Bioenergentics & Metabolism Mitochondria 1
Mitochondria Structure & Function
Generation of metabolic energy is a major activity of all
cells
Two cytoplasmic organelles are specifically devoted to
energy metabolism and production of ATP
• Mitochondria
Generates useful energy derived from breakdown of lipids
& carbohydrates
• Chloroplast
Use energy captured from sunlight to generate ATP and the
reducing power needed to synthesize carbohydrates from
CO2 and H2O
Mitochondria Structure & Function
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Mitochondria
Mitochondria are
◦ Responsible for most of the useful energy derived
from breakdown of carbohydrates and fattyacids
Converted to ATP by oxidative phosphorylation
Mitochondria are unique among cytosolic organelles in
that
They contain their own DNA which encodes
tRNA, rRNA, and some other mitochondrial proteins
Assembly of mitochondria contain
• Proteins encoded by their own genome
• Proteins encoded by nuclear genome and imported from cytosol
Mitochondria Structure & Function
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Structure of Mitochondria
Mitochondria are surrounded by a double
membrane system
• Consist of inner and outer membrane
separated by intermembrane space
• Innermembrane form numerous folds
(cristae) which extends into matrix • Large surface area
• Matrix contains mitochondrial genetic system & enzymes responsible for
oxidative metabolism
• Houses the machinery for aerobic respiration
Mitochondria Structure & Function
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Mitochondria consist of two aqueous compartments
Interior – the matrix
Between the inner and outer membranes – intermembrane space
Inner Membrane
High percentage of proteins(> 70%)
Involved in oxidative phosphorylation
• Transport of metabolites (pyruvate, fatty acids)
Impermeable to most ions and small molecules
a property critical to determining proton gradient that drives oxidative phosphorylation
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Mitochondria Structure & Function
Outer Membrane
Porins – integral proteins
Large internal channel
Allows free diffusion of molecules smaller than
about 1000 daltons
Composition of intermembrane is therefore similar to
cytosol with respect to ions and small molecules
Mitochondria Structure & Function
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Mitochondria are positioned near to locations
of high energy use i-e synapses in nerve cells,
muscle cells
Continuously fusing and dividing , remodel the
network of mitochondria in cell, and affect
function and morphology
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Mitochondria Structure & Function
Endosymbiotic Origin
Mitochondria contain their own genetic system
Mitochondria are thought to have evolved from bacteria
that developed a symbiotic relationship in which they
lived within larger cells (Endosymbiosis)
Genomes of living organisms that are similar to
mitochondria are that of
α-proteobacterium Rickettsia prowazeki
It is able to reproduce only in eukaryotic cells
But unlike mitochondria it transcribes most of its
own genes
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Mitochondria Structure & Function
Mitochondrial Genome
Mitochondrial genome are usually circular DNA
molecules, like bacteria
Present in multiple copies per organelle
Vary considerably in size between different species
Genomes of human and most other animal mitochondria
are only about 16kb
Larger mitochondrial genome are found in yeast (approx
80kb) and plants (more than 200kb)
e.g
Mitochondrial DNA in Arabidopsis is 367 kb, encodes only 31
proteins
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Mitochondria Structure & Function
Contd…
Smallest mitochondrial genome
◦ Protist: plasmodium falciparum 6kb, codes for only 3 proteins
Largest mitochondrial genome
◦ Protozoan Reclinomonas americana
◦ 69 kb and contain 67 genes
Most present day mitochondrial genome encode
◦ Small number of proteins
Mammalian mitochondria (1000 to 5000 different proteins)
representing approx 5% of proteins encoded by mammalian
genome
◦ tRNA, rRNA
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Mitochondria Structure & Function
Human mitochondrial 16 Kb
genome encodes
◦ Circular DNA molecule
◦ Maternal inheritance
Map
Origin of replication and transcriptional promoter sequences (D loop)
16sRNA, 12sRNA, 22tRNA
13 proteins (essential for oxidative phosphorylation)
Electron transfer chain complexes, including I, III, IV and V
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The Human Mitochondrial Genome
Mitochondrial Genetics
Mitochondria use a slightly different genetic code than
in prokaryotic and eukaryotic cells
Human mitochondria encode only 22 tRNAs ,approx
30 different tRNAs are required to translate the
universal code according to wobble rule
Translational of mitochondrial mRNA is accomplished by
extreme form of wobble
U in anticodon of tRNA can pair with any of four bases in
third codon position of mRNA
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Mitochondria Structure & Function
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Nonstandard codon-anticodon base pairing
Base pairing at the third codon position is relaxed, allowing G to pair with U. Abnormal base pairing, allowing phenylalanyl (Phe) tRNA to recognize either UUC or UUU codons
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Contd…
Base pairing at the third codon position is relaxed, allowing inosine (I) in the anticodon to pair with U, C, or A. Abnormal base pairing, allowing alanyl (Ala) tRNA to recognize GCU, GCC, or GCA
Differences between the universal and Mitochondrial Genetic codes
Codon Universal Code Human Mitochondrial code
UGA Stop Trp
AGA Arg Stop
AGG Arg Stop
AUA IIe Met
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Mutations in mtDNA
Mitochondrial DNA can be altered by mutations
Germ-line mutations in mitochondrial DNA are transmitted from mother
• Mutation in tRNA gene
Metabolic syndrome; obesity, diabetes
• Mutation in gene that encode components of electron transport chain
Leber’s hereditary optic neuropathy; blindness
• Progressive Mutations in mitochondrial DNA
Aging
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Mitochondrial proteins
Contain 1000 to 5000 different proteins but nearly half of them remain unidentified (~5% of mammalian encoded proteins)
Mitochondria from different tissues contains different proteins (tissue specific functions)
Genes for mitochondrial proteins are in nucleus(95% of mt proteins)
Some of these genes were transferred to mitochondria by original prokaryotic ancestor
Cytosoloic protein synthesis mt transport
All kerb cycle enzymes/ rep/trans/translation
Complex because of mt double membrane
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Transport & assembly of matrix proteins
Pre-sequence, N-terminal 20-35 a.a target proteins to matrix
Partially unfolded by Hsp70 chaperon ◦ Prevent aggregation as emerge
from free ribosomes
Bind to receptors on Tom protein complex(translocase of outer membrane)
Bind Tim complex(Translocase of inner membrane)
The presequence is cleaved by a matrix protease
a mitochondrial Hsp70 binds the polypeptide chain as it crosses the inner membrane, driving further protein translocation.
