Lecture 11- Pathways that harvest chemical energy II Flashcards
What is oxidative phosphorylation?
Process of ATP synthesis resulting from the reoxidation of electron carriers in the presence of O2
What are the two stages of oxidative phosphorylation?
The electron transport chain
Chemiosmosis
Why doesn’t the cell use one step to oxidize NADH + H+?
The reaction is untameable- it is too exergonic, energy cannot be harvested
Why does the cell use the electron transport chain?
It releases energy in smaller, more manageable amounts
What is the electron transport chain?
Electrons from NADH and FADH2 pass through a series of membrane associated electron carriers to actively transport protons to make a concentration gradient
What is chemiosmosis?
ATP synthase couples proton diffusion to ATP synthesis
What is the name of the four large protein complexes involved in the electron transport chain?
I, II, III and IV
What do the four large protein complexes, I, II, III and IV contain?
Electron carriers and associated enzymes
What are the 4 large protein complexes I, II, III and IV?
Integral proteins on the inner mitochondrial membrane in eukaryotes- three are transmembrane
What is a small peripheral protein in the intermembrane space of the mitochondria involved in the electron transport chain?
Cytochrome c
What is the nonprotein associated with the electron transport chain called?
Ubiquinone (Q)
What is ubiquinone (Q)?
Small, nonpolar molecule that floats within the hydrophobic interior of the phospholipid bilayer of the inner mitochondrial membrane
What is the first step in the electron transport chain?
NADH + H+ passes electrons to the first large protein complex (I) called NADH-Q reductase
Where does NADH-Q reductase transfer this electron to?
Q
What protein passes Q electrons from the oxidation of FADH2?
The second complex (II) succinate dehydrogenase
What is the name of the third complex of the electron transport chain?
cytochrome c reductase
What does cytochrome c reductase do?
Receives electrons from Q
Passes them to cytochrome c
What does the fourth complex do?
cytochrome c oxidase recieves electrons from cytochrome c
passes them to oxygen
What happens when oxygen (1/2O2) receives electrons?
It picks up two hydrogen ions to form H2O
What happens to the protons left over from the electron transport chain?
They are pumped across the mitochondrial membrane
Theoretically, how many molecules of ATP are formed from each pair of electrons passed along the electron transport chain?
3
What is the result of the electron transport chain?
The active transport of protons against their concentration gradient out of the matrix across the inner membrane to the intermembrane space
Why does proton transport occur?
Electron carriers in protein complex I, III and IV are arranged so that protons are taken up on one side of the membrane and transported, along with electrons, to the other side
How do the transmembrane protein complexes act?
As proton pumps
What does pumping of protons cause?
A proton concentration gradient
A difference in electric charge
What is the proton concentration gradient and the charge difference potential energy called?
Proton-motive force
What does proton motive force do?
Drive proteins back across the membrane
How do protons diffuse back across the inner mitochondrial membrane?
though a specific mitochondrial membrane called ATP synthase
What does ATP synthase do?
Couples proton movement with ATP synthesis
What is the coupling of proton movement with ATP synthesis called?
Chemiosmosis
What do the exergonic reactions that occur when electrons move along the electron transport chain drive?
The pumping of H+ out of the mitochondrial matrix to establish a H+ gradient
Where are H+ ions actively transported to?
The intermembrane space
What potential energy is harnessed by ATP synthase?
Proton motive force
What is proton motive force?
The potential energy of the proton gradient
What are the two roles of ATP synthase?
- Channel that allows protons to diffuse back in
2. Uses that energy to make ATP from ADP + Pi
What else can ATP synthase act as?
ATPase
Why does ATP synthase prefer synthesis as opposed to acting as ATPase?
- ATP concentration is low in matrix because ATP moves elsewhere in the cell
- H+ gradient is maintained by electron transport chain
What method can be used to test the hypothesis that a H+ gradient drives ATP synthesis by isolated mitochondria?
- isolate mitochondria, place in pH8 (low H+ concentration outside and inside organelles)
- Move to acidic medium (pH4, high [H+])
What are the results of moving isolated mitochondria from a high pH solution to a low pH solution?
H+ movement into mitochondria drives ATP synthesis in the absence of continuous electron transport.
What can be concluded by H+ movement into the mitochondria driving ATP synthesis in the absence of a continuous electron transport chain?
In the absence of electron transport, an artificial H+ gradient is sufficient for ATP synthesis by mitochondria
What method can be used to test the hypothesis that ATP synthase is needed for ATP synthesis?
- extract proton pump from bacteria, add to lipid vesicle
- H+ is pumped into vesicle, creates gradient
- ATP synthase from a mammal is inserted into the vesicle membrane
What is the result of the experiment to see if ATP synthase is needed for ATP synthesis?
The H+ diffuses out of the vesicle, driving synthesis of ATP by ATP synthase
In the experiment to test if a proton gradient drives ATP synthesis, how was changing the pH of the solution able to change [H+] in the mitochondria?
In outer membrane of mitochondria is freely permeable to protons so they rapidly diffused into the intermembrane space
What is another way to demonstrate the chemiosmotic mechanism?
Show that diffusion of protons and formation of ATP must be tightly coupled
What happens if another type of H+ channel (not ATP synthase) is inserted into mitochondrial membrane?
Energy of H+ gradient is released as heat (not coupled to ATP synthesis)
When might H+ and ATP synthesis be deliberately uncoupled?
To generate heat instead of ATP
What protein uncouples ATP synthesis and H+ gradient energy?
Thermogenin
What is the function of uncoupling ATP synthesis and the H+ gradient energy using thermogenin?
