Bioenergetics Module 3. Sabah

8/3/2019 Bioenergetics Module 3. Sabah

1/69

Prof. Dr. Sabah Abdel-Hady


2/69

Bioenergetics describes the energy changes

during biochemical processes.

Energy is the capacity to perform work


3/69

a) Heat energy that maintains the bodytemperature at 37oC

b) Free energy that performs work e.g.mechanical work muscle contraction

electrical work nerve impulses transmission

chemical work synthetic reactions

osmotic work active secretion & active

absorption


4/69

*** High-Energy compounds liberate on hydrolysismore than 7 Kcal/mole.

High energy phosphate bonds:

Contain high-energy and is written as e.g. 1,3-

biphosphoglycerate , phosphoenol pyruvate,

creatine phosphate, nucleotides as ATP

High energy sulphate bonds:

1- S-adenosyl methionine (SAM).

2- Phosphoadenosine phosphosulfate (PAPS).

*** Low-energy compounds produce, onhydrolysis, 2-4 (below 7) Kcal. of free energy permole, e.g. phosphate ester bond in sugar

phosphates G-6P.


5/69

Storage of energy

1) ATP; the common currency of energy

2) Creatine phosphate

ATP+ Creatine

creatine kinase

ADP+ creatine phosphate.


6/69


7/69

Cells harvest energy by breaking bonds

and shifting electrons from one molecule

to another.

aerobic respiration - final electron acceptor isoxygen

anaerobic respiration - final electron acceptor

isinorganic molecule other than oxygen


8/69

Mechanism of energy collection.

1)Substrate level

2)Oxidative phosphorylation at the

respiratory chain


9/69

1)Substrate level phosphorylation inwhich high-energy phosphate is transferred

from the substrate to ADP to form ATP. e.g.

conversion of 1,3 biphosphoglycerate to 3-

phosphoglycerate by phosphoglycerate kinase

with the formation of ATP.


10/69


11/69


12/69


13/69


14/69


15/69

Mitochondrial Electron Transport

Chain

System of Linked

Electron Carriers


16/69

Components of Electron

Transport Process

Reoxidation of NADH and FADH2

Sequential oxidation-reduction ofmultiple redox centers (four enzymecomplexes)

Production of proton gradient acrossthe mitochondrial membrane


17/69

Oxidative Phosphorylation

Synthesis of ATP driven by free

energy of electrochemicalgradient


18/69

Overall Reaction

(Oxidation of NADH by O2)

N A D H + H + + 1/2 O2

N A D + + H2O

E o ' = + 0.815 V ( 0.315 V ) = 1.130 V

G o ' = nFE o '

G o ' = 218 kJ /m o l


19/69

ATP Synthesis

A DP + P i ATP

G o ' = + 30.5 k J/m ol


20/69

It is final common pathway in aerobiccellsby which electrons derived from varioussubstances are transferred to O2 to form H2O.

ETC is formed of a series of electron carriers,which catalyze the transfer of electrons fromreduced coenzymes to oxygen.

The energy released is utilized for

synthesis of high energy phosphate bonds(conversion of ADP + Pi ATP).

Heat production


21/69


22/69


23/69


24/69

Hydrogen and electrons flow through the

respiratory chain from the more electronegative

components ( NADH) to the more electropositive

O2i.e. in the order of increase in redox potential.

The redox span from NAD+/NADH to O2/H2O is 1.1volts

Electrons move from a carrier with low

reduction potential (high tendency to donateelectrons) toward carriers with higherreduction potential (high tendency to acceptelectrons).


25/69


26/69

The respiratory chain exists in the inner

mitochondrial membrane and consists of a series of

catalysts (= redox carriers) that collect and transportreducing equivalents (hydrogen or electrons) from

NAD-linked dehydrogenase system, through

flavoproteins and cytochromes, and finally to

molecular oxygen to form water.


27/69


28/69

Kinetics and Mechanismsof

Transport


29/69

The components of the respiratory chain

Located in the inner mitochondrial membrane.

