Aerobic respiration

Key reactions for metabolic pathways and biogeochemical cycles are redox reactions

Oxidation • release of electrons by an electron donor Reduction • uptake of electrons by an electron acceptor

1

Reduction equivalents:

Electrons

e-

[H]

or:

H+ + e-

do not occur as free electrons in the cell They only show their presence due to the different redox state of their carrier

The redox potential is dependent on the respective carrier Eo’ [mV] NADH/NAD+

- 320

FADH/FAD +

- 220

Glycolysis

Oxidative decarboxylation of pyruvate

TCA cycle

2

Aerobic respiration

+ O2

CO2 + H2O

glucose ATP

reduction equivalents

NADH

e- transported stepwise via an increasing redox potential to O2

NAD+ NADH

2 pyruvat CO2, NADH ATP

H+

2 e-

Proton gradient

O2 CO2 GTP FADH2

NADH

H2O

respiratory chain free energy is used for ATP formation

The respiratory chain Transport system • located in cytoplasma membrane / inner membrane of mitochondria • uptake of electrons and protons • primary transport of protons (coupled to redox cycles and O2 reduction) • formation of a proton and an electrical gradient (membrane potential) • electrogenic process Electrons • electrons are transfered • transfer only when redox carrier in contact • cytochromes and the FeS complex only change their oxidation number Protons • protons are released to environment • vectorial proton translocation • release only at flavoproteins, quinones (and at terminal oxidase)

3

INSIDE

OUTSIDE

NADH

NADH Oxidase

Periplasm

H+

Cytoplasm

NAD+ + H+

respiratory chain

H+ e-

½ O2 + 2H+

cyt o

[H+] +

H2O

[H+]

ADP + Pi

H+

ATPase

H+

-

ATP

Electron transport chains and their relation to E0' Breaking up the complete oxidation into a series of discrete steps • energy conservation is possible through proton motive force formation • leading to ATP synthesis

electron transport system

Typical for • mitochondria of eukaryotic cells • some Bacteria (e.g. Paracoccus denitrificans)

E. coli lacks cytochromes c and aa3 • electrons go directly from cytochrome b to cytochrome o or d (similar E0' to cytochrome aa3)

4

Orientation of key electron carriers in the membrane

FMN, flavoprotein Q, quinone Fe/S, iron sulfur protein cyt a, b, c, cytochromes

+ and – charges: H+ and OH-

Orientation of key electron carriers in the membrane

Recycling of electrons during the “Q cycle” • electrons from QH2 can be split between the Fe/S protein and the b-type cytochromes

5

Orientation of key electron carriers in the membrane

Increasing the number of protons pumped at the Q-bc1 site • electrons that travel through the b-type cytochromes reduce Q back to QH2 • two, one-electron steps

Orientation of key electron carriers in the membrane

Electrons that travel to Fe/S • proceed to reduce cytochrome c1 cytochrome c a-type cytochromes Reducing O2 to H2O • two hydrogen atoms required

6

Structure and function of ATP synthase (ATPase)

Cytoplasmatic complex (F1) • five polypeptides α, β, γ, ε, δ • conversion of ADP + Pi to ATP

Membrane integrated complex (F0) • three polypeptides a, b2, c12 Subunit a • channeling of protons • protons enter subunit a • proton motive force drives ATP synthesis

The ATPase is reversible in its action • ATP hydrolysis can drive formation of a proton motive force

Measurement of proton translocation • staedy state (release and uptake of protons, pH constant) • small pulse of oxygen in an unbuffered system short imbalance (decrease of pH in medium)

O2 saturation (%)

• adjustment of external pH via proton uptake (ATPase) negative exponential curve (imblance: stronger inflow)

pH

• proton release measurable via a pH electrode

time (s)

Fig.: Cypionka

7

Inhibitors • cyanide, CO (cytochrome oxidase) • Rotenon (oxidation of NADH2) • Antimycin A (oxidation of cytochrome c)

Decoupling substances (protonophores) • permeabilise the membrane for protons • break down of proton gradient and membrane potential • dinitrophenol, CCCP, TCS

Growth of E. coli with Glucose 1.) Formation of biomass, anabolism (specific yield) Sum formular for biomass Protein, nucleic acids, lipids, polysacch. => 10% N, 1% S, 1% P ... Redfield 1963: biomass of algae

C106H263O110N16P1S1

C6H12O6 → 6 ↑ x ATP

How much ATP is needed for biomass formation?

8

2.) Degradation, dissimilation, catabolism (mandatory) a) Aerobic respiration C6H12O6 + 6 O2 → 6 CO2 + 6 H2O ↓ y ATP b) Fermentation: mixed acid fermentation (simplified) C6H12O6 + 1/2 H2O → Lactate + CO2 + H2 + 1/2 Acetate + 1/2 EtOH ↓ z ATP

Question: How big is x, y, z?

