OXIDATIVE PHOSPHORYLATION AND ELECTRON TRANSPORT

 

The Complete Oxidation of Glucose

 

C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O          

DGº’ = -2,823 kJ mol-1 = -680 kcal mol-1

 

1st Half: 

·        C6H12O6 + 6 H2O ® 6 CO2 + 24 H+ + 24 e

·        The 12 e pairs derived from glycolysis and TCA cycle are transferred to 10 NAD+ and 2 FAD (Fig. 17-1)

 

2nd Half:

·        24 H+ + 24 e + 6 O2 ® 12 H2O

·        By oxidative phosphorylation in mitochondria.

·        Produce ATP.

 

Oxidative Phosphorylation

 

·        The process in which ATP is formed as electrons are transferred from NADH and FADH2 to O2 by a series of electron carriers (over 10 redox centers in 4 enzyme complexes).

·        Carried out by respiratory assemblies in the inner membrane of mitochondria.

·        Oxidation of NADH produces 3 ATP.  Oxidation of FADH2 produces 2 ATP.

·        Coupled to the pumping of H+ out of mitochondrial matrix across inner membrane.

·        ATP is formed when H+s flow back to mitochondrial matrix.

 

Mitochondrion

 

·        Site of eukaryotic oxidative metabolism.

·        1948: Eugen Kennedy and Albert Lehninger discovered that pyruvate dehydrogenase, TCA enzymes, F.A. oxidation enzymes, and enzymes and redox proteins for electron transport and oxidative phosphorylation are all in mitochondria.

·        Usually ellipsoid organelles.  Typically 1 μm in length, 0.5 μm in diameter. 

·        Could differ in shape and size considerably, depending on cell type and physiological conditions.

·        A typical eukaryotic cell may contain ~2,000 mitochondria.

·        Occupy a relatively large fraction of the cellular volume.  Liver cells - ~20%.  Heart muscle: >50%.

 

Structure  (Figs. 17-2, 3)

1. Two (lipid bi-layer) Membranes

·        Outer Membrane:  smooth, somewhat porous, contains porins that allow diffusion of molecules <10 kDa.

·        Inner Membrane: Has inward folds (cristae).  Contain ~75% proteins.  Freely permeable only to O2, CO2, and H2O.  Capable of creating and maintaining ionic gradients across inner membrane.

2. Cristae

·        Vary in number and structure depending on cell type.

·        an effective device for increasing the surface area of inner membrane in relation to mitochondrial volume.

·        The blowfly flight muscle mitochondria have ~400 m2 of inner membrane surface per g of mitochondrial protein.

3. Matrix

·        Space inside of inner membrane.

·        Gel-like phase, contains ~50% of proteins.

·        Also contains DNA, RNA, ribosomes.

·        Undergoes dramatic changes in volume and state of organization during changes in respiratory activity.

 

Mitochondrial Transport Systems

1. Transport of cytosolic reducing equivalents into mitochondria

·    Although most NADH molecules are produced by TCA cycle inside of mitochondria, those by glycolysis are in cytosol.

·    Mitochondrial inner membrane does not have any direct NADH transport system. 

·    Must rely on “shuttle” systems for transporting the reducing equivalents of cytosolic NADH into mitochondria.

o     Malate-Aspartate Shuttle (Fig. 15-28):  When run in reverse, (cytosolic OAA + NADH à Malate + NAD+) à Malate into mitochondrion à (mitochondrial malate + NAD+ à OAA + NADH).  Overall:  1 cytosolic NADH ® 1 mitochondrial NADH ® 3 ATP.

o     Glycerophosphate Shuttle (Fig. 17-5). Example:  insect flight muscle.  A flavoprotein dehydrogenase on the outer surface of the inner mitochondrial membrane can accept electrons from 3-phosphoglycerol and subsequently supply electrons to the e-transport chain.  Function similar to that of succinate dehydrogenase.  Overall:  NADH in cytosol is converted to FADH2 in inner mitochondrial membrane.  1 cytosolic NADH ® 1 mitochondrial FADH2 ® 2 ATP.

2. ADP-ATP Translocator

·        ATP utilized in cytosol will produce ADP and Pi as products.  The synthesis of ATP in mitochondria requires ADP and Pi as substrates. 

·        ADP (3- in charge) is transported from cytosol to mitochondria in exchange of the transport of ATP (4- in charge) from mitochondria to cytosol.  See

·        ADP-ATP translocator (Fig. 17-6).  A transmembrane protein dimer and an electrogenic antiport.  Can change from one conformation to another upon binding of ADP or ATP.

·        The net transport of ADP into and ATP out of mitochondria is regulated by membrane potential (more negative inside of mitochondrion).

3. Phosphate Transport

·        The Pi in cytosol is transported into mitochondria by a phosphate carrier, which is a Pi-H+ symport, driven by pH gradient (more acidic outside of mitochondria).