Chapter 5
Microbial Metabolism
Part 3
Fermentation
• First stage: Glycolysis
• Second stage: Reduced
coenzymes (NADH &
NADPH) donate their eand H+ to pyruvic acid and
its derivatives to form a
fermentation end products.
Fig. 5.18a
Fermentation
• Releases energy from oxidation of organic
molecules
– sugars, amino acids, organic acids, purines, and
pyrimidines
• Does not require oxygen
– bun can occur with oxygen
• Does not use the Krebs cycle or ETC
Fermentation
• Uses an organic molecule as the final
electron acceptor
• Produces only small amounts of ATP
– produced only during glycolysis
– much of the energy remain in the chemical
bonds of the organic end-products
Fermentation
• Second stage of
fermentation ensures a
steady supply of NAD+
& NADP+ so that
glycolysis can continue
– regeneration of NAD+
& NADP+ during
fermentation can enter
another round of
glycolysis
Figure 5.19
Fermentation
• Alcohol fermentation
– Produces 2 ethyl alcohol (ethanol) + 2 CO2
• Lactic acid fermentation
– Produces lactic acid; can result in food spoilage
– Homolactic (homofermentative) fermentation:
produces lactic acid only.
– Heterolactic (heterofermentative) fermentation:
produces both lactic acid and other compounds
(e.g. alcohol).
• Use pentose phosphate pathway
Fermentation
Figure 5.18b
Fermentation
Figure 5.23
Lipid and Protein Catabolism
• Lipids and proteins are oxidized for energy
production (sources of electrons & protons
for respiration)
• Lipids (fats) = fatty acids + glycerol (ester
linkage)
• Lipases: extracellular enzymes that degrade
fats into fatty acid and glycerol components
Lipid Catabolism
Beta oxidation:
oxidation of fatty
acids
Oxidation of
glycerol and fatty
acids
Figure 5.20
Lipid and Protein Catabolism
• Proteins = amino acids (peptide bonds)
• Proteases & peptidases: extracellular
enzymes that break down proteins into
amino acids component
• Fig. 5.21 summary of carbohydrates, lipids,
and protein catabolisms
Protein Catabolism
Protein
Extracellular
proteases
Deamination, decarboxylation,
dehydrogenation
Amino acids
Organic acid
Krebs cycle
• Deamination: removal of an amino group
from an amino acid to form an ammonium
(NH4+) (can be excreted from the cell)
Protein Catabolism
Figure 5.22
Biochemical tests
• Used to identify
bacteria and
yeasts.
– Designed to
detect the
presence of
enzymes
Figure 10.8
Photosynthesis
• Used by plants and many microbes to synthesize
complex organic compounds from simple
inorganic substances
• Photo: Conversion of light energy into chemical
energy (ATP)
– Light-dependent (light) reactions
• Synthesis: assembly of organic molecules (using
chemical energy)
– Light-independent (dark) reaction, Calvin-Benson
cycle
Photosynthesis
• Carbon fixation: synthesis of sugars by
using carbons from CO2 gas (from the
atmosphere)
• Recycling of C by cyanobacteria, algae, and
green plants via photosynthesis
• Table 5.6 for summary
The Light-Dependent Reactions:
Photophosphorylation
• Light energy is absorbed by chlorophyll in
the photosynthetic cell
excite some of
the molecules’ electrons
chemiosmotic
proton pump
– Chlorophyll a used by green plants, algae, and
cyanobacteria (in thylakoids)
– Bacteriochlorophylls used by other bacteria
(chlorosomes, intracytoplasmic membrane)
– Bacteriorhodopsin used by Halobacterium
(purple portion of plasma membrane)
Photophosphorylation
• Light-dependent (light) reactions
–
–
–
–
ADP + P + light energy
ATP (chemiosmosis)
NADP reduced to NADPH
cyclic photophosphorylation
noncyclic photophosphorylation
• More common process
Cyclic Photophosphorylation
• Electron eventually return to chlorophyll
Figure 5.24a
Noncyclic Photophosphorylation
• Electrons become
incorporated into
NADPH
Figure 5.24b
The Light-Independent Reactions:
The Calvin-Benson Cycle
• Light-independent (dark) reaction (CalvinBenson cycle)
– use ATP along with electron produced in lightdependent reactions to reduce CO2 to
synthesize sugars (carbon fixation)
– complex cyclic pathway
The Calvin-Benson Cycle
• Go through 6
cycles to
produce one
glucose.
* Shows 3 cycles.
