Biomedical Engineering Reference
In-Depth Information
Piromyces, Orpinomyces, Ruminomyces, Caecomyces,
and
Anaeromyces
genera
,
and anaerobic cellulose-degrading bacteria include 12 genera. Anaerobic bacteria
and fungi are usually attached to the cellulose. Anaerobic bacteria lack the ability to
effectively penetrate the cellulose, so if there is competition from other microbes
and ATP limitation, they search selectively for other mechanisms of cellulose
degradation to obtain cellulose hydrolysis products. Except for a few examples,
anaerobic bacteria mainly produce cellulosome, which decomposes cellulose to
obtain different fermentation end products, such as acetic acid, ethanol, and
succinate [
115
]. For anaerobic bacteria, absorption on cellulose cells is common.
Many raised organelle structures exist on the surface of
Clostridium thermocellum
,
comprising multiple copies of cellulosome. When the cells contact the substrates,
some claws elongate to make the cellulosome settle and arrange along the cellulose
surface. The enzyme on the cellulosome begins to degrade the substrate, and a large
amount of cellobiose is released to the environment. After a period of growth, the
cells are separated from cellulose. The separated cellulosome remains on the surface
of the residue cellulose to degrade continually [
116
].
The concept of the so-called cellulosome was first introduced by Israel Bayer
and Lamed and colleagues after their in-depth study of the anaerobic cellulose-
degrading bacterium
Clostridium thermocellum
. It is the extracellular enzyme
complex that is able to degrade cellulose, hemicellulose, and pectin. It is produced
by anaerobic bacteria such as
Clostridium
spp
.
,
Acetivibrio
spp.,
Bacteroides
spp.,
and
Ruminococcus
spp. Cellulosomes may be the largest extracellular enzyme
complexes found in nature. It is reported that the molecular weight of cellulosome
polymer is up to 100 MDa, but the molecular weight of the cellulosome monomer
is only between 650 kDa and 2.5 MDa. A typical cellulosome is shown in Fig.
3.8
.
Generally, there are two types of cellulase systems of anaerobic cellulose-
degrading microorganisms. The first type is mainly composed of three separate
extracellular enzymes, which are the same as cellulase referred to previously. These
independent extracellular enzymes have their own binding domain and the CD,
degrading cellulose synergistically. The second type uses a multienzyme complex
cellulosome. Initially, it was thought that cellulosome was just a medium-connected
structure between a microorganism and cellulose. Later, it was found that the
cellulosome was a stable polymer multienzyme complex that connected a scaffold
protein and multienzyme subunit through an anchored adhesion domain-domain
interaction. It could effectively degrade fiber material [
117
].
The main catalytic components of the cellulosome are exoglucanases and
endoglucanases, which have a different hydrolysis direction, playing an important
role in hydrolysis. The different enzymes arrange on noncatalytic scaffold protein
to ensure a high concentration in the local region. The correct arrangement of the
respective components and the proper ratio make the cellulosome hydrolyze MCC
efficiently. The cellulosome is incorporated in the cells and on the substrate so that
the cells and cellulose are tightly joined together; thus, it is fit for the degradation of
the substrate and rapid use of the sugar produced by the enzyme of the cellulosome.
It was reported that the integrity of the cellulosome is important for ensuring
