BIOS 301 - ETC 1

Locations of major metabolic processes

• Glycolysis → cytoplasm

• PDC, CAC → mitochondrial matrix

• Electron transport, proton gradient, O2 reactions, ATP synthesis → inner mitochondrial membrane

Structure of a mitochondrion

• Outer membrane

  • Relatively porous, allows passage of small molecules, ions, and metabolites

• Intermembrane space

  • Similar environment to cytosol

  • Higher proton concentration (lower pH)

• Inner membrane

  • Relatively impermeable, with proton gradient across it

  • Location of electron transport chain complexes

  • Convolutions (called cristae) increase surface area

• Matrix

  • Location of CAC and parts of lipid and amino acid metabolism

  • Lower proton concentration (higher pH)

Mitochondrial quality control

• Two pathways for unhealthy mitochondria

  • Sustained depolarization → mitophagy

  • Transient depolarization → fusion with healthy mitochondria → allows for salvation of partly defective mitochondria through dilution

Depolarization

Involves loss of electric potential across inner mitochondrial membrane

Number of protons pumped according to electron carrier

• 10 H+ per NADH + H+

• 6 H+ per FADH2

• 3 H+ pumped yields 1 ATP

What determines flow of electrons in ETC?

• Electrons flow from a negative to a more positive reduction potential

• Delta E must be positive (for negative delta G)

  • Delta E is calculated by: Eacceptor - Edonor

Energy required for ATP synthesis

30.5 kJ/mol or 0.2-0.3 volts

Terminal reaction of ETC

• Requires 4e- transfer to O2 to give 2 H2O

• Complex IV (cytochrome oxidase) stores 4 e- just prior to binding and reducing O2

Actual vs. Theoretical yield of ATP from ETC

• More than enough energy is made available by passing 2e- through ETC to make 3 ATP

• However, this theoretical yield of > 3 ATP is diminished by:

  • Proton gradient used for transport of other molecules

  • Controlled leakage of protons to the cytosol

  • Excess energy dissipated as heat

Relationship of reduction potentials and equilibrium constant

• Oxidant = e- acceptor, Reductant = e- donor

• If reduction potential of A>B, then Keq > 1, and current flows from left to right (electrons move B→A)

• If reduction potential of A<B, then Keq<1, and current flows from right to left (electrons move A→B)

Coenzyme Q/Ubiquinone

• Lipid soluble compound that readily accepts electrons

• Upon accepting 2e-, ubiquinone picks ups two protons to generate an alcohol, ubiquinol

• Can freely diffuse in the membrane, carrying electrons with protons from one side of the membrane to the other

  • Transports electrons from Complexes I and II to Complex III

Chloroplast structural similarities to mitochondrion

• Calvin Cycle (CO2 fixation) occurs in matrix

• Proton gradient, ATP synthesis takes place across inner membrane

• Electron transport and photoreactions occur in thylakoid membranes

Chemiosmotic Theory

• Reduced substrate (e.g. NADH+, FADH2) donates e- fuel to ETC

• Electron carriers pump H+ from matrix to intermembrane space as electrons flow to O2

• Energy of e- flow is stored as electrochemical potential

• ATP synthase uses electrochemical potential (i.e. flow of protons down concentration gradient) to phosphorylate ADP

Characteristics of membranes that perform chemiosmotic energy coupling

• Must be impermeable to ions to stably establish a proton gradient

  • E.g. plasma membrane in bacteria, inner membrane in mitochondria, thylakoid membrane in chloroplasts

• Must contain proteins that couple the downhill flow of electrons in ETC with uphill flow of protons across membrane

• Must contain a protein that couples the downhill flow of protons to the phosphorylation of ADP

Redox centers

• Specific chemical groups within the protein complexes of ETC that physically capture and release electrons

• E.g. flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), cytochromes a-c, iron-sulfur cluster

• Electrons move through centers with low reduction potential to high reduction potential

  • Each jump releases a small amount of energy that is sued by ETC protein complexes to pump H+ against concentration gradient

• FMN exists in Complex I because it can simultaneously accept 2e- from NADH and pass them along one-by-one to subsequent series of Fe-S clusters

NAD+/NADH structure, function

• Dissociate from enzyme after reaction

FAD/FMN structure, function

• Allow for single electron transfers

• Tightly bound to proteins (prosthetic group)

Iron-Sulfur clusters structure, function

• One electron carriers

• Coordinated by cysteines in the protein

• Have a brownish color when oxidized

Cytochromes structure, function

• One electron carriers

• Contain porphyrin derivatives coordinated with iron

• Cytochromes a,b,c differ by addition of chemical groups to the porphyrin ring

• Oxidized and reduced forms differ in relative light absorption

  • The wavelength at which alpha absorption peak occurs characterizes the identity of the reduced cytochrome