ELECTRON CARRIER COMPLEXES

Presented By • Amina Hussain • Zahid Mehboob• Rashida Naseem

Any of various molecules that are capable of accepting one or two electrons from one molecule and donating them to another in the process of electron transport.

As the electrons are transferred from one electron carrier to another, their energy level decreases, and energy is released. Cytochromes and quinones (such as coenzyme Q) are some examples of electron carriers.

Introduction: electron carriers

Protein Electron Carriers Prosthetic Group

Flavoprotein Flavin Mononucleotide (FMN)

Iron-sulfur Protein Iron and Sulfur

Cytochromes Heme Group

Electron carriers of the transport chain

Most electron carriers are proteins. Ubiquinone (Q) is the only electron carrier of the transport chain that is not bound to a protein.

1.) NADH.

This is water-soluble pyridine derivative used by dehydrogenases, enzymes. Important to note that NADH transfers 2 electrons at a time in the form of a hydride.

NAD+ + 2e- + H+ NADH

The electron carriers that ferry electrons from NADH and FADH2 to O2 are associated with the inner mitochondrial membrane. Some of these redox centers are mobile, and others are components of integral membrane protein complexes.

Electron Carriers: NADH, Flavoproteins,

Cytochromes, Iron–Sulfur Proteins, Quinones

2.) Flavoproteins.

Some redox enzymes use redox groups derived from riboflavin; these are called flavoproteins.

The redox groups are the Flavin Mono-Nucleotide (FMN) and the Flavin Adenine Dinucleotide (FAD).

Flavoproteins can accept or donate electrons one at time or two at a time because their semiquinone forms are stable.

FAD + 2e- + 2H+ FADH2; FMN + 2e- + 2H+ FMNH2

3.) Coenzyme Q (CoQ)

Coenzyme Q (CoQ, Q or ubiquinone) is lipid-soluble. It is a soluble electron carrier in the hydrophobic bilipid layer of the inner mitochondrial membrane. The only electron carrier not bound to a protein. It can accept/donate 1 or 2 e-.

O

O

C H 3 O

C H 3C H 3 O

(C H 2 C H C C H 2 )n H

C H 3

O H

O H

C H 3 O

C H 3C H 3 O

(C H 2 C H C C H 2 )n H

C H 3

e + 2 H +

c o e n zy m e Q

c o e n zy m e Q H 2

O

O

C H 3 O

C H 3C H 3 O

(C H 2 C H C C H 2 )n H

C H 3e

c o e n zy m e Q •

3.) Cytochromes.

Cytochromes are proteins that contain heme prosthetic groups which function as one electron carriers.

The heme iron is involved in one electron transfers involving the Fe2+ and Fe3+ oxidation states.

• Some cytochromes (b,c1,a,a3) are part of large integral membrane protein complexes.

• Cytochrome c is a small, water-soluble protein.

This is the structure of iron-protoporphyrin IX, one of the most common hemes.

4.) Iron-Sulfur Proteins

In the ETC we will encounter many iron-sulfer proteins which participate in one electron transfers involving the the Fe2+ and Fe3+ oxidation states.

These are non-heme iron-sulfur proteins.

• The simplest iron-sulfer protein is FeS in which iron is tetrahedrally coordinated by four cysteines.

• The second form is Fe2S2 which contains two irons complexed to 2 cysteine residues and two inorganic sulfides.

• The third form is Fe3S4 which contains 3 iron atoms coordinated to three cysteine residues and 4 inorganic sulfides.

• The last form is the most complicated Fe4S4 which contains 4 iron atoms coordinated to 4 cysteine residues and 4 inorganic sulfides.

5.) Copper Proteins

Copper bound proteins participate in one electron transfers involving the Cu+ and Cu2+ oxidation states.

Overview of the Electron Transport Chain.

Electrons move along the electron transport chain going from donor to acceptor until they reach oxygen the ultimate electron acceptor.

The components of the electron transport chain are organized into 4 complexes.

Each complex contains several different electron carriers.