Formed of a series of 4 complexes ,

coenzyme Q and cytochrome C .


30/69

Mitochondrial Electron Transport

Chain


31/69


32/69

NAD+

FMN

FeS

ubiquinoneFAD FeS

Cyt b

FeS Cyt c1 Cyt c Cyt a Cyt a3

1/2 O2

ubiquinone

NAD+ or FAD

There are 2 sites of entry

for electrons into the

electron transport chain:

Both are coenzymes for

dehydrogenase enzymes

The transfer of electrons is not directly to oxygen but

through coenzymes


33/69

Electron Transport chain(respiratory chain)

The electron transport chain in the innermitochondrial membrane can be isolated infour proteins complexes(I, II, III, IV).

A lipid soluble coenzyme (Q) and a watersoluble protein (cyt c) shuttle betweenprotein complexes

Electrons transfer through the chain - fromcomplexes I and II to complex IV


34/69

Complex I: is (NADH:ubiquinone oxidoreductase,or NADH dehydrogenase.)

It is formed of :

Many polypeptides

FMN coenzyme

Seven iron sulfur centers(fes) which are

necessary for the transfer of hydrogen atoms tothe next member of the chain (coenzyme Q).

It catalyzes the transfer of electrons from NADH

+H+ to coenzyme Q(ubiquinon).

Electrons or hydrogens are transferred fromNADH to FMN then to different fes centers and

finally to UQ to form reduced UQ H2 (ubiqinol) .


35/69


36/69

H3C

H3CN

N

N O

O

N

CH2 CH CH CH CH2

OH OH OH

O P OH

OH

O

riboflavin monophosphate(flavin mononucleotide, FMN)

H3C

H3C NN

N O

O

N

CH2 CH CH CH CH2

OH OH OH

O P O P O

OH

O

OH

O

CH2 O

OH OH

N

N

NH2

N

N

flavin adenine dinucleotide (FAD)

FAD Always a 2-electron reaction transferring 2 e- and 2 H+


37/69

Fe

Fe

SS

S

FeFe

S

S

S

S

S

Cys

Cys

Cys

Cys

S

Fe

S

Fe

S

S

S

S

Cys

CysCys

Cys

Iron-Sulfur CentersTwo iron-sulfur centers

from complex I

4-iron Fe-S

2-iron Fe-S


38/69


39/69

Coenzyme Q(CoQ, Q or ubiquinone) is lipid-soluble.

It dissolvesin the hydrocarbon core of a membrane.

the only electron carrier not bound to a protein.


40/69

Complex II: ( succinate:ubiquinoneoxidoreductase.)

Is formed of:

Succinate dehydrogenase.FAD coenzyme.

Two iron sulfur centers.

It catalyzes the transfer of electrons or

hydrogens from succinate to UQ to form

UQH2.


41/69

COO

C

C

COO

H H

H H

COO

C

C

COO

H

H

Q QH2

via FAD

succinate fumarate

Succinate Dehydrogenase

Com lex II


42/69


43/69


44/69


45/69

The heme iron can undergo 1 e- transitionbetween ferric and ferrous states: Fe3+ + e-

Fe2+

Copper ions besides two heme A groups (aand a3) act as electron carriers in Cyta,a3

Cu2++e- Cu+

Heme is a prosthetic group ofcytochromes.

Heme contains an iron atom in a porphyrin ring

system.


