Mitochondrial Synthesis of ATP Flashcards
What are daily energy needs like of a healthy 70 kg male?
- daily energy needs are around 12 MJ
- blood glucose and glycogen make up near a third of daily energy needs
- there is 400 MJ of triglyceride and 100 MJ of usable protein available
What is meant by “useable protein”?
Protein that could be used without endangering our lives
The loss of muscle protein is never desirable and is the last resort
What is the concentration of ATP in cells?
It is present at a relatively high concentration
This is around 6 mM
What is daily ATP turnover?
How much ATP is found in the average human body?
An average human body contains around 75 g of ATP
ATP turnover is 75 kg/day
How much energy from food is converted into useful energy?
Half the energy from food is converted to ATP
Half of this energy is converted to useful work
25% of energy from food is turned into useful work
What is the composition of the outer mitochondrial membrane?
How permeable is it?
What gradients are present?
Outer membrane is smooth
It is freely permeable to molecules under 5000 Da (including ions)
There are NO ionic or electrical gradients
What are cristae?
Why are they necessary?
The inner mitochondrial membrane is folded into cristae
They protrude into the mitochondrial matrix to increase the surface area for the proteins of electron transport
How permeable is the inner mitochondrial membrane?
It is permeable only to a small number of molecules via specific transporters
What gradients are present across the IMM?
it is a good electrical insulator and is capable of maintaining large ionic and electrical gradients
What is the role of cardiolipin within the IMM?
It is a phospholipid that is capable of maintaining large ionic gradients across the IMM
How does the amount of protein in the IMM compare to the amount of lipid?
The IMM contains more protein than lipid
e.g. respiratory enzymes, transporter proteins etc.
What substances are located within the mitochondrial matrix?
- wide range of enzymes
- high concentrations of substrates, cofactors and ions
- mitochondrial DNA, RNA and ribosomes
What is found within the intermembrane space?
What are metabolite and ion concentrations like?
Cytochrome C is found in the intermembrane space
Metabolite and ion concentrations are similar to in the cytosol
What happens to pyruvate at the end of glycolysis?
It is transported across the inner mitochondrial membrane
It moves from the cytosol into the matrix
What is the main reaction of the link reaction?
Pyruvate dehydrogenase catalyse the conversion of pyruvate to acetyl CoA
What is the most important cofactor of pyruvate dehydrogenase?
Thiamine pyrophosphate (TPP)
Thiamine is a B vitamin that is important in maintaining metabolism
What does a lack of thiamine cause?
Disruption to the function of pyruvate dehydrogenase
This causes Beri-Beri that has neurological and cardiovascular symptoms
What can cause Wernicke-Korsakoff syndrome?
A lack of thiamine seen in alcohol dependency
Alcohol affects the absorption of thiamine in the gut
Wernicke-Korsakoff syndrome affects neurological function
Why is pyruvate dehydrogenase called a “key decision point” in metabolism?
- acetyl CoA cannot be converted back into glucose
2. conversion of pyruvate to acetyl CoA commits the C atoms of glucose to energy production of lipid synthesis
When is pyruvate dehydrogenase inhibited?
When energy levels are high, phosphorylation is promoted
e.g. high levels of acetyl CoA, NADH or ATP
What are the 2 enzymes involved in controlling pyruvate dehydrogenase through phosphorylation?
Pyruvate dehydrogenase kinase phosphorylates PDH to inactivate it
Pyruvate dehydrogenase phosphatase will remove the phosphate from PDH to reactivate it
What happens if there is a sudden energy demand in a cell?
This leads to an increase in free Ca2+ ions
Increase in free Ca2+ leads to the removal of the phosphate group and activation of pyruvate dehydrogenase
Where may the acetyl CoA entering the Krebs cycle be derived from?
- acetate (alcohol breakdown)
- pyruvate (glucose)
- fatty acids
- ketone bodies
- amino acids
What is the first of the 2 stages involved in the Krebs cycle?
