Biology 1002B Lecture Notes - Lecture 7: Hexokinase, Citric Acid Cycle, Nadh Dehydrogenase

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Published on 15 Apr 2013
Western University
Biology 1002B
of 4
Lecture 7: Energy Transformation II
Next class bring paper and pen, drawing lots of stuff
Cellular respiration
o Catabolic… exergonic
Convert C-H bonds into ATP
o Carbs, fats, proteins…
Glucose uses the entire cellular respiration pathway
o Need to know where ATP is made, where NADH is made
Know glycolysis
o Where is it found
o What does it do?
o Free energy?
Where does it go?
o Requires O2?
Huge demand for oxygen, but the whole process does not require oxygen
o Where is the carbon?
We start with carbon, where does it end up?
o Compare with photosynthesis
The differences, where they occur, how is Calvin cycle similar and different than
Citric acid cycle, the electron transport chain
o Splitting of glucose
o Occurs in the cytosol in eukaryotic and prokaryotes
Nothing specific about cellular respiration for eukaryotes
o Nothing catalysis the process, very ancient system
o Did not lose any carbon, did not lose any oxygen
o Reduction of NADH, (from NAD+ reduced to NADH), there is less energy in 2 Pyruvate
because some energy is required to make 2 NADH
o Need to consume 2 ATP, but 4 is generated, so a net of 2 ATP is formed
Energy Coupling
o Glucose catalyzed by Hexokinase, not spontaneous, cannot happen by itself
o Half reactions
Pi + glucose -> glucose 6 P delta G = + 3.3 KJ/mol
ATP + H2O -> ADP + Pi delta G = - 7.3 kcal/mol
o Coupled reaction:
ATP + glucose -> ADP + glucose 6 P delta G = -4 kcal/mol
o Water cannot access the active site
No real hydrolysis happen, the free energy of the phosphate is transferred to
o But why?
To phosphorylate glucose?
A phosphate group is charged, glucose is not charged, so when we add
an charge, we can keep the glucose in the same compartment
Make the glucose more unstable, more reactive, readily to break apart
Substrate-level phosphorylation
o The phosphate group is transferred from Phosphoenol-pyruvate (PEP)
o PEP is very reactive, readily give up phosphate
o With pyruvate kinase PEP can generate ATP
o Phosphoryl transfer potential, readily releases phosphate group to generate ATP
o Compare structure to chloroplast
o Structures:
Outer mitochondrial membrane
Inner mitochondrial membrane
Intermembrane space
Linking Glycolysis and Citric acid cycle
o Cytosol the pyruvate need to move into mitochondrial matrix
o No free energy in the carboxyl group in the pyruvate
o So that it goes through decarboxylation
Release of the CO2
o Dehydrogenase reduces NAD+ to NADH soon after decarboxylation
o Then it adds Coenzyme A, making it more reactive, Acetyl CoA, easier to react with
enzyme to react
o Pyruvate dehydrogenase complex, deals with pyruvate after its being imported from
cytosol to mitochondria
Citric Acid Cycle (CAC)
o When Acetyl-CoA comes in, it’ll loses the CO2
No more carbon from now on
o Get the remaining energy from the acetyl group and linking it to the NADH and FADH2
Consumes NAD+ to produce NADH
o Oxaloacetate is a 4 carbon molecule
o Oxaloacetate binds with the 2 carbons to form a 6 carbon molecule called Citrate
Oxidative phosphorylation
o There are protein complexes on the inner mitochondrial membrane
o 40 proteins to form the complex I of the electron transport chain
o No more carbon, lost in pyruvate conversion and the CAC
o Oxygen is the terminal electron acceptor
One needs a lot of oxygen in mitochondria
o UQ is very similar to PQ in photosynthesis
o Complex I is pumping out the protons into Inner mitochondrial space creating a
electrochemical gradient
o The energies collected by NADH and FADH2 are used to pump protons outside so that it
can come back through ATP synthase
Why do the electrons move?
o Because of the redox potential
o Oxygen is at the end of the chain, very positive redox potential
o It goes from negative redox potential to positive redox potential
o The protein themselves don’t undergo the oxidation/reduction, the cofactors do
o Get proton pumping by electron transport chain
o The protons cannot go through the membrane
o One way to get back is to go through ATP synthase
o But there are other ways to go back:
Chemical uncouplers (ionophores)
Uncoupling proteins (UCP1,2,3)
o Uncoupler, uncouples the electron transport chain from ATP synthase
o It will lower the electrochemical gradient
o If ingest an uncoupler, it is very toxic
Oxygen supply controls the fate of pyruvate
o If there is enough oxygen, the pyruvate goes into the mitochondria
o If there is not enough oxygen, the pyruvate will stay in cytosol and go through
fermentation (ethanol or lactate)
o One of the checks that the cell performs is
1. Redox homeostasis
o If there is high level of NADH, then there is inhibition of electron
transport, not reducing NAD+ to NADH,
So not enough oxygen will generate low ratio
o If there is a lot of NAD+ that means that the electron transport
chain is operating
2. Hypoxia-inducible factor
HIF activates transcription under hypoxia
o HIF 1 (HIF alpha and HIF -1 beta)
HIF-1 is a dimer
o The two sub units come together, only together, they’re functional
o HIF 1 is a transcription factor
Regulate transcription
o HIF -1 beta is found in the nucleus
o If there is lots of oxygen, HIF-1 alpha will become hydroxyl HIF 1 alpha, and then it’ll
be degraded by the proteasome
It’s process is ubiquitination
o If there is insufficient oxygen, (1%) it will not be degraded

Document Summary

Next class bring paper and pen, drawing lots of stuff. Convert c-h bonds into atp: carbs, fats, proteins . Glucose uses the entire cellular respiration pathway: need to know where atp is made, where nadh is made. We start with carbon, where does it end up: compare with photosynthesis. The differences, where they occur, how is calvin cycle similar and different than. Glycolysis: splitting of glucose, occurs in the cytosol in eukaryotic and prokaryotes. Energy coupling: glucose catalyzed by hexokinase, not spontaneous, cannot happen by itself, half reactions. Pi + glucose -> glucose 6 p delta g = + 3. 3 kj/mol. Atp + h2o -> adp + pi delta g = - 7. 3 kcal/mol: coupled reaction: Atp + glucose -> adp + glucose 6 p delta g = -4 kcal/mol: water cannot access the active site. A phosphate group is charged, glucose is not charged, so when we add an charge, we can keep the glucose in the same compartment.