4
1.1
Introduction
World energy consumption is on the rise, and a significant growth in energy demand
of about 48% is projected for the year 2040 from the base value in 2012 (EIA 2016).
Economic growth along with accompanying changes can significantly influence
energy consumption, and improvement in living standards brings with it a rapidly
growing demand for energy. While renewable energy is the world’s fastest growing
source of energy, fossil fuels continue to provide most of the world’s energy, and
liquid transportation fuels represent a major share of this (EIA 2016). Even with
efforts worldwide on alternative energy resources, it is projected that the demand for
fossil fuels will be on the rise at least for the next few decades with serious impacts
on the global environment and climate. It is in this context that the renewable liquid
transportation fuels including biodiesel and bioethanol gain prime importance as the
existing alternatives to petroleum-based transportation fuels. Lignocellulose is inarguably the world’s most abundant renewable source of energy and this justifies the
enormous efforts put into developing plant biomass-based fuels – primarily bioethanol. While several of the second-generation (2G) ethanol programs claim to have
gone commercial, it still is not a reality at consumer level. The major limitation in
commercialization of 2G ethanol is the cost of its production. Lignocellulose contains mainly the sugar polymers – cellulose and hemicellulose, and a significant
fraction as lignin. Both the sugar polymers can be broken down to their component
sugars, which then can be fermented by microbial action to produce bioethanol. The
hydrolysis/saccharification of biomass can be achieved by chemical agents (e.g.,
acids) or through enzymatic hydrolysis. The latter is often much more efficient and
requires only ambient conditions, whereas the former needs higher temperature and
is plagued by issues like generation of sugar breakdown products, and the need to
deal with acidic waste streams (Visser et al. 2015). The seemingly simple enzymatic
process is made difficult by the recalcitrance of lignocellulose and the cost of
biomass- hydrolyzing enzymes. Recalcitrance of biomass to enzymatic hydrolysis
stems from the highly organized structure of lignocellulose, which prevents access
of the enzymes to cellulose. Biomass pretreatments are aimed at making the cellulose more accessible to the enzymes and can bring significant improvements in
digestibility. The chapter is primarily focused on biomass-hydrolyzing enzymes in
the context of bioenergy, and specifically bioethanol. Microorganisms producing
cellulase, their regulation at molecular levels, production strategies, enzyme cocktails for biomass hydrolysis, and the emerging strategies for improving production
and efficiencies of biomass-hydrolyzing enzymes are discussed.
1.2
Biomass-Hydrolyzing Enzymes and Their Role
in Biofuels Production
Plant biomass consists of three major structural biopolymers, namely, cellulose,
hemicellulose, and lignin, each having a unique and complex structure. Cellulose is
the major component and is a homopolymer of β-1,4-linked glucose units which can
have a degree of polymerization (DP) up to 10,000. Often the cellulose chains are
R.K. Sukumaran et al.
1.1
Introduction
World energy consumption is on the rise, and a significant growth in energy demand
of about 48% is projected for the year 2040 from the base value in 2012 (EIA 2016).
Economic growth along with accompanying changes can significantly influence
energy consumption, and improvement in living standards brings with it a rapidly
growing demand for energy. While renewable energy is the world’s fastest growing
source of energy, fossil fuels continue to provide most of the world’s energy, and
liquid transportation fuels represent a major share of this (EIA 2016). Even with
efforts worldwide on alternative energy resources, it is projected that the demand for
fossil fuels will be on the rise at least for the next few decades with serious impacts
on the global environment and climate. It is in this context that the renewable liquid
transportation fuels including biodiesel and bioethanol gain prime importance as the
existing alternatives to petroleum-based transportation fuels. Lignocellulose is inarguably the world’s most abundant renewable source of energy and this justifies the
enormous efforts put into developing plant biomass-based fuels – primarily bioethanol. While several of the second-generation (2G) ethanol programs claim to have
gone commercial, it still is not a reality at consumer level. The major limitation in
commercialization of 2G ethanol is the cost of its production. Lignocellulose contains mainly the sugar polymers – cellulose and hemicellulose, and a significant
fraction as lignin. Both the sugar polymers can be broken down to their component
sugars, which then can be fermented by microbial action to produce bioethanol. The
hydrolysis/saccharification of biomass can be achieved by chemical agents (e.g.,
acids) or through enzymatic hydrolysis. The latter is often much more efficient and
requires only ambient conditions, whereas the former needs higher temperature and
is plagued by issues like generation of sugar breakdown products, and the need to
deal with acidic waste streams (Visser et al. 2015). The seemingly simple enzymatic
process is made difficult by the recalcitrance of lignocellulose and the cost of
biomass- hydrolyzing enzymes. Recalcitrance of biomass to enzymatic hydrolysis
stems from the highly organized structure of lignocellulose, which prevents access
of the enzymes to cellulose. Biomass pretreatments are aimed at making the cellulose more accessible to the enzymes and can bring significant improvements in
digestibility. The chapter is primarily focused on biomass-hydrolyzing enzymes in
the context of bioenergy, and specifically bioethanol. Microorganisms producing
cellulase, their regulation at molecular levels, production strategies, enzyme cocktails for biomass hydrolysis, and the emerging strategies for improving production
and efficiencies of biomass-hydrolyzing enzymes are discussed.
1.2
Biomass-Hydrolyzing Enzymes and Their Role
in Biofuels Production
Plant biomass consists of three major structural biopolymers, namely, cellulose,
hemicellulose, and lignin, each having a unique and complex structure. Cellulose is
the major component and is a homopolymer of β-1,4-linked glucose units which can
have a degree of polymerization (DP) up to 10,000. Often the cellulose chains are
R.K. Sukumaran et al.