1. Complex I also known as the NADH-coenzyme Q reductase or

NADH dehydrogenase.2. Complex II also known as succinate-coenzyme Q reductase or succinate dehydrogenase.3. Complex III also known as coenzyme Q reductase.

4. Complex IV also known as cytochrome c reductase.

Complex I accepts electrons from NADH and serves as the link between glycolysis, the citric acid cycle, fatty acid oxidation and the electron transport chain.

Complex II includes succinate dehydrogenase and serves as a direct link between the citric acid cycle and the electron transport chain.

• Complexes I and II both produce reduced coenzyme Q,CoQH2 which is the substrate for Complex III.

• Complex III transfers the electrons from CoQH2 to reduce cytochrome c which is the substrate for Complex IV.

• Complex IV transfers the electrons from cytochrome c to reduce molecular oxygen into water. Electron carriers operate in sequence.

• Each of these complexes are large multisubunit complexes embedded in the inner mitochondrial membrane.

C | Complex I Accepts Electrons from NADH

oxidoreductase or NADH dehydrogenase, is a large enzyme

composed of 42 different polypeptide chains, including an FMN-

containing flavoprotein and at least six iron-sulfur centers. High-

resolution electron microscopy shows Complex I to be L-shaped, with

one arm of the L in the membrane and the other extending into the

matrix.

Matrix arm.Hydrophilic domain

Membranearm

Complex I Contains Multiple Coenzymes.

• one molecule of flavin mononucleotide• eight or nine iron–sulfur clusters as the prosthetic

groups of iron–sulfur proteins

Complex IFMN

[2Fe–2S][4Fe–4S]

Complex I catalyzes two simultaneous and obligately coupled processes:

• (1) the exergonic transfer to ubiquinone of a hydride ion from NADH and a proton from the matrix and

• (2) the endergonic transfer of four protons from the matrix to the intermembrane space.

The reaction of NADH dehydrogenase is:

NADH + H+ + CoQ + 4H+in → NAD+ + CoQH2 + 4H+

out

Mechanism

All redox reactions take place

in the extramembranous

portion. NADH initially binds to

NADH dehydrogenase, and

transfers two electrons to the

flavin mononucleotide (FMN)

prosthetic group of complex I,

creating FMNH2. The electron

acceptor - the isoalloxazine

ring - of FMN is identical to

that of FAD.

Electrons transfer

path

FMN

Series of iron–sulfur

clusters

hydrophilic peripheral domain(Thermus thermophilus)

The electrons are then transferred

through the second prosthetic group

of NADH dehydrogenase via a series

of iron-sulfur (Fe-S) clusters, and

finally to coenzyme Q (ubiquinone).

This electron flow changes the redox state

of the protein, inducing conformational

changes of the protein which alters the pK

values of ionizable side chain, and causes

four hydrogen ions to be pumped out

of the mitochondrial matrix.

Ubiquinone (CoQ) accepts two electrons

to be reduced to ubiquinol (CoQH2).

CoQ

FMN

Series of iron–sulfur

clusters

Complex II: Succinate to Ubiquinone(membrane-bound enzyme in the citric acid cycle)

• Although smaller and simpler than Complex I, it contains

five prosthetic groups of two types and four different

protein subunits.

• Subunits C and D are integral membrane proteins, each

with three transmembrane helices. They contain a heme

group, heme b, and a binding site for ubiquinone, the

final electron acceptor in the reaction catalyzed by

Complex II.

• Subunits A and B extend into the matrix; they contain

three 2Fe-2S centers, bound FAD, and a binding site for

the substrate, succinate.

