Transcript Cell Chemistry

Readings and Objectives
• Reading
– Russell : Chapter 6
– Cooper: Chapter 3, 11
• Objectives
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Sugar metabolism
Mitochondrion structure
Mitochondrial genome
Proteins
Mitochondrial function
• Krebs cycle
• Oxidative Phosphorylation
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Mitochondria: structure
• Generate energy from lipids & carbohydrates
• Surrounded by a double-membrane system
• The inner membrane has numerous folds (cristae),
which extend into the interior (matrix)
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Mitochondria: dynamic organelles
• Outer membrane: permeable to small molecules
• Porins, form channels (in o.m.), allows free diffusion of small
molecules
• Intermembrane space: composition similar to the cytosol
• Inner membrane: impermeable to most ions and small
molecules
• Helps maintain the proton gradient
• Mitochondria positioned near locations of high-energy
use, ie. synapses in nerve cells, muscle cells
• Continually fusing and dividing, remodels the network
of mitochondria in the cell, and affects function and
morphology
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Endosymbiotic origin
• Mitochondria are thought to have evolved
from bacteria that began living inside
larger cells (endosymbiosis)
• Living organisms that have genomes most
similar to the mitochondrial genome are
free-living α-proteobacteria
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See a review paper for endosymbiosis under literature section
Mitochondrial genome
• Circular DNA 16 kbp,
multiple copies
• Maternal inheritance
Map:
• Origin of replication: D-loop
• Code for rRNAs, tRNAs,
own ribosomes
• Encode 13 proteins
essential for oxidative
phosphorylation
– Electron transfer chain
complexes, including I,
III, IV and V
Human Mitochondrial
genome map
(16 kbp)
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Mitochondrial: Genetics
• mitochondrial genetic code is different from the
universal code
• U in the tRNA anticodon can pair with any of the four
bases in the third codon position of mRNA; thus four
codons are recognized by a single tRNA
• Some codons specify different amino acids in
mitochondria than in the universal code
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Mitochondrial Proteins
• contain 1000 to 1500 different proteins, but nearly half of
them remain unidentified
• mitochondria from different tissues contain different
proteins
• Genes for many mitochondrial proteins are in the nucleus
(95% of mtProteins)
• Some of these genes were transferred to the nucleus from
the original prokaryotic ancestor of mitochondria
• Cytosolic protein synthesis  mit. Transport
• All Krebs enzymes, rep/transcrip/translation
• Complex because of mito. double membrane
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Transport and Assembly of Matrix Proteins
• Presequences, N-terminal 2535 positively charged a.a.
targets proteins to matrix
• Partially unfolded by Hsp70
chaperone
– Prevent aggregation as emerge
from free ribosomes
• bind to receptors on Tom
protein complex (translocase of
outer membrane)
– First to Tom20 then Tom5
– To import poreTom40
– Passage , bind intermembrane
tail of Tom22
• Bind Tim complex (translocase
of inner membrane)
– Bind Tim21/Tim50 of Tim23
complex matrix
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Transport and Assembly of Matrix Proteins
In the Matrix
• Presequence/Hsp70/Tom44
works as a ratchet
• Reversible binding with short
hydrophobic amino acids
• Sequential ATP hydrolysis
• Powers the binding &
dissociation of Hsp70
• Might integrate to membrane, or
• Protein is pulled into matrix
• Matrix processing peptidase
(MPP) cleaves presequence
• Hsp70 binding assists proper
folding
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Mitochondrial Function
• Oxidative catabolism of glucose and fatty acids
• The matrix contains the genetic system and enzymes for oxidative
metabolism
• Pyruvate (from glycolysis) is transported to mitochondria, where
its complete oxidation to CO2 yields the bulk of usable energy
(ATP) obtained from glucose metabolism
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Glycolysis
• Universal pathway
• Glucose starting
substrate
• sequentially broken
down to pyruvate
• 10 steps (all enzymes
are cytosolic)
– Early preparatory
steps uses ATP
– Later steps
produces chemical
energy
Net yeild:
• 2 ATP (4ATP-2ATP)
• 2 NADH
• 2 Pyruvate
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Glycolysis
Glycolysis provides substrates for mitochondrial Krebs cycle
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Krebs Cycle
• In eukaryotic cells, glycolysis takes place in the
cytosol
• Pyruvate is then transported into mitochondria,
where it is completely oxidized
• Pyruvate undergoes oxidative decarboxylation in
the presence of coenzyme A (CoA-SH), forming
acetyl CoA
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Krebs Cycle
• Acetyl CoA enters
the citric acid cycle
or Krebs cycle
• The 2-carbon acetyl
group combines
with oxaloacetate
(4C) to yield citrate
(6 C)
• In the remaining
reactions, 2
carbons of citrate
are completely
oxidized to CO2 and
oxaloacetate is
regenerated
All enzymes are in matrix
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Krebs Cycle
• The citric acid cycle
completes the
oxidation of
glucose to six
molecules of CO2
• yields one GTP,
three NADH, and
one reduced flavin
adenine
dinucleotide
(FADH2), which is
another electron
carrier
All enzymes are in matrix
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Krebs Cycle
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Electron Transport Chain
• High-energy electrons from NADH and FADH2 are transferred through
a series of carriers in the membrane
• e- carriers organized in ET complexes 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
cyt b
Coenzyme Q
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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 and Chemiosmotic coupling
• proton gradient established across the inner membrane
• Chemiosmotic coupling: Energy stored in H+ gradient is
coupled to ATP synthesis (Peter Mitchel 1961)
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Oxidative phosphorylation
• protons can cross the membrane
only through a protein channel
(complex V)
• complex V (ATP synthase), has
two units, F0 and F1, linked by a
slender stalk.
• F0 spans the inner membrane
and forms a channel through
which the protons move
• F1 catalyzes the synthesis of ATP
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Oxidative phosphorylation
• flow of protons through F0 drives
the rotation of part of F1, which
acts as a rotary motor to drive ATP
synthesis
• Four protons are required to
synthesize one ATP
• Oxidation of one NADH yields 3
ATP; oxidation of FADH2 yields 2
ATP
• Krebs and glycolysis: total 38 ATP
per molecule of glucose (ie. 2
pyruvate)
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