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Lecture 15

BIOL 1020 Lecture 15: October 8th.docx


Department
Biological Sciences
Course Code
BIOL 1020
Professor
J.Stacey
Lecture
15

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October 8th 2014
Lecture 14: oxidative phosphorylation, fermentation
Did you get it?
A cell biologist prepares some isolated mitochondria. She measures the rate of
activity an enzyme of the Krebs cycle, citrate synthase, by following the
appearance of some of the products of the reaction with a radiolabelled substrate
that she will add. Since there are no substrates from the cytosol in her isolated
mitochondrial preparation, which of
the following will she label and provide to the mitochondria?
A. FADH2  reduced by the Krebs cycle
B. ATP
C. Pyruvate
D. Glucose  needed by glycolysis
E. NADH
Electron Transport Chain
Electrons from NAHD passed among proteins of the ETC with the help of
Coenzyme Q and cytochromes (iron atom) to O2
ocytochromes is why you need iron in your diet
oremember oxygen is very electronegative
onotice NADH and electrons are at a high free energy level compared to
oxygen at the end has a lower free energy
oETC generates not ATP directly
Oxidation of NADH and FADH2
oNADH (if it has hydrogen it is the reduced form) is oxidized by complex 1
to NAD+
oFADH2 oxidised by complex 2 to FAD
oElectrons travel down the ETC to oxygen (electron acceptor)
oProtons are pumped into the inter-membrane space
Chemiosmosis: use of a H +
gradient to drive work
Generates ATP
Protein complexes pump protons into the inter-membrane space  build up in
intermembrane space
oIf you have a buildup you now have a concentration gradient and because
it is charged you also have an electrical gradient  proton moving force
oYou have a high potential energy, so you have the capacity to do work!
The movement of protons back into the matrix is “coupled” to ATP synthesis and
the protein that does this is by enzyme ATP Synthase
oHow does this happen
Protons pass through channels in our ATP synthase by protein
channels and the energy released is used to phosphorylate ADP
ETC + chemiosmosis = Oxidative phosphorylation
ETC – Chemiosmosis Summary
NADH (glycolysis and TCA) and FADH2 (TCA) transfer electrons to ETC
ETC complexes located on the mitochondrial inner membrane

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Series of redox reactions transfers electrons
H+ pumped into intermembrane space
Electrons delivered to ½ O2
Proton motive force (intermembrane space  mitochondrial matrix) drive ATP
synthesis
ETC summary
Used by ETC Produced by ETC
ATP synthesis by cellular respiration
Energy Flow Organic molecules  ATP generated by substrate phosphorylation
NADH produced
Proton motive force
Produces most ATP from Organic molecules
Most ATP is produced by oxidative phosphorylation in the mitochondrion
Supply of Pyruvate to mitochondria
Transport protein – mitochondrial pyruvate carrier (MPC) yeast mammals
oUniversal protein that is inherited by a common ancestor
Supply can limit rate of oxidative phosphorylation (production of NADH, FADH2
via Krebs cycle)
“Coupled” mitochondria
Normal chemiosmosis: protons re-enter the mitochondrial matric through ATP
synthase, generating ATP
oThe movement of protons is coupled to ATP synthase
These are “coupled”
“Uncoupled” mitochondria
In some situations/ species/tissues:
oProton gradient “uncoupled from ATP synthesis”
Uncoupling proteins
oSometimes called UCP but other agents can do this as well
oProtons enter mitochondrial matrix but not through ATP synthase  No ATP
generated = no chemiosmosis
oFree energy is lost from system (probably as heat)
Also called “leaky” mitochondria
Brown Fat
oIn neonates and hibernators, brown from high density of mitochondria
oHigh density of UCP in mitochondria inner membrane
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oGenerate heat without shivering
Uncoupled mitochondria oxidize NADH from glycolysis and TCA but produce no
ATP
oOnly 4 (from glycolysis and Krebs cycle)/ 32 possible ATP from a glucose
molecule
Inefficiency in conversion of ingested macromolecules to energy
Any given amount of food generates less ATP
oLess excess energy to be stored (as fat)?
Mitochondrial un couplers
Same poisons uncouple mitochondria
Ionophores
oMake the inner membrane permeable to ions including protons, allow
uncoupling (e.g. 2,4 dinitrophenol)
Other metabolic poisons: ETC inhibitors
Rotenone blocks NADH dehydrogenase (complex 1)
Cyanide inhibits cytochrome oxidase (binds to the iron of the heme group)
oProduced by some plants in small amounts: defense against predation?
oApple seeds, almonds, peaches, cassava root
Many poisons inhibit metabolic processes
Reactive oxygen species (ROS)
Sometimes oxygen accepts less than 4 electrons needed to make water
Makes superoxide and hydrogen peroxide = ROS
These can damage biological molecules by oxidizing them
Antioxidant systems prevent some of the oxidizing damage
Some organisms lack these enzymes
To them oxygen is toxic and what we call obligate anaerobes
Catabolism of other substrates
Other organic molecules can be the source of electrons for cellular respiration
Carbohydrates
Glycolysis accepts a wide range of carbohydrates
Proteins
Digested to amino acids
Amino groups – feed glycolysis or the citric acid cycle
Ammonia produced – “nitrogenous waste
Fats/lipids
Digested to
oGlycerol ( glycolysis) and fatty acids (generates acetyl CoA via
beta oxidation)
Review
Should know this is glycolysis, this happens in the cytosol
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