Energy 2: Acetyl CoA, Mitochondria, Oxygen Flashcards
1
Q
Give an overview of the TCA cycle
A
- Prior to this glucose is converted to two (3C) pyruvate molecules. (from now one we’ll be referring to one cycle of citric acid cycle so one of the 2 pyruvates).
- The pyruvate then undergoes a reaction to become (2C) acetyl CoA, during this reaction there is a loss of CO2 and an NADH is generated.
- ACoA then enters the TCA cycle (which occurs in the mitochondrial matrix) and reacts with (4C) oxaloacetate to become (6C) citrate.
- The amount of oxaloacetate remains the same it is continually recycled.
- Citrate undergoes a series of reactions resulting in the loss of 2CO2 becoming a 4C molecule.
- For every turn of the cycle you generate three NADH and one FADH2
- One GTP molecule is formed.
- ATP is NOT produced in the citric acid cycle.
- TCA cycle oxidises acetyl CoA to generate NADH, FADH2 and CO2. The purpose of the TCA cycle is to generate the NADH and FADH2 to be used in the ETC, thus providing most of the electrons for oxidative phosphorylation.
- TCA also integrates carbohydrate, lipid and protein metabolism as these can enter as ACoA (which is an important metabolic intermediate, so it acts as a feeding in point for fatty acid and protein breakdown for energy generation).
- Not only is this cycle the common final pathway of aerobic oxidation it is also the source of building blocks for most important bio-molecules.
- For each glucose molecule – 6 NADH are formed plus 2 NADH, 2 FADH2, 2 GTP, 6 CO2.
2
Q
Describe the TCA cycle in detail and the number of products formed?
A
- Pyruvate is decarboxylated (removal of carboxyl group) → source of some CO2 released.
- Pyruvate is dehydrogenated (removal of hydrogen) to produce an acetyl group.
- Acetyl group combines with coenzyme A to form acetyl CoA (acetyl group carried to Krebs by CoA).
- Coenzyme NAD becomes reduced.
• Overall, from 1 glucose, in the link reaction, 2 red. NAD, 0 red. FAD, 2 CO2 and 0 ATP are produced.
• As 2 molecules of acetyl CoA are formed, there are 2 turns of Krebs cycle for each molecule of glucose. - Acetyl group released from acetyl CoA and combines with oxaloacetate (4C) to form citrate (6C).
- Citrate is decarboxylated and dehydrogenated to produce a 5C compound, 1 CO2 , 1 red. NAD.
- 5C compound is decarboxylated and dehydrogenated to produce a 4C compound, 1 CO2 , 1 red. NAD.
- 4C compound temporarily combines with CoA, substrate-level phosphorylation takes place to produce one molecule of ATP. 4C compound released.
- 4C compound is dehydrogenated, producing a different 4C compound and a molecule of reduced FAD.
- Isomerase enzymes catalyse rearrangement of the 4C compound. Further dehydrogenation, regenerates a molecule of oxaloacetate, so the cycle can continue.
3
Q
How can the TCA cycle be regulated, initially, in muscles and in the liver?
A
- The first point of regulation is the formation of ACoA from pyruvate which is an irreversible reaction.
- This commits the glucose carbon skeleton to either oxidation to CO2 and energy production or fatty acid synthesis.
- The enzyme that converts pyruvate to ACoA is pyruvate dehydrogenase, importantly this enzyme is inhibited by NADH and ACoA (the products of the reaction, note also inhibited by ATP which is a product of whole process), so if these both accumulate the enzyme will be inhibited.
- Also regulated through phosphorylation by a kinase and a phosphatase.
- Build-up of NADH and acetyl CoA inform the enzyme that the energy needs of the cell are being met or that fatty acids are been broken down to produce NADH and acetyl CoA. This has the effect of sparing glucose. (The inhibition of glucose oxidation causes fatty acids and ketone bodies to contribute to a glucose-sparing effect, which is an essential survival mechanism for the brain during times of starvation.)
- In muscle pyruvate dehydrogenase is activated via the action of phosphatase (which removes the phosphorylation), this particular phosphatase in muscle is stimulated by Ca2+.
- This allows the muscle to link contraction to a process that will generate ATP, Ca2+ is important for contraction and ATP is needed for contraction, so increased Ca2+ will cause the contraction and enable more ATP to be supplied.
- In the liver adrenalin increases Ca2+ through the activation of α-adrenergic receptors (hormonal regulation) and IP3.
- In the liver and adipose tissue, insulin (which signifies the fed state) stimulates the phosphatase which funnels glucose to fatty acid synthesis. This is a similar process but is not for producing ATP rather it is for storage.
4
Q
Give further example of how the TCA cycle is inhibited
A
- The first is the conversion of ACoA to Citrate under the action of the enzyme citrate synthase. It is inhibited by citrate itself, product inhibiting enzyme that made it. This means in cases where there is enough ATP the ACoA will be directed to other ways e.g. fatty acid synthesis (if you can’t use it store it).
