A. Mitochondria have two membranes and two compartments
B. Oxidation of Pyruvate
Note: Remember that we get 2X everying listed above for each glucose molecule metabolized
C. Electron Transport-Transfer of Electrons in NADH and FADH2 to O2 producing H2O
A. Mitochondria
1. Two Membranes
a) outer -- has channels to let small hydrophilic molecules through
b) inner -- impermeable to hydrophilic molecules; high protein concentration
2. Compartments -- Matrix contains enzymes, mitochondrial DNA etc.
3. The figure below shows what a mitochondrion really looks like. Click on the picture below to download a Quicktime Movie showing a 3-dimensional model of the mitochondrion rotating.
1. The high potential energy of electrons in reduced nucleotides, NADH and FADH2, reduce O2 during a series of oxidation/reduction reactions in the inner membrane. The overall reaction for NADH is:
2. Each NADH is equivalent to one molecule of H2 but is much higher in potential energy. Thus, this reaction produces more energy than burning hydrogen gas. In order to control such a large amount of potential energy NADH is not allowed to react directly with O2; the electrons pass through a series of molecules producing a sequence of oxidation/reduction reactions liberating the energy in smaller manageable packets.
1. Flavin or Pyridine-Nucleotides:
a) Nicotinamide Adenine Dinucleotide exists in an oxidized form, NAD+, and a reduced form NADH which differ by the addition of 2 electrons and 1 H+. NADH is produced by the dehydrogenases of PDH Complex and of the Krebs (TCA) Cycle; it is also the substrate of NADH Dehydrogenase, one of the primary complexes of mitochondrial electron transport.Figure 7.3 b) Flavin is derived from the vitamin, Riboflavin, and is found in 2 forms: Flavin MonoNucleotide (FMN) and Flavin Adenine Dinucleotide (FAD). They are lower in potential energy than NADH/NAD+
2. Quinones--Ubiquinone (ubiquitous), small organic molecules. The very long hydrophobic tail of 10 isoprene units makes it very hydrophobic so ubiquinone/ubiquinol is concentrated in the hydrophobic region of the lipid bilayer and carries electrons between different proteins. Ubiquinone/Ubiquinol differ in oxidation state by 1 H2. Also called Coenzyme Q
Heme: Fe++ tetracoordinated by a tetrapyrole (protoporphyrin). The 5th and 6th coordination positions can be from amino acid residues and/or binding a ligand such as O2 as is the case for heme a3 of cytochrome oxidase.
3. Iron-Sulfur Proteins (FeS)--components of NADH DeH'ase and Cyt bc 1 Complex. Also called Non- Heme Iron Proteins since iron atoms are coordinated to sulfur atoms from Cys and from H2S
4. Cytochromes--heme proteins, Cyt. b, c 1, c, aa 3. Unlike hemes in hemoglobin and myoglobin, the Fe ions in cytochromes undergo oxidation/reduction between Fe++ and Fe+++.
1. Most are organized into large multisubunit integral membrane protein complexes or coupling sites.
Complex I: NADH Dehydrogenase (MW = 750,000) Coupling Site 1Complex II: Succinate Dehydrogenase -- membrane bound enzyme which catalyzes the oxidation of succinate in the TCA-Cycle producing FADH2 which then reduces Ubiquinone.
Complex III: Cytochrome c Reductase (Cytochrome bc1complex) (MW = 280,000). Coupling Site 2
Complex IV: Cytochrome c Oxidase (Cytochrome aa3) (MW = 200,000) Coupling Site 3
This reaction accounts for more than 90% of all the O2 consumed by living organisms.
2. Small Mobile Electron Carriers:
Ubiquinone/Ubiquinol -- small hydrophobic electron carriers which shuttle electrons between the large complexes and back and forth across the lipid bilayer.Cytochrome c--is a small water soluble protein which is a mobile electron carrier and carries electrons between cytochrome bc1 complex and cytochrome c oxidase
1. Electron transport back and forth across the membrane transfers H+'s from the Matrix to the intermembrane space (cytoplasm); makes use of the impermeable inner membrane. Creates both:
a) DpH -- difference in [H+] across the membrane of about 1.4 pH units
b) DY membrane potential -- difference in charge, negative in matrix, of about -0.14 VoltsDµH+ = DpH + DY Both gradients combine to increase the potential energy of H+ in the intermembrane space. Synthesis of 1 ATP requires the energy from transport of 2 or more H+'s across the membrane.
This was dramatically demonstrated with an experiment in which the F1 head of the enzyme was attached to a microscope slide, and a fluorescent actin filament was attached to the g subunit. The filament turned rapidly when ATP was added to the solution around the enzyme. Click on the picture below to see a video of the motor turning.
The mitochondrial inner membrane must be impermeable to polar and charged molecules in order to maintain the gradient of [H+]. Nevertheless, some polar/charged molecules must cross this membrane; these include pyruvate, ADP, ATP, phosphate, fatty acids, Ca++ etc. The mitochondrial inner membrane has special transport proteins which are integral membrane proteins specifically designed to help these molecules to cross the membrane.
