Anaerobic Respiration
• electron acceptors
other than O2
• generally yields less
energy because E0’
of electron acceptors
are less positive than
E0’ of O2
Table 9.1
1
ETC of Paracoccus denitrificans aerobic
Figure 9.16 (a)
2
ETC of P. denitrificans - anaerobic
Figure 9.16 (b)
3
example of anaerobic respiration
Denitrification
• The biochemical pathway for
dissimilative nitrate reduction has
been well-studied
• Enzymes of the pathway are
repressed by oxygen
Assimilative and
Dissimilative Metabolism
Assimilative metabolism: A small amount of the
electron acceptor is reduced and used as the
source of nutrient for cell growth.
Dissimilative metabolism: A large amount of
the electron acceptor is reduced for energy
and the reduced product is excreted into the
environment.
Denitrification
Sulfate Reduction
Several inorganic sulfur compounds can be used as
electron acceptors in anaerobic respiration
4H2 + SO42- + H+
∆G = -152 kJ
HS- + 4H2O
Major Electron Donors for SulfateReduction
•
•
•
•
•
•
H2
Lactate
Pyruvate
Fumarate
Alcohols
Malate
•
•
•
•
•
•
Acetate
Propionate
Butyrate
Long-chain fatty acids
Benzoate
hexadecane
Fermentations
• Pyruvate or other intermediates are used
as electron acceptor
• NADH is oxidized to NAD+
• Substrate only partially oxidized
• No electron transport chain (no oxidative
phosphorylation)
– ATP formed by substrate-level
phosphorylation
9
Fermentations
1. In most fermentations intermediates with an
energy-rich phosphate bond or a molecule of
coenzyme-A are generated
2. Hydrolysis of these energy-rich compounds can be
coupled to ATP synthesis by Substrate-Level
Phosphorylation
10
The Essentials of Fermentation
Figure 21.1
11
Energy-Rich Compounds Involved in SLP
12
Fermentations
Figure 9.17
13
homolactic
fermenters
heterolactic
fermenters
food
spoilage
yogurt,
sauerkraut,
pickles, etc.
Figure 9.18
14
alcoholic
fermentation
alcoholic
beverages,
bread, etc.
Table 9.2
15
Fermentations of amino acids
• Stickland reaction
– oxidation of one
amino acid with
use of second
amino acid as
electron acceptor
– carried out by
some Clostridium
spp.
Figure 9.19
16