Download Aerobic/Anaerobic Respiration

Aerobic/Anaerobic Respiration
The physical chemistry of electron transfer
Respiration
Generation of Proton Motive Force (via catalytic reactions) created by the transfer of
protons across the plasma membrane to the exterior, and transfer of electrons in an
electron transport chain to an external terminal electron acceptor
2H+
2H+
Oxidation-reduction (redox) reactions
Electron donor;
becomes oxidized
2H+
H2 Æ 2e- + 2H+
6H+
H+
e-
2H+
2H+
(oxidation half-reaction)
needs to be coupled with a reduction half-reaction e.g.
½O2 + 2e- +2H+ Æ H2O
2H+ + ½H2O Æ H2O
(reduction half-reaction)
Electron acceptor;
becomes reduced
NADH + H+ Æ NAD+
2ADP + 3P Æ 3ATP
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
1
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
2
UG0’ (free energy) and Eo’
Redox
The Electron Tower
Reduction potential (Eo’)
-0.5
The tendency to become either reduced or oxidized
-0.3
-0.4
-0.2
At pH 7
2H+/H2 (-0.42)
NAD+/NADH (-0.32)
H2 + fumarate2- Æ succinate2UG0’ = 86 kJ
Pyruvate/Lactate (-0.19)
-0.1
Eo’ for H2 Æ 2e- + 2H+
is –0.421 volts
Eo’ for ½O2 + 2e- +2H+ Æ H2O
is +0.816 volts
0.0
0.1
Fumarate/Succinate (+0.03)
H2 + NO3- Æ NO2- + H2O
UG0’ = 163 kJ
0.2
0.3
Redox couples
0.4
0.5
oxidized form reduced form
Couple
2H+/H2
½O2/H2O
0.6
o
E ’
-0.421 V
+0.816 V
0.7
NO3-/N2 (+0.74)
H2 + ½O2 Æ H2O
UG0’ = 237 kJ
0.8
½O2/H2O (+0.82)
0.9
x Strong reductants at the top of the tower
x Strong oxidants at the bottom of the tower
x Electron transfer moves exergonically from more negative to more positive Eo’ couples
The more negative the Eo’ the more the tendency to donate electrons
The positive the Eo’ the more the tendency to accept electrons
Dr. Clem Kuek
NO3-/NO2- (+0.42)
ZIP/Lect+Prac/MPAG/Metabolism5.doc
3
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
4
Electron transport chains in microbes 2
Electron transport chains in microbes
Series of linked redox couples move electrons from compounds with more
negative Eo’ to those with less negative Eo’
x Transport e- and H+ between energy sources and energy storage or
biosynthesis
ƒ Respiratory ETC: from catabolism to oxidative phosphorylation of ADP
ƒ Photosynthetic ETC: from light source to phosphorylation of ADP and/or oxidation of NADP+ to
NADPH
ƒ Lithotrophic ETC: from inorganic compounds to reduce NAD+ to NADH
x Found the plasma membrane of prokaryotes and mitochondria of
eukaryotes
x Movement of protons and electrons/ATP production
ƒ Electron and protons may enter ETC at different stages from e.g. NADH, FADH, lactate
ƒ Protons excreted at various stages of chain
ƒ Electrons transferred to external acceptors via oxidases/reductases creates Proton Motive
x Contain
ƒ Primary electron donor e.g. NADH
ƒ Flavoproteins, cytochromes, ferredoxins, quinones (redox couples)
ƒ NADH dehydrogenases and electron transfer enzymes
ƒ Terminal electron acceptor e.g. O2; NO3-
Force to phosphorylate ADP
Respiratory Electron Transport Chains
x Two types of respiration
ƒ Aerobic: When only O2 is the external, terminal electron acceptor
ƒ Anaerobic: External, terminal e- acceptors other than O2
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
5
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
6
Aerobes and anaerobes using respiration
Anaerobic respiration with nitrate reduction
x Aerobes
ƒ Only O2 as terminal, external e- acceptor
ƒ have cytochrome oxidase as terminal enzyme in the ETC
ƒ need to protect cell against oxidation by superoxide (O2-), H2O2, hydroxyl radicals
x Nitrate
ƒ assimilated to organic N by aerobic heterotrophs under aerobic conditions
o e.g. into amino acids and used for growth
ƒ dissimilated by denitrifying bacteria for energy
(by-products of reduction of O2 to H2O in respiration)
o used for energy production in respiration to form nitrite and products returned to environment
o superoxide dismutase, catalase, peroxidase
x Dissimilatory nitrate reductase
ƒ enzyme biosynthesis repressed by O2,
x Facultative anaerobes and strict anaerobes
