LEHNINGER PRINCIPLES OF BIOCHEMISTRY David L. Nelson and Michael M. Cox LEHNINGER PRINCIPLES OF BIOCHEMISTRY Sixth Edition CHAPTER 19 Oxidative Phosphorylation © 2013 W. H. Freeman and Company

Dehydrogenases produced NADH and FADH2 in catabolism via Glycolysis and the Citric Acid Cycle Q: What happens to the electrons stored in NADH and FADH2? A: They get transferred to O2 through a series of electron carriers located in the inner membrane of mitochondria.

The inner membrane contains Mitochondria contain two membranes, an outer membrane permeable to small molecules and ions and an impermeable inner membrane. The inner membrane contains components of the respiratory chain for transfer of electrons to O2. The inner membrane also contains ATP synthase, an enzyme that synthesizes ATP.

Electron Carriers in the Respiratory Chain NADH – nicotinamide adenine dinucleotide soluble molecule used by dehydrogenases Flavoproteins – contain FAD or FMN which can be reduced to FADH2 and FMNH2 Ubiquinone – also known as coenzyme Q lipid-soluble molecule with isoprenoid chain Cytochromes – proteins that contain a heme prosthetic group Iron-sulfur proteins – coordinate a non-heme iron atom(s)

NADH and NADPH are soluble electron carriers used by enzymes called dehydrogenases Niacin is a vitamin that is a precursor to NADH and NADPH

Some enzymes use flavin nucleotides for oxidation-reduction

Ubiquinone (Coenzyme Q) Lipid-soluble molecule with a long isoprenoid side chain. Diffusible in the membrane. Can accept one electron to become the semiquinone radical QH•. Can accept two electrons to become ubiquinol QH2.

Cytochromes are proteins that contain heme prosthetic groups

Iron-sulfur proteins have Fe-S centers containing Fe atoms Each Fe is surrounded by four sulfurs

Complex I is NADH dehydrogenase Complex I transfers a hydride ion from NADH to ubiquinone and pumps 4 protons across the inner mitochondrial membrane

H+ H+ H+ are pumped out of the mitochondrial matrix. Therefore a H+ gradient is being built up. The matrix becomes negatively charged and the intermembrane space becomes positively charged H+ H+

Complex II Succinate Dehydrogenase

Complex III: cytochrome bc1 complex Transfers electrons from ubiquinol (QH2) to cytochrome c

The Q Cycle of Complex III

Complex IV

Complex IV

H+ H+ H+ are pumped out of the mitochondrial matrix. Therefore a H+ gradient is being built up. The matrix becomes negatively charged and the intermembrane space becomes positively charged H+ H+

Chemiosmotic Model

The release of ATP from ATP synthase is the major energy barrier to catalysis

Mitochondrial ATP synthase complex

The F1 subunit of ATP synthase has 3 a and 3 b subunits. Alternating a and b subunits are arranged in a hexamer. The g subunit is a central shaft. Each of the b subunits has a catalytic site for ATP synthesis.

Binding-change model for ATP synthase

ATP synthase is a “molecular motor” The three b subunits are each in a different conformation, b-empty, b-ADP, or b-ATP. Each b subunit alternates between conformations, b-empty  b-ADP + Pi  b-ATP. When protons pass through Fo the c subunits of Fo rotate and rotate the attached g subunit of F1 along with them. With each 120º rotation of the g subunit of F1, the g subunit comes in contact with another b subunit, forcing it to release ATP and enter the b-empty conformation. The empty subunit then binds ADP and Pi and catalyzes the synthesis of another ATP molecule.

a – bovine b - yeast Different organisms have different numbers of c subunits in the C ring of the Fo complex

Active transport processes for oxidative phosphorylation

The malate-aspartate shuttle carries NADH electrons into the matrix (liver, kidney, heart)

The glycerol 3-phosphate shuttle carries NADH into the matrix (skeletal muscle, brain)

Regulation of the ATP-producing pathways

Regulation of the ATP-producing pathways

Regulation of the ATP-producing pathways

mitochondrial matrix, producing heat rather than ATP. In adipose tissue of newborn mammals, an inner membrane protein, thermogenin, allows protons to flow back into the mitochondrial matrix, producing heat rather than ATP.