570 Produce Degradation: Reaction Pathways and their Prevention
19.2.2 CELLULOSE
As opposed to starch, which serves as energy storage molecules, cellulose mainly
confers structural integrity to plant cells. The high tensile strength of cellulose allows
plants cells to withstand osmotic pressure and resist mechanical stress. For example,
woody plants and plants used as a source for textile fibers, such as cotton, exhibit
high mechanical strengths. Chemically, cellulose is a linear polymer of glucose
subunits (100 to 14,000) linked together via –1,4 bonds forming biological structures
that are highly crystalline and water-insoluble (Figure 19.7). Because of their capac-
ity to form inter- and intramolecular hydrogen bonds, adjacent cellulose molecules
adhere strongly to one another in parallel arrays of 60 to 70 cellulose chains, all
having the same polarity, to form long, rigid, and highly ordered crystalline aggre-
gates called microfibrils (Figure 19.8). Microfibrils range, in lateral dimension,
anywhere from 3 to 4 nm in higher plants and up to 20 nm for the microfibrils of the
alga Valonia macrophysa, which contains up to several hundred cellulose chains [1].
Also, depending on the botanical origin, the degree of crystallinity of cellulose could
vary from 0% for totally amorphous cellulose, to 70% for cotton [17] to 100% in
Valonia macrophysa. Because microfibrils are highly crosslinked they exhibit a high
degree of resistance to enzymatic breakdown, often requiring the combined action
of enzymes with multiple specificities for complete hydrolysis.
It is generally assumed that the hydrolysis reaction catalyzed by glycosidases,
including cellulases and xylanases, proceeds via an acid–base mechanism involving
two residues. The first residue acts as a general acid catalyst and protonates the
oxygen of the glucosidic bond. The second residue acts as a nucleophile, which
either interacts with the oxocarbonium intermediate or promotes the formation of a
hydroxide ion from a water molecule. Reactions leading to retention of configuration
involve a two-step mechanism, with a double inversion of configuration at the
FIGURE 19.7Structure of cellulose (a) and cellobiose (b) indicating β-glucosidic bonds.
O
HO
HO OHOHOOHO OHHO
O
O
HO OHOHHOnO
HO
HO
OHOHOOHO OHOHHO(a)(b)