To search the entire book, enter a term or phrase in the form below
Diversity of Metabolism in Procaryotes (page 1)
(This chapter has 8 pages)
© Kenneth Todar, PhD
Introduction
A lot of hoopla is made about microbial diversity. The unicellular
eucaryotes (protista) exhibit a fair amount of structural diversity,
but the procaryotes (bacteria and archaea) lack this distinction. There
are but a few basic morphologies, the possibilities of motility and
resting cells (spores), and a major differential stain (the Gram stain)
that differentiates procaryotes microscopically. So
what is all the hoopla about regarding procaryotes? It is about
biochemical or metabolic diversity,
especially as it relates to energy-generating metabolism and
biosynthesis
of secondary metabolites. The procaryotes, as a group, conduct all the
same types of basic metabolism as eucaryotes, but, in addition, there
are
several types of energy-generating metabolism among the procaryotes
that
are non existent in eucaryotic cells or organisms. The diversity of
procaryotes
is expressed by their great variation in modes of energy generation and
metabolism, and this feature allows procaryotes to flourish in all
habitats suitable for life on earth.
Even within a procaryotic species, there may be great versatility in
metabolism. Consider Escherichia coli. The bacterium can
produce
energy for growth by fermentation or respiration. It can respire
aerobically
using O2 as a final electron acceptor, or it can respire
under
anaerobic conditions, using NO3 or fumarate as a terminal
electron
acceptor.
E. coli can use glucose or lactose as a sole carbon source
for growth, with the metabolic ability to transform the sugar into all
the necessary amino acids, vitamins and nucleotides that make up cells.
A relative of
E. coli, Rhodospirillum rubrum, has all the
heterotrophic capabilities as E. coli, plus the ability to
grow
by
photoautotrophic, photoheterotrophic or lithotrophic means. It does
require
one growth factor, however; biotin must be added to its growth media.
Fundamentally, most eucaryotes produce energy (ATP) through alcohol
fermentation (e.g. yeast), lactic acid fermentation (e.g. muscle cells,
neutrophils), aerobic respiration (e.g. molds, protozoa, animals) or
oxygenic
photosynthesis (e.g. algae, plants). These modes of energy-generating
metabolism
exist among procaryotes, in addition to all the following types of
energy
production
which are virtually non existent in eucaryotes.
Unique fermentations proceeding through the Embden-Meyerhof
pathway
Other fermentation pathways such as the phosphoketolase
(heterolactic)
and Entner-Doudoroff pathways
Anaerobic respiration: respiration that uses substances other
than O2 as a final electron acceptor
Lithotrophy: use of inorganic substances as sources of energy
Photoheterotrophy: use of organic compounds as a carbon
source
during bacterial photosynthesis
Anoxygenic photosynthesis: photophosphorylation in the
absence
of O2
Methanogenesis: an ancient type of archaean metabolism that
uses
H2 as an energy source and produces methane
Light-driven nonphotosynthetic photophosphorylation: unique
archaean
metabolism that converts light energy into chemical energy
In addition, among autotrophic procaryotes, there are three ways to
fix CO2, two of which are unknown among eucaryotes, the CODH
(acetyl CoA pathway) and the reverse TCA cycle.
Energy-Generating Metabolism
The term metabolism refers to the sum of the biochemical
reactions
required for energy generation AND the use of energy to
synthesize
cell material from small molecules in the environment. Hence,
metabolism
has an energy-generating component, called catabolism,
and
an
energy-consuming, biosynthetic component, called
anabolism.
Catabolic reactions or sequences produce energy as ATP, which
can
be utilized in anabolic reactions to build cell material from nutrients
in the environment. The relationship between catabolism and anabolism
is
illustrated in Figure 1 below.
Figure 1. The relationship
between
catabolism and anabolism in a cell. During catabolism, energy is
changed
from one form to another, and keeping with the laws of thermodynamics,
such energy transformations are never completely efficient, i.e., some
energy is lost in the form of heat. The efficiency of a catabolic
sequence
of reactions is the amount of energy made available to the cell (for
anabolism)
divided by the total amount of energy released during the reactions.
ATP
During catabolism, useful energy is temporarily conserved in the
"high
energy bond" of ATP -
adenosine triphosphate. No matter what
form
of energy a cell uses as its primary source, the energy is ultimately
transformed
and conserved as ATP - the universal currency of energy exchange in
biological
systems. When energy is required during anabolism, it may be spent as
the
high energy bond of ATP which has a value of about 8 kcal per mole.
Hence,
the conversion of ADP to ATP requires 8 kcal of energy, and the
hydrolysis
of ATP to ADP releases 8 kcal.
Figure 2. The structure of
ATP.
ATP is derived from the nucleotide adenosine monophosphate (AMP) or
adenylic
acid, to which two additional phosphate groups are attached through
pyrophosphate
bonds (~P). These two bonds are energy rich in the sense that their
hydrolysis
yields a great deal more energy than a corresponding covalent bond. ATP
acts as a coenzyme in energetic coupling reactions wherein one or both
of the terminal phosphate groups is removed from the ATP molecule with
the bond energy being used to transfer part of the ATP to
another
molecule to activate its role in metabolism. For example, Glucose + ATP
----->
Glucose-P
+ ADP or Amino Acid + ATP ----->AMP-Amino
Acid + PPi.
Because of the central role of ATP in energy-generating metabolism,
expect to see its involvement as a coenzyme in most energy-producing
processes
in cells.
chapter continued
