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BM11 Mitochondria.ppt
1. What is a Mitochondrion?
• A cellular organelle probably of
endosymbiotic origin that resides in the
cytosol of most nucleated (eurkaryotic) cells.
• This organelle produces energy by oxidising
organic acids and fats with oxygen by the
process of oxidative phosphorylation and
generates oxygen radicals (reactive oxygen
species ROS )as a toxic by-product
2. Generation of Energy
• Millions of years ago there was no O2 available for oxidative
phosphorylation to occur
• Organisms produced energy from fermentation, still see this
today
• As O2 became available, a more efficient method of energy
production developed
– Based on the transfer of e- along the membrane
3. Organism’s Energy Source
• Small amount of ATP from glycolysis in the cytosol of cells
• Majority made by a membrane based process in 2 stages
– Stage 1 – e- transport chain
• e- transferred along e- carriers in the membrane
– Stage 2 – flow of H+ down an electrochemical gradient
to produce ATP
• Use a complex called ATP synthase
4. Stage 1
• NADH (from the Kreb’s cycle)
brings in the e- and transfers
them to the carrier molecules
• The e- moves down the chain
and looses energy at each step
– as this happens, H+ are
pumped across the membrane
• This creates an electro-chemical
gradient across the membrane
5. Stage 2
• The electrochemical gradient
is a form of stored energy – it
has the potential to do work
• The H+ can now move down
the gradient and return to the
other side of the membrane
thru ATP synthase – in this
process, generates ATP from
ADP and Pi
6. Chemiosmotic Coupling
• Once called the chemiosmotic hypothesis
– Chemi from making ATP, osmotic because of crossing the
membrane
• Now known as chemiosmotic coupling
7. Mitochondria
• Produce most of a cells ATP – acetyl groups in the Kreb’s
cycle producing CO2 and NADH
• NADH donates the e- to the electron transport chain and
becomes oxidized to NAD+
• e- transfer promotes proton pump and ATP synthesis in
process called oxidative phosphorylation
• Cells that require large amounts of energy such as the
heart have large numbers of mitochondria
8. Mitochondria
• Contain their own copies of DNA and RNA along with
transcription and translation system (ribosomes)
• Are able to regenerate themselves without the whole cell
undergoing division
• Shape and size dependent on what the cell’s function is
9. Mitochondria
• Double membrane creates 2 spaces
Matrix: large internal space
Intermembrane space:
between the membranes
Outer membrane
Inner membrane
11. Inner Membrane
• Inner membrane is the site of the e- transport chain,
across which the proton pump occurs and contains ATP
synthase
• Inner membrane is highly folded – called cristae –
increasing the surface area on which the above reactions
can take place
12. High Energy e-
• Mitochondria use pyruvate and fatty acids and convert it
to acetyl CoA in the matrix
• Citric acid cycle generates NADH and FADH2 which carry
the e- to the electron transport chain
17. Electron Transport Chain
• Resides in the inner mitochondrial membrane – also
called respiratory chain
• 15 proteins involved in the chain – grouped in 3 large
respiratory enzyme complexes
– NADH dehydrogenase complex
– Cytochrome b-c1 complex
– Cytochrome oxidase complex
• Pumps protons across the membrane as e- are
transferred thru them
19. Proton Gradient
• e- transfer is an oxidation/reduction reaction
• NADH has high-energy e- has a low electron affinity so
the e- is readily passed to NADH dehydrogenase and so
on down the chain
• Each transfer couples the energy released with the
uptake of a H+ from the matrix to the intermembrane
space setting up the electrochemical gradient
20. Proton Gradient
• Gradient of proton (H+) concentration across the inner
mitochondrial membrane – a pH gradient with the pH in
the matrix higher than in the intermembrane space
• Proton pumping also generates a membrane potential
– matrix side is negative and intermembrane space is
positive
24. Oxidative Phosphorylation
• ATP synthase is the protein complex responsible for making
ATP by creating a path for H+ thru the membrane
• ATP synthase is an enzyme
30. Mitochondrial Genetics
• Each cell contains many mitochondria,
each of which contains multiple copies
of 16.5-k-b circular DNA molecule
• The mitochondrial genome is subject to
a number of peculiarities of inheritance
31. Mitochondrial Genetics
• Interest in mitochondrial genetics
comes mostly from:
• interest in diseases caused by
mutations in mDNA
• interest in human history
• Doug Wallace.(mitochondrial
enthusiast)
32.
33. The nuclear and Mitochondrial
genetic codes are similar but not
identical
34.
35.