- synthesis of citrate (6C) which then loses 2C as CO2 to become succinyl CoA (4C)
oxaloacetate reacts with acetyl CoA to produce citrate
What is the second of the 2 stages involved in the Krebs cycle?
- oxidation of succinyl CoA to regenerate oxaloacetate and initiate another round of the cycle
What are the main outputs of the Krebs cycle?
- the electron carriers, NADH and FADH2
- CO2 - waste product
- 1 GTP (which is equivalent to 1 ATP)
What are NADH and FADH2?
They are reduced coenzymes
They are carriers of hydrogen ions and high energy electrons
What controls the entry of pyruvate into the Krebs cycle?
- the need for energy
2. the availability of acetyl CoA from fat oxidation
What 2 factors are involved in controlling the entry of pyruvate into the Krebs cycle depending on the need for energy?
- ATP:ADP ratio
2. NADH:NAD+ ratio
What is the first point at which the Krebs cycle can be controlled?
What can this stage be inhibited by?
Incorporation of acetyl CoA into the Krebs cycle by citrate synthase
Inhibited by:
- ATP
- NADH
- succinyl-CoA
- citrate
What is the second point at which the Krebs cycle can be controlled?
What can this stage be inhibited and stimulated by?
Conversion of isocitrate into a-ketoglutarate by isocitrate dehydrogenase
Inhibited by ATP and NADH
Stimulated by ADP and NAD+
What is the third point at which the Krebs cycle can be controlled?
What can this stage be inhibited by?
Conversion of a-ketoglutarate to succinyl-CoA by a-ketoglutarate dehydrogenase
Inhibited by:
- ATP
- NADH
- succinyl-CoA
At which stages may amino acid carbon skeletons be fed into the Krebs cycle?
They can be added or removed from the cycle as oxaloacetate and a-ketoglutarate
These molecules are important in amino acid synthesis
What can enter the Krebs cycle as succinyl-CoA?
- odd-chain fatty acids
2. Amino acids - Ile, Met and Val
What can enter the Krebs cycle as fumarate?
- Amino acids - Asp, Phe and Tyr
What are the compounds involved in the Krebs cycle?
- acetyl CoA joins with oxaloacetate to form citrate
- isocitrate
- a-ketoglutarate
- succinyl-CoA
- succinate
- fumarate
- malate
- back to oxaloacetate
Why may citrate be transported from the mitochondria into the cytosol?
It is used to synthesise fatty acids and cholesterol in the cytosol
Why may oxaloacetate be converted back to malate?
This allows it to be moved into the cytosol for gluconeogenesis
When are ketones synthesised by an individual?
Ketones are synthesised from fats in times of starvation
They provide a vital source of energy to the brain
What condition leads to people synthesising ketones?
Why do they not use glucose?
Type 1 diabetes
They cannot use glucose effectively due to the absence of insulin
What happens in type 1 diabetes regarding glycolysis and gluconeogenesis?
- glycolysis is inhibited meaning that pyruvate levels are low
- gluconeogenesis is NOT inhibited and is sped up to try and increase blood glucose levels
Oxaloacetate and malate are removed from the Krebs cycle to form glucose
What happens to oxaloacetate levels in type 1 diabetes and why?
Oxaloacetate levels drop as it is being removed from the Krebs cycle to form glucose
It cannot be regenerated from pyruvate as glycolysis is inhibited
What happens to acetyl CoA levels in type 1 diabetes and why?
Acetyl CoA levels are HIGH
In the absence of insulin, fatty acids are mobilised from adipose tissue and oxidised to acetyl CoA
Why are ketones synthesised in type 1 diabetes?
A lack of oxaloacetate prevents acetyl CoA from entering the Krebs cycle
Ketones are synthesised instead
What can happen if type 1 diabetes is poorly controlled?
Ketones can build up in the body and cause ketoacidosis
what are the 2 tightly coupled processes involved in oxidative phosphorylation?
Where do they occur?
- electron transport (oxidation)
- ATP synthesis (phosphorylation)
Both processes take place in or across the IMM
What is involved in the oxidation stage of oxidative phosphorylation?