To regulate temperature
In what organisms does thermogenin play an important role?
Newborn human infants (no hair to keep warm)
Hibernating animals
What two parts is ATP synthase composed of?
F0 unit
F1 unit
What is the F0 unit of ATP synthase?
A transmembrane region that is the H+ channel
What is the F1 unit of ATP synthase?
The lollipop of interacting subunits that constitute the active site for ATP synthesis
What type of energy does ATP synthase turn the potential energy from the H+ gradient into?
Kinetic energy of movement
How does ATP synthase use the kinetic energy of movement to produce ATP?
The subunits of F1 rotate, exposing the active site for ATP synthesis
What is the maximum yield of ATP through glycolysis followed by cellular respiration?
32 gross 30 net
Why is the net yield only 30?
Inner membrane of some animal cells is inpermeable to NADH, one ATP is paid for each NADH to enter the mitochondrial matrix
How many molecules of ATP are produced in the electron transport chain per glucose molecule?
28
How do carbon skeletons enter the metabolic pathway?
Catabolic interconversions- other molecules are broken down to release their energy
what catabolic interconversions occurs for polysaccharides?
They are hydrolyzed into glucose which passes through glycolysis and cellular respiration- energy is captured as NADH and ATP
What catabolic interconversions occurs for lipids?
Broken down into glycerol and fatty acids
What is glycerol converted into?
dihydroxyacetone phosphate (DAP)
What is dihydroxyacetone phosphate?
An intermediate in glycolysis
What are fatty acids converted into?
acetyl CoA in the mitochondria
How do proteins enter the metabolic pathway?
They are hydrolyzed into amino acids
20 different amino acids feed into glycolysis and citric acid cycle at different points
How does the amino acid glutamate enter the metabolic pathway?
converted into alpha-ketoglutarate- an intermediate in the citric acid cycle
What are anabolic interconversions?
When catabolic pathways operate in reverse
What process forms glucose?
Gluconeogenesis
What molecules are reduced to form glucose?
glycolytic and citric acid intermediates
What can be used to form fatty acids?
Acetyl CoA
What are the most common number of carbon atoms in fatty acids?
14,16,18
How are the most common fatty acids formed?
By adding two carbon acetyl CoA units one at a time until correct length is reached
What molecule can be used as the starting point for purines?
alpha-ketoglutarate
What molecule can be used as the starting point for pyrimidines?
oxaloacetate
What is alpha-ketoglutarate the starting point for?
purines
chlorophyll synthesis
What molecule is oxaloacetate the starting point for?
Pyrimidines
What is acetyl CoA a building block for?
fatty acids various pigments plant growth substances rubber steroid hormones of animals
What are all of these substances called?
The metabolic pool
How does the level of substances in the metabolic pool vary?
They don’t- they are constant
What is metabolic homeostasis?
The cell regulates anabolism and catabolism to maintain balance
Give an example of the metabolic homeostasis being upset.
Undernutrition
Why are proteins not preferentially broken down?
They have essential roles as enzymes and structural elements- using as energy deprives these vital roles
Why do fats weigh less in water than polysaccharides?
Fats are nonpolar, they do not bind as much water
Why do fats store more energy in their bonds?
They are more reduced than carbohydrates (C-H vs C-OH)
The level of what molecule in the blood rises as fatty acids are broken down?
Acetyl CoA
Why can fatty acid not be the bodies only source of energy?
Fatty acids cannot cross the blood-brain barrier
The brain must use glucose as its energy source
How does the body, depleted of glucose, convert something else to make glucose for the brain?
Gluconeogenesis from amino acids from the break down of proteins
What happens after several weeks of starvation?
Proteins and fats are used up Essential proteins (such as muscle and antibodies) are broken down
What happens when essential proteins are broken down?
can lead to illness and eventual death
How is glycolysis, the citric acid cycle and the electron transport chain regulated?
Allosteric control of enzymes involved
How can excess product effect a metabolic pathway?
- suppress action of enzymes that catalyze earlier reaction
2. speed up reactions in another pathway
How does speeding up reactions in another pathway control the amount of product being formed?
Diverts raw materials away from synthesis of the first product
What type of mechanisms control metabolic pathways?
Positive and negative control mechanims
What is the main control point in glycolysis?
Phosphofructokinase
What reaction does phosphofructokinase catalyze?
The third reaction of glycolysis from fructose-6-phosphate to fructose-6-bisphosphate
How is phosphofructokinase controlled?
Inhibited by ATP
Activated by ADP or AMP
What is the main control point of the citric acid cycle?
The enzyme isocitrate dehydrogenase
What does isocitrate dehydrogenase do?
Interconverts isocitrate into alpha ketoglutarate (reaction 3)
What are the feed back inhibitors and activators of isocitrate dehydrogenase?
NADH + H+ and ATP are inhibitors
NAD+ and ADP are activators
What builds up when isocitrate dehydrogenase is inhibited?
isocitrate and citric acid
What stops the build up of isocitrate and citric acid when isocitrate dehydrogenase is inhibited?
Acetyl CoA conversion to citrate is inhibited by ATP and NADH + H+
What else does a build up of citrate do to the metabolic pathway?
feedback inhibitor to slow fructose-6-phosphate reaction early in glycolysis
How does Acetyl CoA act as a control point?
When too much ATP is made the accumulation of citrate diverts acetyl CoA to fatty acid synthesis for storage
What is the final control point for metabolic pathways?
Cell differentiation- for example, PPARδ which controls proliferation of slow twitch muscle fibers
