Dr. Aga Syed SameerCSIR Lecturer

Department of Biochemistry,

Medical College,

Sher-I-Kashmir Institute of Medical Sciences,

Bemina, Srinagar, Kashmir, 190018. India.

Biological

Oxidation

Respiratory Chain

ComponentsFlow of ElectronsComplexes

Mitochondria

Electron Transport ChainLocated in selective permeable – inner

mitochondrial membrane

Transports reducing equivalents (e-/H+) to O2for their final oxidation into CO2 & water

It consists of five distinct e- carriers;

All five e- carriers are arranged in order of their increasing redox potentials

All act in sequence to transfer e-s from substrate (NADH & FADH2) to O2

And Consists of five different components

Nicotinamide Nucleotides

Derived from vitamin niacinNAD+ and NADP+

Are first components of ETC

NADH serves as electron donor to FMN-Linked NADH dehydrogenase (Complex I)

Of the two, NAD+ is more actively involved in ETC

Flavoproteins

Component of Two Complexes of ETC

FMN-Linked NADH dehydrogenase (Complex I)

Succinate:CoQ Oxidoreductase (Complex II)

In complex I: FMN accepts e-/H+ from NADH

In complex II: FAD accepts e-/H+ from FADH2

The mobile acceptor CoQ accepts e-/H+ from both complexes I & II)

Iron-Sulphur Proteins

Non-heme iron proteins containing 1-4 or more iron atoms per protein

The iron component of Fe-S proteins is bound to cysteine residues of protein and to acid labile sulphur

The iron atoms of Fe-S proteins are alternatively switched between oxidised (Fe3+) and reduced (Fe2+)

Coenzyme Q

Lipophilic Carrier, which acts as mobile diffusible electron carrier

Lipid-soluble benzoquinone, remains confined to lipid bilayer of inner mitochondrial membrane

CoQ accepts e-/H+ from flavoproteins (of complexes I & II) and transfers them to another mobile carrier CytC

The quinone portion of CoQ switches between Oxidisedand Reduced form via Semi ubiquinone intermediate

Cytochromes

Iron containing heme proteins:: Cyt b, c1, c, a and a3

They accept e-/H+ from CoQ to penultimate carrier - Cytochrome c Oxidase (Cyta/a3) of ETC

Cyta/a3 transfers e-s from Cytc to molecular O2 because of its high affinity for O2

Cyta/a3 has two atoms of copper in its electron acceptor/donating core that oscillate between Cu2+

and Cu+ during e- transfer

Cyta/a3 is inhibited by CN-, CO and H2S

Info

NAD/FMN/CoQ::

Transfers both e-s and H+

Fe-S Proteins/Cyt::

Transfers only e-s

Cytochrome c

Cytochrome a.a3

2e-

+ve

Re

do

x p

ote

ntia

l-v

e

Reaction co-ordinate

2e-

2e-

Flavoprotein/Cytochrome b

NAD+

NADH + H+

FAD

FADH2

1/2O2 + 2H+

H2O

G=- 84.5 kJ

G=- 38.5 kJ

G=- 51 kJ

Redox Potential

Nicotinamide Nucleotides

Complex I

NADH CoQ Oxidoreductase:: mw = 850kD

Consists of NADH dehydrogenase and an Fe-S Centre

Catalyzes the oxidation NADH

Transfers e-s from NADH to CoQ via FMN and an Fe-S centre

In addition, It also serves as proton pump; transporting H+s from inner chamber to Inter Membrane Space of Mitochondria

Results in generation of One ATP per two e-s transfer

Complex II

Succinate CoQ Oxidoreductase:: mw = 125kD

Consists of Succinate dehydrogenase and an Fe-S Centre

Catalyzes the oxidation Succinate to Fumarate

Transfers e-s from Succinate to CoQ via FAD and an Fe-S centre

Complex III

CoQ:Cytc Oxidoreductase:: mw = 250kD

Consists of Cytb, Cytc1 and an Fe-S Centre Catalyzes the oxidation CoQH2 back to its oxidsed form

Transfers e-s from CoQH2 to Cytc via Cyt b/c1 and Fe-S centre

In addition, It also serves as proton pump; transporting H+s from inner chamber to Inter Membrane Space of Mitochondria

Results in generation of One ATP per two e-s transfer

Complex IV

Cytc:O2 Oxidoreductase :: mw = 300kD

Contains Iron containing heme protein core :: Cyt a and a3

Transfers e-s from Cytc to molecular O2

It also serves as proton pump; transporting H+s from inner chamber to Inter Membrane Space of Mitochondria

Results in generation of One ATP per two e-s

transfer

Electron Transference

NADH→FMN →FeS →CoQ

Succinate →FAD →FeS →CoQ

CoQ →FeS →Cytb →Cytc1 →Cytc

Cytc →Cytaa3 →O2

Questions?