Intermembranespace (P side)

CD

Matrix(N side)

Fe-S centers

Substratebinding

site

BFAD

Heme b

Ubiquinone

Phosphatidyl ethanolamine

QH2

Intermembranespace (P side)

CD

Matrix(N side)

Fe-S centers

Substratebinding

site

BFAD

Heme b

Ubiquinone

Phosphatidyl ethanolamine

QH2

Electron Transfer Pathway

Structure of Complex II (succinate dehydrogenase).• (PDB ID 1ZOY) This complex (shown here is the porcine

heart enzyme) has two transmembrane subunits, C and D; the cytoplasmic extensions contain subunits A and B. Just behind the FAD in subunit A is the binding site for succinate. Subunit B has three sets of Fe-S centers; ubiquinone is bound to subunit B; and heme b is sandwiched between subunits C and D. Two phosphatidylethanolamine molecules are so tightly bound to subunit D that they show up in the crystal structure. They serve to occupy the hydrophobic space below the heme b

• Electrons move (blue LINE) from succinate to FAD, then through the three Fe-S centers to ubiquinone. The heme b is not on the main path of electron transfer but protects against the formation of reactive oxygen species (ROS) by electrons that go astray.

Complex III: Ubiquinone to Cytochrome c

• The next respiratory complex, Complex III, also called

cytochrome bc1 complex or ubiquinone:cytochrome c

oxidoreductase, couples the transfer of electrons from

ubiquinol (QH2) to cytochrome c with the transport of

protons from the matrix to the intermembrane space.

• The functional unit of Complex III is a dimer, with the two

monomeric units of cytochrome b surrounding a “cavern” in

the middle of the membrane, in which ubiquinone is free to

move from the matrix side of the membrane (site QN on

one monomer) to the intermembrane space (site QP of the

other monomer) as it shuttles electrons and protons across

the inner mitochondrial membrane.

Cytochrome bc1 complex (Complex III).

The complex is a dimer of

identical monomers, each with

11 different subunits.

(a) The functional core of each

monomer is three subunits:

cytochrome b (green) with its

two hemes (bH and bL); the

Rieske iron-sulfur protein

(purple) with its 2Fe-2S

centers; and cytochrome c1

(blue) with its heme (PDB ID

1BGY).

Heme bHHeme bH

Heme bLHeme bL

Cytochrome b

2Fe-2S

Rieske ironsulfurprotein

Heme c1

Cytochrome c1

Cavern

P" side (inter membrane space),

"N" side (matrix)

Reaction Mechanism

The reaction mechanism for complex III is known as the ubiquinone ("Q")

cycle. In this cycle four protons get released into the Positive "P" side (inter

membrane space), but only two protons get taken up from the Negative "N"

side (matrix). As a result a proton gradient is formed across the membrane.

In the overall reaction, two ubiquinols are oxidized to ubiquinones and one

ubiquinone is reduced to ubiquinol. In the complete mechanism, two

electrons are transferred from ubiquinol to ubiquinone, via two cytochrome

c intermediates.Overall:•2 x QH2 oxidised to Q •1 x Q reduced to QH2 •2 x Cyt c1 reduced •4 x H+ released into intermembrane space •2 x H+ picked up from matrix

• Round 1:

• Cytochrome b binds a ubiquinol and a ubiquinone. The 2Fe/2S

center and BL heme each pull an electron off the bound ubiquinol,

releasing two hydrogens into the intermembrane space.

• One electron is transferred to cytochrome c1 from the 2Fe/2S

centre, whilst another is transferred from the BL heme to the BH

Heme.

• Cytochrome c1 transfers its electron to cytochrome c (not to be

confused with cytochrome c1), and the BH Heme transfers its

electron to a nearby ubiquinone, resulting in the formation of a

ubisemiquinone.

• Cytochrome c diffuses. The first ubiquinol (now oxidised to

ubiquinone) is released, whilst the semiquinone remains bound.

Round 2:

• A second ubiquinol is bound by cytochrome b. The 2Fe/2S

center and BL heme each pull an electron off the bound

ubiquinol, releasing two hydrogens into the intermembrane

space.

• One electron is transferred to cytochrome c1 from the 2Fe/2S

centre, whilst another is transferred from the BL heme to the BH

Heme.

• Cytocrome c1 then transfers its electron to cytochrome c, whilst

the nearby semiquinone picks up a second electron from the BH

Heme, along with two protons from the matrix.

• The second ubiquinol (now oxidised to ubiquinone), along with

the newly formed ubiquinol are released.

