Produce Degradation Pathways and Prevention

Structure and Function of Complex Carbohydrates in Produce 579

lignin content. Complete lignin degradation is an oxidative process that is only
performed by a few microorganisms, such as the white-rot fungus Phanerochaete
chrysosporium. However, a larger variety of organisms, in particular actinomycetes
[114], are capable of carrying out partial delignification in order to gain access to
the cellulosic substrate. Trichoderma reesei, an aerobic and highly cellulolytic fun-
gus, is probably the microorganism whose cellulase system has been most thoroughly
investigated. Among aerobic, cellulolytic soil bacteria, several species belonging to
the genera Cellulomonas, Pseudomonas, Thermomonospora, and Microbispora have
been studied in detail [1–5]. Restricted cellulolytic action may facilitate the access
of phytopathogens to plant tissues and may contribute to the leakage of plant cell
sap. Due to the inability of cellulose-degrading enzymes to penetrate the cell wall,
enzymes are usually secreted or bound to the outside surface of cellulolytic micro-
organisms. Additionally, enzyme systems generally display a set of typical proper-
ties. The systems contain a multiplicity of enzyme components showing a marked
synergism against crystalline cellulose. These components often possess a substrate-
binding site independent of the catalytic site and are often associated with each other
and with the surface of cellulolytic microorganisms.

19.3.3 PECTIN-DEGRADING ENZYMES

Up to 75% of the carboxyl groups in a pectin polymer are generally esterified.
Enzymes that degrade pectins are absent in animals but are commonly found in
plants and microorganisms. These enzymes are also responsible for softening of
many fruits and vegetables, especially citrus fruits and tomatoes. Pectinesterases,
polygalacturonases, and pectate lyases are the three main enzymes that catalyze the
degradation of pectic substances. Pectinesterases from citrus fruits have been well
studied for their role in maturation, ripening, and fruit softening [71–76, 81–84,
95,96,102,105]. Pectinesterase (EC 3.1.1.11), also known as pectin methylesterase,
removes methoxy groups from the methylated pectin substrate at pH 7.5, with
concomitant release of a proton as the ester bond is hydrolyzed (Figure 19.13). These
enzymes typically attack on a position adjacent to a preexisting free carboxyl group
in pectin molecules.
The other pectin-degrading enzyme, polygalacturonases (poly-α-1,4-galactur-
onide glycanohydrolase, EC 3.2.1.15), cleaves glycosidic bonds in hydrated pectic
substances (Figure 19.14). These enzymes work best at pHs between 4.5 and 6.0,
and their mode of action could either be exo- or an endo-splitting, depending on the
substrate type.

FIGURE 19.13Enzyme action of pectinesterase (also known as pectin methylesterase,
EC 3.1.1.11) on a pectin molecule. The enzyme removes methoxy groups from methylated
pectin substrate at pH 7.5, and a proton is released upon hydrolysis of an ester bond.

O

HO

MeOOC

OHO

O

O

HO

HOOC

OHO hydrolysis

H 2 O MeOH

O

HO

HOOC

OHO

O

O

HO

HOOC

OHO