The reduction potential (energy) of the electrons in NADH/FADH2 is used to create a proton gradient across the inner mitochondrial membrane
What is involved in the phosphorylation stage of oxidative phosphorylation?
The energy from the proton gradient is used to phosphorylate ADP
This synthesises ATP
What are the major complexes involved in the electron transport chain?
What are the electron carriers?
The electron transport chain consists of:
- complex I
- complex II
- [ubiquinone]
- complex III
- [cytochrome c]
- complex IV
Ubiquinone and cytochrome c are electron carriers
Other than the electron transport chain, what 3 other proteins in the IMM are involved in oxidative phosphorylation?
- F0F1 ATPase is involved with ATP synthesis
- Adenine dinucleotide transporter
- Pi transporter
The transporters are needed to facilitate ATP synthesis
Where will electrons from NADH and FADH2 enter the electron transport chain?
What are the reactions that occur?
NADH enters at complex I
NADH –> NAD+ + e-
FADH2 enters at complex II
FADH2 –> FAD + e-
What is different about complex II compared to the other complexes in the electron transport chain?
Complex II does NOT traverse the whole membrane
What happens to the electrons once they enter the electron transport chain?
They are transferred from one electron carrier to the next
When they reach complex IV, they are donated to oxygen
Why are electrons donated to oxygen at complex IV?
What is the reaction that occurs at complex IV?
Donation of electrons reduces oxygen to water and removes electrons from the chain
O + 2H+ –> H2O
How does cyanide affect the electron transport chain?
Cyanide inhibits complex IV
This means electrons cannot be donated from complex IV to oxygen
Why does cyanide lead to the death of cells?
Cells die due to lack of oxygen as they are no longer able to make ATP
Electrons cannot be fed into the electron transport chain until the electrons at the end of the chain are removed
What happens as the electrons are transferred between complexes?
Energy is released
Each reduction step (transfer) results in the release of energy
What is the energy released as electrons move between complexes used for?
It is retained within the different complexes
It is used to pump H+ ions across the inner mitochondrial membrane from the matrix into the intermembrane space
How many H+ ions are pumped across the IMM by the various complexes into the intermembrane space?
Complex I and III will pump 4 H+ each
Complex IV will pump 2 H+
For each pair of electrons from NADH, a total of 10 H+ ions are translocated
Why are more protons pumped from a molecule of NADH than FADH2?
The electrons from FADH2 bypass complex I and only go through complexes III and IV
What is the result of the movement of H+ ions into the intermembrane space?
It creates a large proton gradient across the well-insulated inner mitochondrial membrane
Where will protons flow once they have accumulated in the intermembrane space?
They flow across the IMM down their concentration gradient, into the matrix
They pass through the F0F1 ATPase
What is the consequence of the flow of protons through the F0F1 ATPase?
The flow of protons through the ATPase generates the energy required to synthesise ATP
How many H+ ions must flow through the F0F1 ATPase in order to generate ATP?
3 H+ ions must flow through the ATPase to generate 1 molecule of ATP
What is the structure of the F0F1 ATPase within the membrane?
It has a ring structure within the membrane that is made up of identical subunits
This is the F0 component
What happens when H+ ions are funnelled from the intermembrane space into the rota of the ATPase?
H+ ions cause the rota to move around
The H+ ions can then leave, via an exit channel, on the other side of the membrane
How does the rota (axle) connect the ATPase molecule?
The rota is attached to a stalk that connects the F0 and the F1 subunits
Where is the F1 component of the ATPase found?
It is held in place in the matrix, whilst the axle rotates inside it
What is the action that causes ATP to be generated by the F0F1 ATPase?
The movement of the axle alters the conformation of the subunits that make up the F1 component
This changes the conformation of the active sites, allowing ATP to be synthesised
What is the antiport in the IMM involved in ADP and ATP transport?