ƒ Have other than O2 as terminal, external e- acceptor
ƒ Have no cytochrome oxidase; other terminal enzyme
synthesised under anoxia
o e.g. nitrate reductase, nitrite reductase
-
x Further reduction of NO2 by series
of various nitrogen reductases in
different bacteria
ƒ Denitrification in soil, water, effluent
x Various kinds of anaerobic respiration processes
ƒ Various external terminal electron acceptors
o nitrate, sulphate, CO2 , fumarate
ƒ Reducing power differentials: more negative = more anaerobic
-
ƒ
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
7
NO2 ´ N2O ´ NH3 ´ N2
Nitrogen lost to organisms for assimilation
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
8
Anaerobic Respiration:
Energy Production from Respiration
Other terminal electron acceptors
x Proton/Oxygen (P/O) ratios
ƒ moles of ATP per atom of O2 utilised (aerobes)
ƒ also relates to moles of ATP per 2 protons excreted by ETC
ƒ depends on stage at which protons/electrons enter ETC
ƒ often P/O ratio = 3 (aerobes) but may only be 1 - 2 in strict anaerobes and facultative anaerobes
x Sulphate
ƒ oxidised to organic S compounds e.g. methionine, (assimilation) concurrently with
ƒ reduction (dissimilation) to sulphite, H2S generates ATP
ƒ Specialised electron transport chains
ƒ electrons donated from acetate, H2, lactate, not glycolysis and TCA cycle
x Calculations based on oxidation of NADH + H+ via aerobic respiration:
ƒ 2e- transferred through ETC to ½O2 (1 atom O)
ƒ 3 u 2H+ excreted for each transfer of electron pair
ƒ 2H+ + ½O2 ´ H2O external to membrane
ƒ 3H+ used to produce 3 ATP; 3H+ released to environment
x Organic electron acceptors
ƒ CO2 reduced in methanogens to methane and in homoacetogens to acetate
ƒ Fumarate ´ succinate; trimethylamine oxide ´ trimethylamine
x Nutrient cycling in the environment
ƒ Reduction of nutrients is anaerobic part of biogeochemical cycles of elements
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
9
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
10
Energy Production from Catabolism 2
Energy Production from Catabolism
x ATP yield from aerobic catabolism of one glucose molecule
Based on P/O ratio = 3 for NADH; 2 for FADH2 (in oxidative phosphorylation)
x Substrate level phosphorylation of ADP
ƒ ADP + Pi ´ ATP directly during oxidation of organic substrate
ƒ Of lesser importance as source of energy in respiratory dependent micoorganisms
ƒ Occurs in
x Glycolytic pathway
Substrate level phosphorylation
Oxidative phosphorylation with 2 NADH
o EMP, Entner-Doudoroff, Phosphoketolase Pathways not Pentose Phosphate Pathway
o TCA cycle with GDP ´ GTP / ADP ´ ATP
x 2 pyruvate to 2 acetyl-CoA
Oxidative phosphorylation with 2 NADH
x Oxidative Phosphorylation of ADP: Respiration
ƒ ATP generated from Proton Motive Force
ƒ Of greater importance as source of energy in respiratory dependent microorganisms
ƒ Occurs as a result of all glycolytic pathways and TCA cycle (NADH + H/FADH + H)
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
2 ATP
6 ATP
6 ATP
x Tricarboxylic acid cycle
11
Substrate level phosphorylation (GTP)
Oxidative phosphorylation with 6 NADH
Oxidative phosphorylation with 2 FADH2
2 ATP
18 ATP
4 ATP
Total aerobic yield
38 ATP
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
12
Identification of various respiratory types
x (Cytochrome) Oxidase test
ƒ Positive reaction identifies organisms which use aerobic respiration
x Nitrate reductase test
ƒ Positive reaction identifies organisms which use nitrate as terminal electron acceptor in
anaerobic respiration
x Catalase test
ƒ Positive reaction identifies organisms which are facultatively anaerobic, i.e. tolerate O2, or
use aerobic respiration
x Organisms not using nitrate or oxygen
ƒ are negative for (cytochrome) oxidase and nitrate reductase
ƒ may be positive or negative for catalase
x Catalase negative organisms
ƒ may still be O2 tolerant
ƒ may be aerobes, facultative or strict anaerobes
Dr. Clem Kuek
ZIP/Lect+Prac/MPAG/Metabolism5.doc
13