ELECTRON TRANSPORT AND CLINICAL CORRELATION

ADVANCE BIOCHEMISTRY

OUTLINE• MITOCHONDRIA• ELECTRON TRANSPORT CHAIN• CLINICAL CORRELATIONS

MITOCHONDRIAL MEMBRANE: Structure and Compartmentation

Mitochondria• Site of oxidative phosphorylation• Powerhouse of the cell• Number and activity reflects the role of tissue in

aerobic metabolic functions and its need for energy– Example is the Cardiac Muscle

• ½ of the cytoplasmic membrane consist of mitochondria

• Inner membrane contain numerous invaginations (cristae)

MITOCHONDRIA - Shapes depend on cell type• Oblong or cylindrical• spherical

• Cardiac cells• Liver

- Hepatocyte contain 800- 2000 mitochondria

• RBC- no mitochondria and obtain energy from glycolysis

Intermembrane space

OUTER MEMBRANE INTERMEMBRANE SPACE INNER MEMBRANE

• Rich in integral protein (porin or VDAC, voltage dependent anion channel

• Monoamine oxidase & Kynurenine hydroxylase for removal of neurotransmitter

• Enzymes involve in the trasnfer of energy

• 80 % protein• Rich in unsaturated fatty

acids• Cardiolipin is present in

high concentrations• Enzyme complexes and

oxidative phosphorylation

MATRIX• mtDNA• Ribosomes• Proteins for transcription • Enzymes of the TCA except succinate dehydrogenase

ELECTRON TRANSPORT CHAIN

• NADH and FADH2 are oxidized by the ELECTRON TRANSPORT CHAIN

• Reducing equivalents are converted to utilizable energy as ATP

• Done by Oxidative Phosphorylation

OXIDATION- REDUCTION REACTIONS• Mitochondrial electron transport consists of a

sequence of linked oxidation- reduction reactions

• Transfer electrons from a suitable electron donor ( reductant) to a suitable electron acceptor (oxidant)

• REDOX COUPLE or PAIR- an oxidant and its reductant

OXIDATION- REDUCTION REACTIONS

Oxidation- Reduction Potential The ease with which an electron donor (reductant)

gives up its electron to an electron acceptor (oxidant) Measured in volts as electromotive force (emf) of a

half cell made up of an oxidation-reduction couple when compared to a standard reference half cell hydrogen electrode reaction

OXIDATION- REDUCTION REACTIONS• Standard hydrogen electrode is set at 0.0 V at pH 0.0

• At pH 7.0, the reference hydrogen potential is – 0.42 V

MULTICOMPONENT SYSTEM• Electron transport, also known as aerobic

respiration, is the last stage of aerobic metabolism• After glycolysis and TCA cycle, 10 NADH and 2 FADH2

are generated from the oxidation of one glucose molecule.

• When there is sufficient O2 supply, NADH and FADH2 enter electron transport chain to become reoxidized

MULTICOMPONENT SYSTEM• Large amount of energy is recovered, when

electrons are passed from NADH and FADH2 to O2.• This is accomplished by a series of carrier protein in

the inner mitochondrial membrane • Not organized in linear arrangement but are grouped

into four large complexes

Electron Transport and Oxidative Phosphorylation

• In ETS, electrons are transferred from NADH and FADH2 to O2 step by step.

• In the mean time, energy released from electron flow is coupled to ATP synthesis.

• Here, phosphorylation of ADP is coupled with the oxidation of NADH or FADH2.

Composition of the Electron Transport ChainLarge protein complexes.

• Complex I - NADH-ubiquinone oxidoreductase• Complex II - Succinate-Ubiquinone oxidoreductase• Complex III – Ubiquinol -Cytochrome c reductase• Complex IV - Cytochrome c oxidase• Complex V- ATP synthase

Complex I - NADH-ubiquinone oxidoreductase

• Entry Point of NADH• Transfers electrons from NADH to obiquinone (co-

enzyme Q) coupled with the transport of four protons across the membrane required for the synthesis of ATP

CoQ – ubiquinone

Highlighted region serves as an anchor to inner mitochondrial membrane.

Reduction of CoQ

Complex I - NADH-ubiquinone oxidoreductase

• FMN (Flavin Mononucleotide- located in different subunits of the hydrophilic arm of complex I

NADH + H+ + FMN ------> NAD+ + FMNH2

• Series of FeS center (2 Fe2 S and 4Fe S type)reduce ubiquinone to ubiquinol

• 4 protons are translocated across the inner membrane to the intermembrane space

Complex II - Succinate-Ubiquinone oxidoreductase

• Known as Succinate dehydrogenase• Entry point for FADH2

• Succinate dehydrogenase (from the citric acid cycle) directs transfer of electrons from succinate to CoQ via FADH2.

• Acyl-CoA dehydrogenase (from -oxidation of fatty acids) also transfers electrons to CoQ via FADH2.