A mitochondrial Hsp60 then facilitates folding of the imported polypeptide within the matrix
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Insertion of mitochondrial membrane proteins
Proteins targeted for the mitochondrial membranes contain hydrophobic stop-transfer sequences (second sorting signals) that halt their translocation through the Tom or Tim complexes and lead to their incorporation into the outer or inner membranes, respectively.
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Sorting proteins to the intermediate space
Proteins can be targeted to the intermembrane space by several mechanisms. Some proteins (I) are translocated through the Tom complex and released into the intermembrane space. Other proteins (II) are transferred from the Tom complex to the Tim complex, but they contain hydrophobic stop-transfer sequences that halt translocation through the Tim complex. These stop-transfer sequences are then cleaved to release the proteins into the intermembrane space. Still other proteins (III) are imported to the matrix. Removal of the presequence within the matrix then exposes a hydrophobic signal sequence, which targets the protein back across the inner membrane to the intermembrane space.
Mitochondrial Function
Oxidative catabolism of glucose and fatty acids
The matrix contains genetic system and enzymes for
oxidative metabolism
Pyruvate is transported to mitochondria, where its
complete oxidation to CO2 yields bulk of useable
energy (ATP) obtained from glucose metabolism
22 Mitochondria Structure & Function
Glycolysis
Universal pathway
Glucose starting material
Sequentially broken down to pyruvate
10 steps (all enzymes are cytosolic)
◦ Early preparatory step uses ATP
◦ Later steps produces chemical energy
Net Yield
◦ 2ATP(4ATP-2ATP)
◦ 2NADH
◦ 2Pyruvate
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Kerb Cycle
• In eukaryotic cell glycolysis took place in cytosol
• Pyruvate is then transported to mitochondria where it is completely oxidized
• Pyruvate undergoes oxidative decarboxylation in presence of coenzyme A (CoA-SH) forming acetyl CoA
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Kerb Cycle
Acetyl CoA enters the kerb cycle or citric acid cycle
The 2-carbon acetyl group combines with oxaloacetate
(4C) to yield citrate (6C)
In the remaining reactions the 2-carbons of citrate are
completely oxidized to CO2 and oxaloacetate is
regenerated
The citric acid cycle completes the oxidation of glucose
to six carbon molecule of CO2
Yields one GTP, three NADH and one reduced flavin
adenine dinucleotide (FDAH) which is another electron
carrier
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Electron Transport Chain
High-energy electrons from NADH and FADH are transferred
through a series of carriers in the membrane
e- carriers organized in ET complex as I, II, III, IV
Low energy electrons from IV carried on O2+ 2H to form H2O
Energy from ETC is used to pump protons to intermembrane
space
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Electron Transport Chain
Electrons from FADH2 are transferred through
complex II
Then carried by coenzyme Q to complex III and
IV
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Proton gradient & Chemiosmotic coupling
Proton gradient established across the inner membrane
Chemiosmotic coupling: Energy stored in H+ gradient is coupled to ATP synthesis
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Mitochondria Structure & Function
Oxidative Phosphorylation
Protons can cross through the membrane only through a proton channel (complex V)
Complex V (ATP synthase) has two units, F0 and F1, linked by slender stalk
F0 spans the inner membrane and forms a channel through which the proton move
F1 catalyzes the synthesis of ATP
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Oxidative Phosphorylation
Flow of electron through F0 drives the rotation of part of F1, which act as rotatory motor to drive ATP synthesis
Four protons are required to synthesize ATP synthesis
Oxidation of one NADH yield 3ATP; oxidation of FADH2 yield 2ATP
Kerbs and glycolysis: total 38 ATP per molecule of glucose.
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