It allows the exchange of ATP and ADP
It transports ATP out of the matrix after synthesis
It transports ADP into the matrix to supply the ATPase
What is the symporter involved in phosphate transport across the IMM?
It transports phosphate into the matrix for ATP synthesis
This is coupled with the movement of a proton
The proton is needed to ensure the movement is electrically neutral as phosphate is negatively charged
In total, how many protons must cross the IMM to synthesise 1 molecule of ATP?
4 protons must cross the inner membrane
3 H+ are required to flow through the ATPase
1 proton is required to move the phosphate
Why can NADH produced in the cytosol not be directly reoxidised by electron transport?
NADH cannot cross the inner mitochondrial membrane
It must be oxidised in the cytosol and reduced in the matrix
How is NADH oxidised in the cytosol?
NADH is oxidised in the cytosol by the conversion of oxaloacetate to malate
This produces NAD
What happens to the malate produced from oxidation of NADH?
The malate moves out into the matrix space
It is converted back into oxaloacetate, which reforms the NADH
Why might the electron transport chain not be working?
How does this affect ATP synthesis?
- There is a lack of reduced substrates or oxygen
2. There is no proton gradient to drive the ATPase enzyme
Why might ATP synthesis be blocked?
There may be a lack of substrate (ADP) or inhibition of the ATPase enzyme
What happens when ATP synthesis is blocked?
The proton gradient builds up to a level where the complexes have insufficient energy to pump more protons across the membrane
A lot more energy is needed to move protons against their concentration gradient
How does blocking ATP synthesis affect electron transport?
Energy cannot be released from the electron carriers due to the large proton gradient
The electron carriers cannot accept anymore electrons, so electron transport stops
What are uncouplers?
They are weak acids that are soluble in the membrane
They will uncouple oxidation from phosphorylation
What happens to uncouplers when they penetrate the IMM?
At the intermembrane interface, they associate with protons (driven by high proton conc.)
At the matrix surface, they release protons (driven by low proton conc.)
What is the overall effect of the actions of uncouplers?
They dissipate the proton gradient
This means that electron transport can continue WITHOUT ATP synthesis
What is the action of 2,4-dinitrophenol as an uncoupler?
- when it reaches the intermembrane space, there is a high concentration of H+ ions so it takes up H+ ions
- it releases H+ ions when it passes over the membrane and into the matrix
How does the action of 2,4-dinitrophenol affect the properties of the IMM?
How does this affect oxidative phosphorylation?
The IMM becomes “leaky” to H+ ions
Oxidation can continue, even though phosphorylation has stopped
There is another mechanism for moving H+ ions from the intermembrane space into the matrix, without using ATPase
How is uncoupling used to generate heat in newborns?
Through non-shivering thermogenesis
What is significant about newborns possessing brown adipose tissue?
Brown adipose tissue contains more mitochondria
The mitochondria in the brown adipose tissue contain thermogenin (uncoupling protein-1)
What happens when core body temperature drops in a newborn?
- sympathetic system releases noradrenalin
- noradrenalin leads to increased concentrations of fatty acids in the cytosol
- free fatty acids activate thermogenin
What is the action of free fatty acids activating thermogenin?
This uncouples electron transport from ATP synthesis
This leads to energy being released as heat
What type of scan is used to detect brown adipose tissue in adults?
PET scan using 18F-fluorodeoxyglucose in response to cold
Which types of adults will have less brown adipose tissue?
Brown adipose tissue/activity decreases with age and in obesity
What is beige adipose tissue?
Why is activating beige/brown adipose tissue a valuable therapeutic target?
Beige adipose tissue switches between brown and white forms
Activating brown/beige adipose tissue could promote triglyceride clearance and weight loss
Activating brown adipose tissue means more ATP would be used
Why was dinitrophenol used as a weight loss product?
It significantly increases energy usage in the same way as activating brown adipose tissue
Why was dinitrophenol withdrawn as a weight loss product?
Due to the side effects:
- hyperthermia
- tachycardia
- excess sweating
- blindness due to cataracts
5 fatalities