Formation of biomass: x

C6H12O6 → 6 ↑ x ATP

Experience: YATP ≈ 10.5 g dry mass/mol ATP (depending on C-source, N-source, pH ...) a dry mass with MW = 30 results in: 1 mol ATP gives 10.5 g 2.86 mol ATP give 30 g 17.1 mol ATP give 180 g for the assimilation of 1 mol Glucose

x = 17

9

Aerobic respiration: y [ 2 FADH2 + 10 NADH2 ] C6H12O6 + 6 H2O → 6 CO2 + 24 [H] ↓ 4 ATP Glycolysis + TCA Substrate level phosphorylation (depending on pathway) 12 O2 →12 H2O ↓

"Electron transport phosporylation" no ATP gained through respiratory chain! "Textbook"

ATP E.coli

FADH2 NADH2

4 30

2 20

Sum

38

26

y = 26

Fermentation: z

C6H12O6 ← Transport + Phosphorylation ↓ ↑ ↓ → 2 ATP ↓ → 4 [H] ↓ → 2 ATP 2 [H] ↓ ↓ ↓ 1 Lac ← 2 Pyr ↓ 1 AcCoA + Formiat → CO2 + H2 ↓ Pi →↓ ½*2 [H] ½*2 [H] ↓ ↓ ↓ ½ Ac-P → ½ Acetaldehyde → ½ EtOH ↓ ½ ADP ADP →↓ ↓ ½ Acetat + ½ ATP

z = 2.5

10

Expected yield (Y) with O2 (10-a)*17 ATP 10 Gluc. ----------------> (10-a) ↓ ↑ → a * 26 ATP ↓ a * 6 CO2 a * 26 = (10-a) * 17 a * 26 = 170 - 17 a 43 a = 170 a = 3.95 ≈ 4 Expectation: 40 % of Glucose is dissimilated, 60 % assimilated

Expected yield (Y) without O2 (fermentation) (10-b)*17 ATP 10 Gluc. ----------------> (10-b) ↓ ↑ → b * 2.5 ATP ↓ b * Fermentation products 2.5 b = (10-b) * 17 2.5 b = 170 - 17 b 19.5 b = 170 b = 8.95 ≈ 9 Expectation: 90 % of Glucose is dissimilated, 10 % assimilated

11

Respiration of alternative electron acceptors NAD+ glucose

NADH H+

ATP

2 e-

Proton gradient

X

NADH

Xred.

pyruvate CO2 ATP

Nitrite Nitrate TCC

CO2

Thiosulfate IronIII

GTP NADPH FADH2

ManganeseIV NADH

UraniumVI

Electron transport processes in E. coli

More protons are translocated aerobically during electron transport reactions than with nitrate as electron acceptor

The terminal oxidase (cyt o) can pump one proton

O2

NO3-

12

Free Energie (ΔG0’) depend on

“Nernst equation”

• Number of electrons transferred

ΔG0 ' = − z ⋅ F ⋅ ΔE0 '

• Difference of redox potentials (ΔEo’) Eo’ [mV] aerobic respiration

O2/H2O

+820

ΔG0’ = -2870 kJ/mol Glucose

denitrification

NO3-/N2

+751

ΔG0’ = -2715 kJ/mol Glucose

+363

ΔG0’ = -1800 kJ/mol Glucose

nitrate ammonification NO3-/NH4

NADH/NAD+ - 320

Energy yield

FADH/FAD + - 220

Energy conservation at extreme pH

pHi

PMF: Proton motive force PMF = -F·ΔΨ + R·T·ΔpH

ΔpH

ΔΨ

F ΔΨ R T

Faraday constant Membrane potential Gas constant Temperature [K]

Acidophiles: large ΔpH compensated via ΔΨ Alkaliphiles: Proton gradient? Na+ instaed of H+

13

Introduction of Oxygen into sediments Water layer • transport • Eddy diffusion Diffusive boundary layer • 0,2-0,5 mm • low consumption • linear concentration gradient Within the sediment • diffusion, but high O2 consumption • parabolic curve

Waste water: biochemical oxygen demand, BOD (German: BSB5) Measure for the amount of dissolved O2 thet is needed for the degradation of dissolved organic compounds in a (waste) water sample

BSB5 indicates how much oxygen is consumed after an incubation of a given sample at 20 °C for five days to degrade the containing organics. • Respiration by heterotrophic microorganisms • Comparizon to CSB (chemical oxygendemand): measure for degradability of organic compounds • BSB up to 50% of CSB: good biological degradability • CSB much higher than BSB: recalcitrant organic compounds

14

Measuring the BSB5 Oxygen saturated water sample (contains heterotrophic bacteria)

Measuring the oxygen content

Sample is sealed (gas tight) and incubated at 20 °C --> oxygen is consumed due to microbial degradation of organic compounds Second oxygen measurement to determine the oxygen consumption --> BSB5 in mg O2/l Source: Folienserie des Fonds der Chemischen Industrie - Umweltbereich Wasser (modified)

15