Hemicellulases are key components in the degradation of
plant biomass and carbon flow in nature. The substrates of
these enzymes, the hemicelluloses, are a heterogeneous
group of branched and linear polysaccharides that are
bound via hydrogen bonds to the cellulose microfibrils in
the plant cell wall, crosslinking them into a robust network.
Hemicelluloses are also covalently attached to lignin, forming
together with cellulose a highly complex structure.
The degradation of cellulose and hemicellulose is carried
out by microorganisms that can be found either free in
nature or as part of the digestive tract of higher animals.
The variable structure and organization of hemicellulose
require the concerted action of many enzymes for its
complete degradation. In many niches, this process is very
slow because of the insoluble rigid structure of the plant
cell wall and the limited availability of efficient cellulolytic
and hemicellulolytic microorganisms. As a large part
of the polymers are either insoluble or closely associated
with the insoluble cellulose matrix, many of the hemicellulases are modular proteins, and, in addition to their
catalytic domains, include other functional modules. The
most important modules are carbohydrate-binding modules
(CBMs), which facilitate the targeting of the enzymes
to the insoluble polysaccharides, and dockerin modules
that mediate the binding of the catalytic domains via
cohesin–dockerin interactions, either to the microbial cell
surface or to large enzymatic complexes, such as the
cellulosome