure having only one branch. The native amylopectin having many more branching points would be more open than this structure, in fact it would have very little curvature. The is colored white and the non-reducing termini are colored orange. In a native amylopectin molecule the distance between the branching points is 12 to 30 glucose residues which is a greater distance than in this model, and there would be many more branched chains so that the non-reducing termini would greatly out number the one reducing terminus.

Glycogen^[5] is another example of a branched chain polysaccharide, and the major structural difference between glycogen and amylopectin is that glycogen has 8 to 12 glucose units between branching points.

Unbranched, β(1→4) glycosidic bonds

Cellulose is a polysaccharide with glucose units connected only by β(1→4) linkages, and therefore is unbranched. A theoretical model of illustrates the structural features. Notice that every other glucose residue is flipped 180° with respect to the preceding residue. This configuration can be best identified by observing on what side of the chain C-6 of glucose is located. Since the linkage is β, this configuration is necessary in order for the to have its normal angular geometry. Observe that of the two residues highlighted the lower right one is flipped. Moving down the chain to the notice that the oxygen on C-1 having the β configuration projects down because the glucose residue has been flipped 180° and that the next residue in its normal orientation is properly positioned to bond with the oxygen forming the angular geometry for oxygen. is in the flipped position.

Cellulose is present in a cell wall as a microfibril which is a paracrystalline structure containing 36 strands of cellulose. A brief description of the biosynthesis of this microfibril is available.^[6] During the synthesis of a single chain in order to achieve the configuration as described above,every other residue flipped 180°, the active site of the sythase has to accommodate two glucose molecules. One glucose molecule is flipped 180° with respect to the other one during bind to the active site, and the active site has the facility to catalyze the formation of two glycosidic bonds extending the chain by two glucose residues.^[7]