- Also control of isocitrate to α-ketoglutarate, by enzyme isocitrate dehydrogenase. Inhibited by NADH, ATP. Stimulated by ADP.
- Also control of α-ketoglutarate –> Succinyl CoA by α-ketoglutarate dehydrogenase. It is inhibited by NADH, ATP, Succinyl CoA. (note that the control of entry into TCA spoken of earlier (pyruvate to ACoA) is inhibited by NADH, ATP, ACoA. It is stimulated by ADP and pyruvate).
- These two control points integrates citric acid cycle with other pathways. Inhibition of isocitrate dehydrogenase and ketoglutarate dehydrogenase leads to the build-up of citrate. Citrate is transported out of the mitochondria where it inhibits PFK which stops glycolysis. The citrate will also act as a source of acetyl CoA for FA synthesis.
- These enzymes can be important clinically.
5
Q
Describe the fate of NADH and FADH2
A
- The NADH and FADH2 are used in the electron transport chain.
- ETC involves the removal of hydrogen atoms from oxidizable substrates, notably NADH and FADH2. The hydrogen atoms enter the ETC and each is split to give an electron and proton.
- The electron is passed through a series of enzymes called cytochromes from left to right, going from high energy to low energy state. It finally reacts with molecular oxygen.
- The proton is pumped across the inner mitochondrial membrane into the IMS, which generates a proton gradient (pH gradient). This gradient can be harnessed to produce ATP.
- Electron transport is coupled to ATP synthesis. It needs 3 protons to make one ATP. As one proton is consumed to transport ATP out of the matrix four protons in total are needed to generate one ATP.
- For every NADH we get 3 ATP. For every FADH2 we get 2 ATP. (these are approximations, not always 100% efficient). 10H+ are pumped out for every NADH. 6H+ are pumped out for every FADH2.
6
Q
Describe OF
A
- Red. NAD and red. FAD are reoxidised when they deliver their hydrogen atoms to the ETC.
- Hydrogen atoms released from the reduced coenzymes split into protons and electrons.
- The protons go into solution in the mitochondrial matrix, and electrons pass along the ETC.
- Each electron carrier contains iron ion - the iron ion can gain an electron to become reduced Fe2+. The reduced iron ion then donates the electron to the next iron ion in the chain, becoming reoxidised Fe3+.
- Energy released (as electrons pass along chain) used to pump protons into intermembrane space.
- As protons accumulate in the intermembrane space, a proton gradient forms across the membrane.
- Proton gradient across inner membrane generate a chemiosmotic potential, known as a proton motive force (pmf). Source of potential energy, which is used in the formation of ATP.
- Protons cannot diffuse through the lipid bilayer of the membranes, as the outer membrane has a low degree of permeability to protons, and the inner membrane is completely impermeable to protons.
- Instead, protons diffuse through protein channels associated with ATP synthase enzymes.
- The action of protons diffusing down their gradient and flowing through the ATP synthase enzymes causes a conformational change in the ATP synthase enzyme, which allows ADP and Pi to form ATP.
- Oxygen is the final electron acceptor - it combines with electrons coming off the electron transport chain and combines with protons which have diffused down the ATP synthase channel, forming water.
7
Q
What regulates the electron transport chain?
A
- Governed by the need for ATP
- Electron transport tightly coupled to phosphorylation ie ADP to ATP
- Regulated uncoupling leads to the generation of heat
8
Q
What type of protein is ATP synthase
A
• ATP synthase is a transmembrane protein which acts as a motor (bottom part rotates).
9
Q
Describe mitochondria and heat generation in newborns
A
- There are instances where proton movement across IMM is no longer coupled with ATP synthesis.
- Neonates do not have the ability to shiver and obviously shivering is a good way of generating heat, so a newborn who cannot generate heat will lose heat from their surface.
- Neonates possess brown fat, which is predominantly around neck and shoulders. Brown fat contains a large no. of mitochondria, giving the brown fat its colour.
- The mitochondria in an infant are different to adult. One difference is they contain a protein called uncoupling protein, which uncouples the proton gradient with ATP synthesis.
- Uncoupling protein is an alternative route by which H+ can move down its conc. gradient and in doing so it doesn’t generate ATP it generates heat. So this is what we believe happens in order for them to generate heat, note that obviously not all their mitochondria will be have uncoupling protein, only those in brown fat tissue.
10
Q
Describe OXPHOS Diseases
A
Degenerative diseases
Caused by mutations in genes encoding
proteins of ETC
Lead to a number of symptoms, including
fatigue, epilepsy, dementia
Dependent on the mutation, symptoms may
be evident near birth to early adulthood
Metabolic consequence can be congenital
lactic acidosis
