Molecular & Cell Biology

S. Rahgozar,PhD

University of Isfahan

Faculty of Science

5. Bioenergetics and Metabolism

5.1. Mitochondria and Oxidative Phosphorylation

93-1392

Structure of a mitochondrion

Organization and

Function of mitochondria

Metabolism in the matrix of mitochondria

The Oxidation of Acetyl CoA to CO2 is coupled to the reduction of NAD+

and FAD to NADH and FADH2, respectively.

Selectively positioning of mitochondria to locations of high energy use

Constant fusion and fission

The genetic system of mitochondria

The 16 kb human mitochondrial genome (larger genomes are found in yeasts, 80kb, and

plants,200kb)

Endosymbiosis

Circular DNA

Human mitochondrial genome

encodes

13 protein

(16S, 12S) rRNA

22 tRNA

Human mitochondrial DNA

encodes only 22 tRNA

species, and these are the only

tRNAs used for translation of

mitochondrial mRNAs, not

61, not even 30.

How this may happen?

There are 64 possible triplet codons, of which 61 encode the 20 aa

30 different tRNAs, but not 61, are

required to translate the universal code

according to the “wobble” rules;

Base pairing at the third codon position is

relaxed, allowing G to pair with U, and

inosine (I) in the anticodon to pair with U,C,

or A.

Mitochondrial Mutations are

associated with a number of diseases

Mutations in one mitochondrial

tRNA gene are associated with

metabolic syndrome, the condition

associated with obesity and diabetes.

Mutations in mitochondrial genes that encode components of the electron

transport chain are associated with Leber’s hereditary optic neuropathy, a

disease that leads to blindness.

The progress of accumulation of mutations in mitochondrial DNA during the

lifetime of individuals has been suggested to contribute to the process of aging.

Protein

import and

mitochondrial

assembly

Approximately 1000 proteins

encoded by nuclear genes (more

than 95% of mitochondrial

proteins) are synthesized on free

cytosolic ribosomes and imported

into mitochondria as completed

polypeptides chains.

Import of mitochondrial matrix

proteins through Tom and Tim

complexes mediated by

aminoterminal presequences

(Mitochondrial matrix

processing peptidase)

Hsp70

(Hsp60)

Some polypeptides are

then transferred to a

chaperone of the Hsp60-

family (a chaperonin)

within which additional

protein folding takes

place

Protein imports requires

ATP both outside and inside

the mitochondria as well as

the electric potential across

the inner membrane.

Sorting of proteins containing

presequences to different

mitochondrial compartments

Mitochondrial proteins with N-

terminal presequences can be

imported to the outer

membrane, inner membrane, or

intermembrane space.

Recognition happens by the

Tom20.

Proteins destined to the outer

membrane halt translocation in

the Tom40 and pass laterally

into the membrane.

Some proteins destined to the

intermembrane space are

translocated through Tom40 but

are recognized by cystein

chaperones and remain in the

intermembrane space.

Other proteins are transferred

through Tim23 into the

mitochondrial matrix.

(Translocase)

Import of small molecule

transport proteins into the

mitochondrial inner membrane

The internal signal sequences

in association with Hsp90

chaperons interact with the

Tom70 receptors, from which

the transmembrane protein is

transferred to the Tom 40

channel.

In the intermembrane space, the

protein is bound by small

mobile Tim proteins that guide

it to the Tim 22 complex in the

inner membrane. The Tiny Tim

proteins transfer the protein to

Tim54 and then to the Tim22

import pore. Internal stop

sequences halt translocation,

and the protein is transferred

laterally into the mitochondrial

inner membrane.

Insertion of β-barrel proteins

into the mitochondrial outer

membrane

Phosphatidylcholine and

phosphatidylethanolamine are

synthesized in the ER, extracted

by phospholipid transfer

proteins, burried in a

hydrophobic binding site of the

protein and released when the

complex reaches a new

membrane, such as

mitochondria.

Phosphatidylserine is

synthesized from

phosphatidylethanolamine by

mitochondria in addition to

synthesis of cardiolipins.

Sorting assembly machinery

Cardiolipin

The mechanism of oxidative phosphorylation

During oxidative phosphorylation, electrons derived from NADH and

FADH2 combine with O2, and the energy released from these

oxidation/reduction reactions is used to drive the synthesis of ATP from ADP.

The transfer of energy from NADH/FADH2 to O2 is a very energy-yielding

reaction with ΔG=-52.5 kcal/mol from each pair of electrons transferred.

To be harvested in usable form, this energy must be produced gradually by the

passage of electrons through a series of carriers, which constitute the electron

transport chain.

These carriers are organized into four complexes in the inner mitochondrial

membrane called: complex I (consisting of 40 polypeptide chains), coenzyme

Q (ubiquinone), complex III, complex IV (cytochrome oxidase). complex

V(ATP synthase) then serves to couple the energy-yielding reactions of

electron transport to ATP synthesis.

Transport of electrons from NADH

ΔG=-16.6 kcal/mol

ΔG=-10.1 kcal/mol

ΔG=-25.8 kcal/mol

Transport of electrons from FADH2

This electric potential is about 0.14V

across the membrane.

Because the phospholipid bilayer

is impermeable to ions, protons

are able to cross the membrane

only through a protein channel.

This restriction allows the energy

in the electrochemical gradient to

be harnessed and converted to

ATP as a result of ATP synthase.

Transport of metabolites across the inner

membrane

In addition to

driving the synthesis

of ATP, the potential

energy stored in the

electrochemical

gradient drives the

transport of small

molecules into and

out of mitochondria.

This transport is

mediated by

membrane-

spanning proteins.