Succinate- fumarate + 2 H+ + 2 e

All electrons from FADH2 and NADH must pass through CoQ.

• Known as cytochrome bc1 complex• Catalyzes transfer of 2 electrons from ubiquinol to

cytochrome c with translocation of four protons across the membrane.

• Consist of 3 prosthetic groups that serve as redox center

A. Cytochrome bB. Cytochrome c1C. Rieske iron –sulfur proteins

Complex III – Ubiquinol -Cytochrome c reductase

CYTOCHROMES• Proteins that contain a heme group tightly bound• Iron in heme of a cytochrome is alternately oxidized (

Fe3+) or reduced ( Fe2+) as it functions in the transport of electrons

• Designated as a, b, and c on the basis of alpha band of their absorption spectrum and the type of heme group attached to the protein

Complex III – Ubiquinol -Cytochrome c reductase

Cytochrome B• Heme b562• Heme 566Cytochrome c1• 1 heme groupRieske- iron sulfur protein• 2 Fe2S clusters

Complex III – Ubiquinol -Cytochrome c reductase

• Cytochrome c is a mobile electron carrier

• Bind to cytochrome c1 of complex III and accept electron

• Heme iron is coordinated to a nitrogen of histidine and a sulfur iron of a methionine

Complex III – Ubiquinol -Cytochrome c reductase

Complex IV - Cytochrome c oxidase

Components of ATP synthase

• These are knob-like projections into the matrix side of the inner membrane.

• Two units• F1 contains the catalytic site for ATP

synthesis.• F0 serves as a transmembrane

channel for H+ flow.

• F1-F0 complex serves as the molecular apparatus for coupling H+ movement to ATP synthase.

ATP is transported from the matrix of mitochondria to cytosole by ATP-ADP translocase.

ATP and ADP cannot diffuse through the mitochondria membrane freely.

The exit of ATP is coupled with the entry of ADP into mitochondria.

Regulation of oxidative phosphorylation

• Electrons do not flow unless ADP is present for phosphorylation

• Increased ADP levels cause an increase in the activity of various enzymes including:

• glycogen phosphorylase• phosphofructokinase• citrate synthase

• ATP is transported from the matrix of mitochondria to cytosole by ATP-ADP translocase.

• ATP and ADP cannot diffuse through the mitochondria membrane freely.

• The exit of ATP is coupled with the entry of ADP into mitochondria

Regulation of oxidative phosphorylation

• Electrons do not flow unless ADP is present for phosphorylation

• Increased ADP levels cause an increase in the activity of various enzymes including:

– glycogen phosphorylase– phosphofructokinase– citrate synthase

Recycling of cytoplasmic NADH(?)

• Different methods are used to recycle NADH. This accounts for the different energy productions from glucose.

• Glycerol-3-phosphate shuttle• Used by skeletal muscles and the brain

• Malate-aspartate shuttle• Used by the heart and liver

Glucose-3-phosphate shuttle

Malate-aspartate shuttle

MITOCHONDRIAL GENES AND DISEASES

• Contain their own genome• Contains 13 proteins of

electron transport chain• Not self replicating

organelles

• 90% mitochondrail proteins are encoded in nuclear DNA

• Synthesized in the cytosol• Imported in the mitochondria

MITOCHONDRIAL GENES AND DISEASES

MITOCHONDRIAL DEFECTS• Degenerative disease of aging (Parkinson and

Alzhiemer Disease)• Point mutation in mtDNA• Deletion of large portions of mtDNADECREASED ACTIVITY OF THE ELECTRON TRANSPORT CHAIN which leads to the accumulation of pyruvate and fatty acids= lactic acidosis and triglyceride accumulation

Complex I • Mutations in subunits have

been shown to lead to a number of nuerode degenerative disease.

• Source of reactive oxygen species – damage mitochondrial DNA and may be cause of aging.

Leber Hereditary Optic Neuropathy( OMIM 53500)

• Maternally inherited• Affects the nervous system• Including optic nerves causing sudden onset blindness

in early adulthood due to the death of optic nerve• Results from single base changes in the mitochondrial

genes encoding 3 subunits o complex I (ND1, ND4, and ND6)

• Lower the activity o complex 1

MITOCHONDRIAL MYOPATHIES FROM MUTATIONS IN MITOCHONDRIAL tRNA genes

• Mutation in the tRNA gene for lysine- myclonic epilepsy and ragged red fibers ( MERRF)

( OMIM 545000)Symptoms: myoclonus and ataxia with generalized seizurees and myopathy Skeletal muscle contain abnormally shaped mitochnodria that contain paracrystalline structures giving an appearance of ragged red fibers

• Mutation in the tRNA for leucine- common mitochondrial encephalopathy- lactic acidosis- stroke like activity

( MELAS) (OMIM 540 000)- Skeletal muscles contain ragged red fibers but retain cytochrome c oxidase activity

MITOCHONDRIAL MYOPATHIES FROM MUTATIONS IN MITOCHONDRIAL tRNA genes