Metabolic modes of energy generation Respiration – couple substrate oxidation to the ultimate reduction of an extrinsic chemical such as O 2, DMSO, etc. Fermentation – couple substrate oxidation to reduction of internally generated substrates Photosynthesis – harvest light energy to facilitate electron transport in energy generating mechanism

Chemiosmotic Theory The transmembrane differences in proton concentration are the reservoir for energy extracted from biological oxidation reactions - Peter Mitchell Fermentation utilizes substrate level phosphorylation, more on that later

Oxidative phosphorylation and photophosphorylation have similarities and differences In eukaryotes, both are organellar processes; mitochondria and chloroplast Both involve flow of electrons through membrane components Free energy from electron flow is used to pump protons across a membrane Flow of protons back drives ATP synthesis In aerobic systems, Oxidative phospho. Reduces O 2 to H 2 O, while photophospo. Can oxidize H 2 O to O 2

Oxidative energy generation leads to ATP, water, and oxidized electron carriers

Eukaryotic cells

Oxidative phosphorylation is a mitochondrial process Although the mitochondria imports some biomolecules from the cytoplasm, it contains Citric acid cycle enzymes, it’s own genome, etc. Notice the cristae which increase membrane surface area.

Electron carriers initiate oxidative phosphorylation Pools of electrons linked to carriers such as NAD, NADP, FMN, and FAD are generated by catabolic mechanisms (mostly NADH is generated) Note when depicted as NAD +, the intent is to reflect oxidation state, NOT charge on the molecule

Electrons are passed to membrane components For instance, NADH is oxidized by a membrane bound enzyme NADH dehydrogenase, which subsequently passes electrons to quinones, and so on. Various steps are linked to proton translocation out of the mitochondrial inner membrane

A membrane is a prerequisite for biological energy generation This serves as an impermeable barrier to many solutes such as H +, even H 2 O. Polar molecules cannot traverse the hydrophobic layer.

Cells can alter the fatty acid content of their membranes Sterols modify fluidity also

Ubiquinone (CoQ) is a lipid soluble two electron, two proton carrier Plants – Plastoquinone Bacteria – menaquinone Freely diffusible in Lipid bilayer

Cytochromes classified on basis of porphyrin ring

Another example of a spectroscopic bioassay

Iron-sulfur proteins carry electrons and do more… At least eight iron-sulfur proteins act in mitochondrial electron transfer Also Fe-S centers have been shown to be sensors of aerobic/anaerobic gene expression

Determining the order of electron transfer Standard reduction potentials Spectroscopic measure of carrier oxidation Inhibitors –Rotenone inhibits NADH dehydrogenase –Antimycin A inhibits cytochrome b –Cyanide, azide, or Carbon monoxide inhibit cytochrome oxidase aa 3

Experimental evidence for electron transfer order

The order of electron transfer under aerobic conditions

Cytosolic-derived NADH must be shuttled into the mitochondria Although citric acid cycle and fatty acid oxidation occur in the “right” place (mitochondrial matrix), glycolysis is cytoplasmic and NADH from this pathway must be shuttled into the matrix of the mitochondria (membrane is impermeable to this compound; no transporter) –Glycerol-3-phosphate shuttle –Malate-Aspartate shuttle

Glycerol-3-phosphate shuttle 2 e - from NADH to FADH 2 Get only 2 ATP from FADH 2 vs 3 from NADH More on that later

Malate-Aspartate Shuttle

General class of transporters

Electron transport is accomplished by enzyme complexes

NADH:ubiquinone oxidoreductase utilizes NADH generated from catabolic reactions This is a huge protein complex ~900,000 kDa Electron transfer from NADH to ubiquinone is coupled to the translocation of protons through the protein, with a stoichiometry of 2H+/e- –NADH + H + + Q  NAD + + QH 2

An electron’s path through this complex Oxidation of NADH transfers two electrons to FMN bound to the enzyme, and releases a proton into the matrix The electrons are passed from FMN through a series of Fe-S centers the last one being called N-2 (Six in the case of the mitochondrial enzyme, but only four appear to be universally conserved) N-2 reduces ubiquinone

Proton pumping by this complex Experiments suggest one proton is pumped into the intermembrane space during NADH to N-2 transfer of one electron, and a second proton during N-2 to ubiquinone transfer of one electron Recall the stoichiometry, 2 protons per electron – this means four protons in total are pumped for each NADH oxidation