Thompson 2 Flashcards
What effect does phosphorylation of intermediates have on metabolic pathways?
- Creates a negative charge at physiologic pH, trapping the molecule in the cell.
- Conserves the energy that is released from phosphoanhydride bonds (ATP->ADP) by forming phosphoester bonds.
- Sets up for further metabolism (“committed” steps)
What are the functional differences between hexokinase and glucokinase?
Hk is found in most tissues, has a low Km (low capacity), and a low Vmax (low rate). Gk is in the liver and pancreatic beta cells, has a high Km and a high Vmax. Gk is also not inhibited by G6P, which further promotes glucose metabolism (glycolysis, glycogen synthesis and other pathways)
Glycolysis takes place in the ____ and the TCA cycle takes place in the _____.
cytoplasm, mitochondrial matrix
What are the three irreversible steps of glycolysis? Which is the rate-limiting step?
Hexo/glucokinase (#1), PFK-1 (#3, rate-limiting), pyruvate kinase (#10)
What is the oxidizing agent in glycolysis?
NAD+ (gets reduced to NADH)
What is substrate-level phosphorylation?
In glycolysis, the substrates themselves phosphorylate ADP to produce ATP (unlike ox-phos where ATP synthase is an enzyme designated for ATP production). In SLP, oxidation and phosphorylation are not coupled. SLP is a fast source of ATP, independent of external electron acceptors and respiration (O2).
Substrate level phosphorylation is important in _____ because they do not have _______, and in oxygen-deprived _____.
RBCs, mitochondria, muscle tissue
Substrate level phosphorylation occurs at which steps in glycolysis?
Phosphoglycerate kinase (#7), pyruvate kinase (#10)
What is the net ATP production of glycolysis?
2.
One is invested at step #1 and one at #3 (hexo/glucokinase, PFK-1) and one is returned at #7 and one at #10 (phosphoglycerate kinase, pyruvate kinase) but that happens twice for every glucose molecule. [-2 + 2(2) = 2]
What are the “high energy intermediates” of glycolysis? Why are they called that?
1,3 BPG and phosphoenolpyruvate. It is very energetically favorable for them to lose a phosphate.
Standard free energy of hydrolysis = -11.8, -14.8 (most negative of all the intermediates of glycolysis)
What glycolytic side-reaction product is important in RBCs?
Isomerization of 1,3BPG to 2,3BPG (mutase) before becoming 3BPG (phosphatase), instead of 1,3BPG to 3BPG via phosphoglycerate kinase. 2,3BPG binds with greater affinity to deoxygenated Hb than to oxygenated Hb and weakens Hb affinity for O2, thus promoting oxygen transfer from oxygenated lung tissue to deoxygenated respiring tissue.
Why might F1,6P up-regulate pyruvate kinase?
F1,6P is after the committed step (actually the product of that step) so, in order for it (and the other downstream intermediates) to proceed down the pathway, the final irreversible reaction must be going at a good rate. (Prevents bottlenecking)
Explain the G-protein-coupled receptor/cAMP pathway.
Transmembrane receptor and nearby G protein-GDP complex… Extracellular hormone/substrate binds receptor, changes shape, interacts with G protein, causing it to release GDP and bind GTP. Alpha subunit-GTP complex dissociates from rest of G protein and activates adenylyl cyclase, which makes cAMP from ATP.
How is PKA activated and what does it do?
Its two regulatory subunits bind a total of 4 cAMP molecules, causing dissociation and activation of its two catalytic subunits. Active C subunits catalyze the phosphorylation of other proteins, using ATP.
How does insulin oppose the cAMP cascade?
Insulin binds insulin receptor which self-phosphorylates and also phosphorylates other kinases. Some of these other kinases inhibit glucagon-stimulated phosphorylation (mechanisms utilizing cAMP).
What is insulin resistance?
IR is a condition in which the body produces insulin (pancreatic beta cells) but the cells in the liver and the rest of the body fail to use it effectively, leading to hyperglycemia. This is caused by post-insulin-receptor defects in the insulin pathway.
As a result, beta cells increase their production of insulin, further contributing to hyperinsulinemia. This often remains undetected and can contribute to a diagnosis of DM2.
Describe the pathway of insulin-regulated glycolysis.
High insulin/glucagon ratio causes low cAMP and thus low PKA. Low PKA promotes the dephosphorylated state of the PFK2-FBP2 bifunctional enzyme, in which PFK2 is activated. PFK2 phosphorylates F6P to F2,6BP. F2,6BP up-regulates PFK-1, which increases the rate of glycolysis.
In ____ glycolysis, pyruvate gets _____ to lactate. ______ catalyze the reaction.
Anaerobic, reduced (NADH is the reducing agent), lactate dehydrogenase
What are the cofactors required for oxidative decarboxylation of pyruvate?
CoA, TPP, lipoic acid, FAD, NAD+
Acetyl CoA and NADH down-regulate pyruvate dehydrogenase. How?
They promote phosphorylation of pyruvate dehydrogenase, which inactivates it.
Which enzyme catalyzes the decarboxylation of pyruvate to Acetyl CoA?
Pyruvate dehydrogenase
At which steps of the TCA cycle does oxidative decarboxylation occur?
Isocitrate dehydrogenase (#3), alpha-ketoglutarate dehydrogenase (#4)
Which steps of the TCA cycle are just oxidation steps? (No decarboxylation)
6 uses FAD as the oxidizing agent; the other 3 oxidation steps (#3, #4, #8) use NAD+.
Succinate dehydrogenase (#6), malate dehydrogenase (#8)
Substrate-level phosphorylation occurs at which step of the TCA cycle?
Succinyl CoA synthetase (#5), catalyzes production of GTP from GDP + Pi