Reaction

It catalyzes the reduction of cytochrome c by oxidation of coenzyme Q

(CoQ) and the concomitant pumping of 4 protons from the mitochondrial

matrix to the intermembrane space:

QH2 + 2 cytochrome c (FeIII) + 2 H+in → Q + 2 cytochrome c (FeII) + 4 H+

out

In the process called Q cycle, two protons are consumed from the matrix

(M), four protons are released into the inter membrane space (IM) and

two electrons are passed to cytochrome c.

Complex IV: Cytochrome c

to O2

In the final step of the

respiratory chain, Complex IV,

also called cytochrome oxidase,

carries electrons from

cytochrome c to molecular

oxygen, reducing it to H2O.

Complex IV is a large enzyme

(13 subunits; Mr 204,000) of

the inner mitochondrial

membrane. Heme a3

Heme a

Structure of cytochrome oxidase (Complex IV).

This complex from bovine mitochondria has 13 subunits, but only four

core proteins are shown here (PDB ID 1OCC). (a) Complex IV, with four

subunits in each of two identical units of a dimer. Subunit I (yellow) has

two heme groups, a and a3, near a single copper ion, CuB (green

sphere). Heme a3 and CuB form a binuclear Fe-Cu center. Subunit II

(purple) contains two Cu ions complexed with the —SH groups of two

Cys residues in a binuclear center, CuA, that resembles the 2Fe-2S

centers of iron-sulfur proteins. This binuclear center and the

cytochrome c–binding site are located in a domain of subunit II that

protrudes from the P side of the inner membrane (into the

intermembrane space). Subunit III (light blue) is essential for rapid

proton movement through subunit II. The role of subunit IV (green) is

not yet known.

BiochemistrySummary reaction:

4 Fe2+-cytochrome c + 8 H+in + O2 → 4 Fe3+-cytochrome c + 2 H2O + 4 H+

out

Two electrons are passed from two cytochrome c's, through the CuA and

cytochrome a sites to the cytochrome a3- CuB binuclear center, reducing the

metals to the Fe+2 form and Cu+1. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O2. The oxygen is rapidly

reduced, with two electrons coming from the Fe+2cytochrome a3, which is

converted to the ferryl oxo form (Fe+4=O). The oxygen atom close to CuB picks up

one electron from Cu+1, and a second electron and a proton from the hydroxyl of Tyr(244), which becomes a tyrosyl radical: The second oxygen is converted to a hydroxide ion by picking up two electrons and a proton. A third electron arising from another cytochrome c is passed through the first two electron carriers to the cytochrome a3- CuB binuclear center, and this electron and two protons convert the

tyrosyl radical back to Tyr, and the hydroxide bound to CuB+2 to a water molecule.

The fourth electron from another cytochrome c flows through CuA and cytochrome

a to the cytochrome a3- CuB binuclear center, reducing the Fe+4=O to Fe+3, with the

oxygen atom picking up a proton simultaneously, regenerating this oxygen as a hydroxide ion coordinated in the middle of the cytochrome a3- CuB center as it was

at the start of this cycle. The net process is that four reduced cytochrome c's are used, along with 4 protons, to reduce O2 to two water molecules.

StructureSubunit I and II of Complex IV excluding all other subunits, PDB 2EIKThe complex is a large integral membrane protein composed of several metal prosthetic sites and 13 protein subunits in mammals. In mammals, ten subunits are nuclear in origin, and three are synthesized in the mitochondria. The complex contains two hemes, a cytochrome a and cytochrome a3, and two copper centers, the CuA and CuB centers.[1] In fact, the cytochrome a3 and CuB form a binuclear center that is the site of oxygen reduction. Cytochrome c reduced by the preceding component of the respiratory chain (cytochrome bc1 complex, complex III) docks near the CuA binuclear center, passing an electron to it and being oxidized back to cytochrome c containing Fe3+. The reduced CuA binuclear center now passes an electron on to cytochrome a, which in turn passes an electron on to the cytochrome a3- CuB binuclear center. The two metal ions in this binuclear center are 4.5 Å apart and coordinate a hydroxide ion in the fully oxidized state.