46/69

N

N

N

N

CH3 HC

CH3

S CH2

CH3

CH S CH2

CH3

CH2

CH2

COO

CH3

H3C

CH2CH2

OOC

protein

protein

Fe

Heme c


47/69


48/69

NAD+, flavins and Q carry electrons and H+

Cytochromes and non-haem iron proteins carry only

electrons

NAD+ FAD undergoes only a 2 e- reaction;

cytochromes undergo only 1e- reactions

FMN Q undergoes 1e- and 2 e- reaction

Electron carriers


49/69


50/69

NAD+

FMN

FeS

ubiquinoneFAD FeS

Cyt b

FeS Cyt c1 Cyt c Cyt a Cyt a3

1/2 O2

ubiquinone

I

II

III IV

NADH Dehydrogenase

Succinate

dehydrogenase

CoQ-cyt c Reductase

Cytochrome Oxidase


51/69


52/69


53/69


54/69

When a substrate is oxidized via NAD-linked

dehydrogenase, three molecules of ATP are

formed.

when a substrate is oxidized via aflavoprotein-linked dehydrogenase, only

two molecules of ATP are Formed


55/69

Mechanism of oxidative phosphorylation (Mitchell'schemiosmotic theory)

Oxidation of components of the respiratory chainleads to generation of protonsinside themitochondrial matrix that are pumped to the outsideof the inner membrane.

ComplexesI,III and IV act as proton pumps

This resultsin accumulation of protons outside theinner membrane which in turn creates anelectrochemical potential difference across the innermembrane leading to formation of ATP from ADP andPi by the enzyme ATP synthase

ATP synthase is formed of two subunits F0 & F1 .

The F0 subunit protrude into mitochondrial matrixand acts as a gate or channel for protons

F1 subunits present in the inner mitochondrialmembrane, it catalyzes the synthesis of ATP.


56/69


57/69

As protonscross the membrane through the channel

in the base of ATP synthase

the FO proton-driven motor rotate

This movement provides the energy

for the active sites of F1

that produces and then releases ATP


58/69


59/69

ADP/ATP Exchanger

Electrogenic Antiporter

Driven by electrochemical gradient


60/69

Control of Respiratory Chain

The respiratory chain iscontrolled by thelevels of ADP and Pi as well as by the

availability of ADP/ATP transporter.


61/69

Inhibitors of the respiratory

chain.A) Inhibitors of the respiratory chain proper.

B) Inhibitors of oxidative phosphorylation.

C) Uncouplers of oxidative phosphorylation.


62/69

A) Inhibitors ofthe respiratory chain proper.

They inhibit the respiratory chain at four sitesSite 1: between FeS and Qin complex I. It is

inhibited by barbiturates, piericidin A(antibiotic) and rotenone (insecticide and fishpoison)

Site 2: between cytochrome b and cytochromec1 in complex III. It isinhibited by antimycinand dimercaprol.

Site 3: at cytochrome oxidase in complex IV. Itisinhibited by hydrogen sulfide (H2S), carbon

monoxide (CO), and cyanide.Site 4: between succinate dehydrogenase and Q

in complex II. It isinhibited by carboxin.


63/69

B) Inhibitors of oxidative phosphorylation.

They include Oligomycin that inhibits thetransport of ADP into and the transport of ATPout of the mitochondria. Atractyloside isanother inhibitor.

C) Uncouplers of oxidative phosphorylation.

They dissociate oxidation in the respiratory

chain from phosphorylation. Oxidation becomes unlimited since it is not

controlled by the concentration of ADP or Pi.

They include 2,4 dinitrophenol, dinitrocresol,

pentachlorophenol, thyroxine, Ca2+

and m-chlorocarbonyl cyanide phenyl hydrazone(CCCP).


64/69


65/69

Oxidation of extramitochondrial NADH.

NADH is produced in the cytosol duringglycolysisin the reaction catalyzed by

glyceraldehydes -3- phosphate

dehydrogenase.

This NADH can not enter the respiratorychain in the mitochondria because the inner

mitochondrial membrane isimpermeable toit.

reducing equivalents are transferred fromextramitochondrial NADH to the

mitochondria by one of two shuttles


66/69

No NADH Transporter


67/69


68/69


69/69

Aerobic glycolysis yeilds

8 ATPs if malate shuttle is used

6 ATPs if glycerophosphate shuttle is used

and accordingly complete oxidation of 1mol of glucose yields 38 or 36 ATPs.

Documents

Bioenergetics Module 3. Sabah