Produce Degradation Pathways and Prevention

Structure and Function of Complex Carbohydrates in Produce 571

anomeric carbon, and the formation of an oxocarbonium intermediate [18]. A par-
adigm of this type of reaction is the mechanism of lysozymes [19]. Reactions leading
to inversion of configuration proceed via a single nucleophilic substitution [18].
The complete degradation of crystalline cellulose requires the combined inter-
action of cellulolytic enzymes of different specificities. Based on the biochemical
studies of the cellulase systems derived from many microbial sources [20–32], a
cellulose degradation model has been developed to show how cellobiohydrolases,
endoglucanases, and β-glucosidases function together (Figure 19.9). According to
this model, the endoglucanases would first hydrolyze amorphous regions of the
cellulose fibers. The nonreducing ends generated could then be attacked by cello-
biohydrolases, which would then proceed with the degradation of the crystalline
regions. The β-glucosidases would prevent the accumulation of cellobiose, which
inhibits cellobiohydrolases. This model captures the functional role of β-glucosidase
enzymes working together without any physical interactions between these enzymes.
Studies have shown that these enzymes are capable of behaving as individual proteins
without forming stable complexes. However, contrary to these assumptions, it has
been shown that a purified stable multienzyme complex (or cellulosome) from
T. reesei, upon treatment with urea and octylglucoside, dissociated into endogluca-
nase, a xylanase, and β-glucosidase components [33]. It is not clear whether the
physical association between multiple cellulolytic enzymes has any advantage as far
as their catalytic efficiency is concerned.
The enzymatic action on native cellulose can be evaluated by a combination of
biochemical and spectroscopic techniques. Catalytic activity is usually assayed by
measuring the release of soluble reducing sugars, which are expressed as glucose
equivalents. The residual insoluble cellulose can also be determined gravimetrically
[34], calorimetrically [35], or by turbidimetry [36]. The physical and chemical
properties can be determined by crystallinity index, degree of polymerization, alter-
ation of available surface area, or small particle formation. Electron microscopy can
reveal surface morphology, showing enzyme adhesion sites [37], patterns of erosion
[38], and alterations of fiber or microfibril structure [39–42].

FIGURE 19.8Inter- and intraspecific hydrogen bonding in two β-linked glucose chains of
cellulose.

O

O

O

O O

O

O

O

O O

O

O

O

O O

O

O O

O

O O

O

O

O

O O

O

O

O

O O

O

O

O

O O

O

O O

O

O O

H H H H H H

H H

H

H

H H H

H

H H

H H H H H H H H