Electron transport chains

Electrons move from a carrier with a lower standard reduction potentials (EO) to a carrier with a higher EO

Mitochondrial electron transport chain

Electrons eventually combine with 1/2 O2 and 2 H+ to form H2O

Protons pumped across the membrane at various points during electron transport

E. coli electron transport chain

Electrons move from:

NADH FAD Coenzyme Q

Terminal oxidase varies depending on growth conditions

Amount of protons pumped out depends on growth conditions

P. denitrificans electron transport chains

Has both aerobic and anaerobic electron transport chains

Anaerobic chain uses NO3- as the

final electron acceptor

Oxidative phosphorylation

Is dependent on the proton motive force and chemiosmosis

The proton motive force

Protons are pumped from the interior to the exterior of the membrane resulting in a gradient of protons and a membrane potential

The roles of proton motive force

Powers rotation of bacterial flagella

Required for some types of active transport

Generation of ATP

The roles of proton motive force

Flagella rotation Active transport

Chemiosmosis

Diffusion of protons back across the membrane

drives the formation of ATP by ATP synthase

ATP synthase

Composed of 2 components:

F0 - membrane embedded

F1- attached to inner membrane

F0 component

Composed 1 a subunit, 2 b subunits and 9-12 c subunits

Electrons pass through a channel in F0 a subunit

F1 component

Appears as a sphere on the inner membrane

Composed of 3 subunits, 3 subunits 2 subunits and 1 subunit

F1 component

Passage of electrons through F0 causes subunit to rotate

Rotation causes conformational changes in subunits that results in the synthesis of ATP

F1 component

Yield of ATP in eukaryotic cells

1 NADH generates 2-3 ATPs

1 FADH2 generates 2 ATPs

Actual yield can be closer to 30 ATPs

Yield of ATP in prokaryotic cells

Prokaryotic cells generate less ATP

Amounts vary depending on growth conditions

Anaerobic respiration

Final electron acceptor is an inorganic molecule other than oxygen

Major electron acceptors are nitrate, sulfate and CO2

Metals and certain organic molecules can also be reduced

Anaerobic respiration

Reduction of nitrate in respiration known as dissimilatory nitrate reduction

Nitrate often reduced sequentially to nitrogen gas (N2)

Process referred to as denitrification

Carbohydrate catabolism

Glucose, fructose and mannose can enter glycolytic pathway after phosphorylation

Galactose is modified before being transformed into glucose-6-P

Carbohydrate catabolism

Disaccharides and polysaccharides must be cleaved into monosaccharides

Can be cleaved by hydrolysis or phosphorolysis (results in the addition of a phosphate group)

Carbohydrate catabolism

Reserve polymers like glycogen and starch are degraded by phosphorolysis to release glucose-1-P

Converted to glucose-6-P and enters glycolytic pathway

Poly--hydroxybutyrate converted to acetyl-CoA and enters the TCA cycle

Lipid catabolism

Triacylglycerides are composed of glycerol and three fatty acids

Lipases separate glycerol from fatty acids

Glycerol phosphorylated and converted to dihydroxyacetone phosphate glyceraldehyde-3-P glycolysis

Lipid catabolism

Fatty acids are converted to CoA esters and oxidized by the -oxidation pathway

Fatty acids degraded to acetyl-CoA TCA cycle

Lipid catabolism

Fatty acids are converted to CoA esters and oxidized by the -oxidation pathway

Fatty acids degraded to acetyl-CoA TCA cycle

-oxidation pathway

Produces

1. Acetyl-CoA

2. NADH

3. FADH2

Protein and amino acid catabolism

Proteases hydrolyze proteins and polypeptides into amino acids

Removal of amino group referred to as deamination

Deamination

Usually accomplished by transamination

Amino group transferred to an -keto acid acceptor

Deamination

Organic acid oxidized for energy or used as carbon source

Deamination

Excess nitrogen excreted as ammonium ion

Oxidation of inorganic molecules (chemolithotrophy)

Chemolithotrophs derive energy from the oxidation of inorganic molecules

Most common electron donors are hydrogen, reduced nitrogen compounds, reduced sulfur compounds and ferrous iron (Fe2+)

Oxygen, nitrate and sulfate can be used as the final electron acceptor

Oxidation of inorganic molecules (chemolithotrophy)

Hydrogen oxidation

Several bacteria possess a hydrogenase enzyme that catalyzes the reaction:

H2 2H+ + 2e-

Electrons can be donated to an electron transport chain or NAD+

Nitrogen oxidation

Species of Nitrosomonas and Nitrosospira oxidize ammonia to nitrite

NH4+ + 3/2 O2 NO2

- + H2O + 2H+

Species of Nitrobacter and Nitrococcus oxidize nitrite to nitrate

NO2- + 1/2 O2 NO3

-

Nitrogen oxidation

Two genera working together can oxidize ammonia to nitrate

NH4+ + 2 O2 NO3

-

Process referred to as nitrification

Nitrogen oxidation

Proton motive force can be used to produce ATP and NADH

Sulfur oxidation

Some microorganisms can use reduced sulfur compounds as a source of electrons

Species of Thiobacillus oxidize sulfur-containing compounds to sulfuric acid (important environmental consequences)

Sulfur oxidation

Can generate ATP by oxidative phosphorylation and substrate level phosphorylation

Substrate level phosphorylation requires the formation of adenosine 5-phosphosulfate (APS)

Oxidation of inorganic molecules

Much less energy is available from the oxidation of inorganic molecules than from the oxidation of organic molecules