OXIDATIVE PHOSPHORYLATION AND ELECTRON TRANSPORT
C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O
DGº’ = -2,823 kJ mol-1 = -680 kcal mol-1
· 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)
· 24 H+ + 24 e + 6 O2 ® 12 H2O
· By oxidative phosphorylation in mitochondria.
· Produce ATP.
· 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.
· 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.
· 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.
· 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).