6 CO2
18 ATP
+ 12 NADPH
= 1 Glucose
Figure 5.25
Summary
Fig. 5.26
Metabolic diversity Among Organisms
• All organisms can be classified
metabolicaly according to their nutritional
pattern
– energy source: phototrophs vs. chemotrophs
– carbon (C) source: autotrophs vs. heterotrophs
• autotrophs (lithotrophs): self-feeders; use CO2 as C
source
• heterotrophs (organotrophs): feed on others; require
an organic source of C
Phototrophs
• Use light as energy source.
Chlorophyll
Chlorophyll
oxidized
ETC
ADP + P
ATP
• Photoautotrophs use energy in the CalvinBenson cycle to fix CO2; oxygenic &
anoxygenic.
• Photoheterotrophs use organic compounds as
C source; anoxygenic.
Photosynthetic process in
photoautotrophs
• Oxygenic (produces O2):
– H atoms of H2O are used to reduce CO2 to form
organic compounds, and O gas is given off
• Anoxygenic (does not produce O2):
– typical of cyclic photophosphorylation;
anaerobic reaction
– use sulfur, sulfur compounds, or hydrogen gas
to reduce CO2 to form organic compounds
Chemotrophs
• Use chemical compounds as energy source.
– Redox reactions of inorganic or organic compounds
• Chemoautotroph e.g. Thiobacillus ferroxidans
2Fe2+
NAD+
2Fe3+
NADH
ETC
ADP + P
ATP
2 H+
– Inorganic source of energy; CO2 is C source
• Energy used in the Calvin-Benson cycle to fix CO2.
Chemotrophs
• ATP produced by oxidative phosphorylation
• Chemoheterotroph (fungi, protozoa, animals, & most
bacteria)
Glucose
Pyruvic acid
NAD+
NADH
ETC
ADP + P
ATP
– Energy source and C source are usually the same
organic compound e.g. glucose
• saprophytes (use dead organic matter) vs. parasites (need
living host)
• electrons from H atoms = energy source
Metabolic Diversity Among
Organisms
Nutritional type
Energy source
Photoautotroph
Light
Carbon source
Example
CO2
Oxygenic: Plants,
cyanobacteria, algae
Anoxygenic: Green,
purple bacteria.
Photoheterotroph Light
Organic
compounds
Green, purple
nonsulfur bacteria.
Chemoautotroph Chemical
CO2
Iron-oxidizing
bacteria.
Chemoheterotroph
Organic
compounds
Fermentative
bacteria.
Animals, protozoa,
fungi, bacteria.
Chemical
Metabolic Pathways of Energy Use
• Most of the ATP used in the production of
new cellular components
– Also used to provide energy for active transport,
and flagellar motion
• Anabolism in autotrophs
– carbon fixation via Calvin-Benson cycle require
both ATP& electrons
• Anabolism in heterotrophs
– need ready source of organic compounds + ATP
Metabolic Pathways of Energy Use
• Polysaccharide Biosynthesis
• Use intermediates
produced during
glycolysis and the
Krebs cycle & from
lipids or amino acids.
Figure 5.28
Metabolic Pathways of Energy Use
• Lipid Biosynthesis
– synthesized by
variety of routes
– used for structural
component of
membranes (e.g.
phospholipids,
cholesterol, waxes,
carotenoids)
– also used in energy
storage
Figure 5.29
Amino Acid and Protein
Biosynthesis
• Microbes with the necessary enzymes can
either synthesize all amino acids directly or
indirectly from intermediates of
carbohydrate metabolism
• Others need preformed amino acids
– Supplied from Krebs cycle
• amination: addition of an amino group
Metabolic Pathways of Energy Use
• Amino Acid and Protein Biosynthesis
• Protein synthesis from
amino acids involves
dehydration and ATP.
Figure 5.30a
Metabolic Pathways of Energy Use
• Amino Acid and Protein Biosynthesis
– transamination: transfer of amino group from a
preexisting amino acid
Figure 5.30b
Metabolic Pathways of Energy Use
• Purine and
Pyrimidine
Biosynthesis
• C and N atoms derived
from amino acids form the
purine & pyrimidine rings
Figure 5.31
Integration of Metabolism
• Catabolic and anabolic reactions are joined
through a group of common intermediates
& share some metabolic pathways (e.g.
Krebs cycle)
Amphibolic pathways
• Are metabolic pathways that have both catabolic
and anabolic functions.
– Bridge the reactions that lead to the breakdown and
synthesis of carbohydrates, lipids, proteins, and
nucleotides
Figure 5.32.1
Amphibolic pathways
Figure 5.32.2