36. The human nuclear and mitochondrial genomes
Nuclear Genome Mitochondrial Genome
Size 3200 Mb 16.6 kb
No. of different DNA
molecules
23 (in XX cells) or 24
(in XY cells); all linear
One circular DNA
molecule
Total no. of DNA
molecules per cell
46 in diploid cells, but
varies according to
ploidy
Often several
thousands (but variable
Associated protein Several classes of
histone & nonhistone
protein
Largely free of protein
No. of genes ~ 30 000 ~35-000 37
Gene density ~ 1/100 kb 1/0.45 kb
37. Table continued……..
Repetitive DNA Over 50% of genome Very little
Transcription The great bulk of genes are
transcribed individually
Co-transcription of
multiple genes from both
the heavy and light strands
Introns Found in most genes Absent
% of coding DNA ~ 1.5% ~ 93%
Codon usage Slightly different see slide
Recombination At least once for each pair
of homologs at meiosis
No evidence for this
occurring naturally
Inheritance Mendelian for sequence on
X and autosomes; paternal
for sequence on Y
Exclusively maternal
38. The limited autonomy of the mitochondrial
genome
Mitochondrial
component
Encoded by
Mitochondrial genome
Encoded by nuclear
genome
Components of
oxidative
phosphorylation
system
13 subunits 80 subunits
Components of
protein synthesis
apparatus
24 approx 80
39. An affected woman transmits the trait to
all her children. Affected men
(represented by squares do not pass
the trait to any of their offspring
Maternal genetic
transmission
41. Concept of heteroplasmy. Both wild-type and mutant
(gray) mitochondria are included in the hundreds of
mitochondria in a cell. These mitochondria segregate
passively when the cell divides. This can lead to
variation in the proportion of affected mitochondria in
different tissues or different individuals in a family
42. Number of Mitochondria per cell
• Most somatic cells 100-10,000
• Lymphocyte 1000
• Oocytes 100,000
• Sperm few hundred
• No mitochondria in red cells and some
terminally differentiated skin cells
43. Some diseases associated with
mitochondrial mutations
MERRF (Myoclonic Epilepsy with Ragged Red
Fibres
MELAS (Myopathy, Epilepsy, Lactic
acidosis,Stroke-like episodes
LHON (Leber’s Hereditary Optic atrophy)
Kearn-Sayre (eye problems,heart
block,ataxia ie loss of coordination
Leigh syndrome(rare severe brain disease
in infancy,also heart problems)
44. Myoblasts were isolated from muscle cells obtained from an
individual with MERRF. They were fused to make myotubes.Protein
production was normal in those with about 16% normal
mitochondria
45. Michael presented with muscle problems, epilepsy,lack of progress at
school,difficulty with vision and hearing.
Diagnosed as MERRF aged 12 after muscle biopsy.At postition 8344 he
has a change from A-G in most of the mitochondrial DNA from muscle
and lymphocytes.The other relatives have different proportions of the
same mutation,which is in the tRNA for lysine (MT-TK)
46. Another mtDNA synthesis mutation
3243(A>G) in the tRNALeu gene
(MT-TL1)
If this mutation is present in 10-30% of the
mtDNA in white blood cells the patient may have
type II diabetes with or without deafness .
If the same mutation is in more than 70% of the
mtDNA the full MELAS syndrome is likely
47. Deletions of mitochondrial DNA
in muscle biopsies from
individuals with Kearns-Sayre
syndrome. DNA was digested
with Pvull, which cuts the
mitochondrial genome at one
site, resulting in a 16.5-kb
fragments that is detected with
a probe to the mitochondrial
DNA by southern analysis.
Each individual with the
syndrome has two populations
of mitochondrial DNA: one of
normal size and one of smaller
size form Zeiani M, Moraes CT
DiMauro S et al. Deletions of
mitochondrial DNA in Kearns-
Sayre syndrome. Neurology
1988; 38: 1339-1346)
48. Three pedigrees of rare families having infants with fatal mitochondrial
disorders showing mtDNA depletion;caused by mutations in nuclear
encoded mitochondrial genes eg TK2 encoding mitochondrial Thymidine
kinase
49.
50. mtDNA contribution to the reconstruction
of human history
Depends on:-
•High mutation rate (especially in
D loop region)
•Maternal transmission
•No recombination
This allows the origins of female ancestors to be deduced
54. Adaptive mutations
Some of the mtDNA variants are found more
frequently in humans in cold climates such as Siberia
and they are thought to alter the balance of
production of energy (ATP) versus Heat per calorie
consumed.
It is also suggested that that the selection of mtDNA
variants which allowed energy production even in
time of food shortage (tight coupling to maximum
ATP production) may now expose us in the presence
of excess calories in food to excess ROS (reactive
oxygen species). This in turn may cause mtDNA
damage and mitochondrial decline that contributes to
metabolic and degenerative diseaes,ageing and
cancer(Wallace 2005)
57. References
Strachan and Read HMG3 p240-244
Bruce Korf. Human Genetics A Problem-based
approach. chapter 7
and for the enthusiast
http://www.mitomap.org/
and Wallace DC. A Mitochondrial Paradigm of
Metabolic and degenerative diseases,Ageign and
cancer: A Dawn for Evolutionary Medicine
Ann Rev Genet 2005;39.359-407
