Session 2: Carbohydrates Flashcards
(36 cards)
What happens to pyruvate before entering the TCA cycle?
Pyruvate dehydrogenase oxidises pyruvate (using NAD+ as oxidising agent) and adds CoA to form acetyl CoA and CO2 (and NADH)
Where does the link reaction take place?
Mitochondrial matrix
Name a cofactor that pyruvate dehydrogenase requires.
Vitamin B1
Is the link reaction reversible or irreversible?
Irreversible because CO2 is released
What activates PDH activity? (5)
Low energy substrates activate PDH by dephosphorylation:
- Pyruvate
- CoA
- NAD+
- ADP
- Insulin
What inhibits PDH activity? (4)
High energy substrates and products inhibit activity by phosphorylation:
- Acetyl CoA
- NADH
- ATP
- Citrate
What does PDH deficiency lead to?
Lactic acidosis
Where does the TCA cycle take place?
Mitochondrial matrix
What molecules does the TCA cycle require? (3)
NAD+
FAD
Oxaloacetate
What is the function of the TCA cycle? (2)
- Break the C-C bond in acetate
2. Oxidise C atoms to CO2
Does the TCA cycle function in the absence of oxygen?
No
Approximately how many molecules of ATP doe the TCA cycle produce?
32 molecules of ATP
Describe the events of the TCA cycle. (6)
- Acetyl CoA (2C) join oxaloacetate (4C) to form citrate (6C)
- Citrate undergoes isomerisation to form Isocitrate
- Isocitrate gets oxidised (NAD+ -> NADH) to a-ketoglutarate (5C) by Isocitrate dehydrogenase
- A-ketoglutarate gets oxidised to succinyl-CoA (4C) (NAD+ -> NADH) with the addition of CoA by a-ketoglutarate dehydrogenase
- Succinyl-CoA releases CoA to form Succinate (4C) - this releases energy in the form of GTP
- Succinate undergoes isomerisation and oxidation to form oxaloacetate (along with FADH2 and NADH)
What are the products of one TCA cycle?
Multiply by 2 for each glucose because 2 pyruvate molecules:
6 NADH
2 FADH2
2 GTP
Which enzymes and molecules are involved in the regulation of the TCA cycle?
- Isocitrate dehydrogenase
- a-ketoglutarate dehydrogenase
- ATP/ADP ratio
- NADH/NAD+ ratio
- Succinyl-CoA
How does ATP regulate the TCA cycle?
High ATP/ADP ratio down-regulates the TCA cycle
How does NADH regulate the TCA cycle?
High NADH/NAD+ ratio down-regulates the TCA cycle
What regulates Isocitrate dehydrogenase?
- Down-regulated by NADH and ATP
2. Up-regulated by ADP
What regulates a-ketoglutarate dehydrogenase?
NADH, ATP and succinyl CoA (product inhibition) down-regulates the enzyme activity
What is the significance of Succinate?
Used in haem synthesis and can be interconverted to amino acids
Why does the TCA cycle not function in the absence of O2?
- Oxygen is the final acceptor of electrons in electron transport chain
- Therefore, in the absence of oxygen, oxidative phosphorylation is not carried out as effectively
- NAD+ and FAD not regenerated from donation of electrons
- NADH and FADH therefore builds up and down-regulates the TCA cycle
Which stage of carbohydrates metabolism produces most energy?
Electron transport chain (stage 4)
Where does the electron transport chain and oxidative phosphorylation take place?
Inner membrane of the mitochondria
What happens in the electron transport chain?
- carrier molecules transfer high-energy electrons from NADH and FADH2 into a series of 4 highly specialised complexes which span the inner mitochondrial membrane
- oxygen acts as the terminal electron acceptor;
- 3 of the complexes act as proton translocating complexes (PTCs) which use the free energy from electron transport to move protons from inside the matrix to the inter-membrane space
- the PTCs help create a proton concentration gradient across the inner mitochondrial membrane
