Lecture 16+17: Glycolysis, PDH, TCA Flashcards
Possible fuel sources for ATP and their storage forms
Glucose (glycogen), fatty acids (triglycerides), amino acids (proteins)
How does alcohol inhibit gluconeogenesis (clinical case)? How can it be treated?
Treatment with thiamine, glucose infusion
Key regulated steps in glycolysis
Hexokinase, PFK, pyruvate kinase
ATP investment and return steps in glycolysis
Net = 2 ATP (2 in, 4 out)
High energy phosphate donors in glycolysis
1,3-bisphosphoglycerate and phosphoenolpyruvate (PEP)
What does iodoacetate inhibit?
Generation of NADH by G3P DH in glycolysis
How is NADH re-oxidized to NAD in aerobic conditions?
Reducing equivalents from NADH are transported by shuttles to the ETC and finally to O2
How is NADH re-oxidized to NAD to anaerobic conditions?
NADH reduces pyruvate to lactate via lactate DH
Pasteur effect
Anaerobic fermentation of pyruvate into acetaldehyde then reduction by alcohol DH and NADH into EtOH in yeast. Inhibited by oxygen.
How is glucokinase different from hexokinase?
What roles does glucokinase play compared to hexokinase?
Because glucokinase is most sensitive to changes in [gluc.] at physiological ranges, it acts as a glucose sensor for the liver and pancreas. Hexokinase instead wants to bring in glucose as quickly as possible, especially with high energy demand (e.g. working muscle).
How does glucokinase play a role in glucose dysregulation in diabetes?
Liver glucokinase is positively controlled by insulin, and so is low in diabetes -> impaired blood glucose response.
Mature Onset Diabetes of the Young (MODY2)
Inhibitory mutation in beta cell glucokinase resulting in insufficient insulin release despite high blood glucose.
Persistent Hyperinsulinemic Hypoglycemia of Infancy (PHHI)
Activating mutation in beta cell glucokinase leading to insulin production even with low blood glucose.
GLUT2 vs GLUT4
GLUT4 is found in muscle and fat and is activated by insulin and anoxia.
GLUT2 acts independent of insulin and is part of glucose sensing in the liver/pancreas by achieving rapid equilibrium with blood glucose.
How is PFK regulated?
Activation: ADP, AMP, F-2,6-bisPi
Inhibition: ATP (feedback), citrate
Why are AMP and ADP more sensitive indicators of changes in [ATP]?
ADP is generally at a lower concentration -> larger percent changes
[AMP] is 2nd order to [ADP] by myokinase (adenylate kinase), so a 3x increase in ADP leads to a 9x increase in AMP
Why does citrate inhibit PFK?
Citrate buildup is an indicator of available alternate fuel, i.e. fatty acid oxidation produces acetyl-CoA. Acetyl-CoA can’t be transported out of the mitochondria, so it is instead converted to citrate which can be transported to the cytosol where PFK is located.
What role does fructose-2,6-bisphosphate play? How is it regulated?
F26BP is important in signaling for control of glycolysis/gluconeogenesis in the liver. Glucagon (low blood gluc.) activates Protein Kinase A, inhibiting PFK2 and activating F26BPase -> decreased PFK activity in liver
How is fructose-2,6-bisphosphate regulated in muscle?
PFK2/F26BPase don’t have the PKA phosphorylation site in muscle, and so are not affected by glucagon.
Role of the liver in glucose levels
The body mainly uses the glycogen stores in the liver to regulate blood glucose in close coordination with the pancreas via insulin/glucagon.
Why is PFK the primary regulated step in glycolysis and not hexokinase?
Glucose-6-P can go to glycolysis, glycogen, or the PPP. Inhibition of hexokinase by G6P means, with little flux to glycogen/PPP, hexokinase activity follows PFK. When PFK activity increases/decreases, so does hexokinase activity.
How is pyruvate kinase regulated in muscle?
Weak competitive inhibition by ATP
How is pyruvate kinase regulated in the liver?
Strong allosteric inhibition by ATP; also inhibited by Ala and PKA phosphorylation. Depends on high F16bP levels.
What is PKM2?
PKM2 is the fetal form of pyruvate kinase
What’s bad about high fructose in food?
Fructose bypasses early regulated steps of glycolysis in the liver via aldolase B, which can lead to obesity.
Aldolase B deficiency
Aldolase B deficiency in the liver leads to fructose intolerance due to as phosphate groups get tied up in F1P from fructose/fructokinase.
Glucokinase Inhibitory Protein (liver)
Binds glucokinase for nuclear retention. Inhibited by F1P, which stimulates glucokinase.
Galactosemia
Deficiency in galactose-1-P uridyl transferase, trapping P in galactose-1-P
PDH Complex enzymes
- Pyruvate DH
- Dihydrolipoyl transacetylase
- Dihydrolipoyl DH
How is PDH allosterically regulated?
Activated by CoA, NAD
Inhibited by acetyl-CoA, NADH
(ratios)
Why is PDH complex regulation important?
Pyruvate can be converted back into glucose, but acetyl-CoA creation is irreversible.
How is PDH complex covalently regulated?
PDH can be inactivated by phosphorylation via PDH kinase; reversed by PDH phosphatase
How is PDH kinase regulated?
Acetyl-CoA:CoA, NADH:NAD, ATP:ADP ratios
How is PDH phosphatase regulated?
Activated by insulin, pyruvate, and calcium
PDH deficiency
X-linked E1-alpha gene, inefficient E1 import to mitochondria leads to elevated lactate, pyruvate, and Ala due to low pyruvate -> acetyl-CoA. Can be treated sometimes with thiamine or a ketogenic diet (PDH cofactor or bypassing PDH)
PDH cofactors and their vitamin precursors
Thiamine (B1) -> TPP
Riboflavin (B2) -> FAD
Nicotinate (Niacin) -> NAD
Pantothenate -> CoA
How does arsenic impact glucose metabolism?
Arsenic interacts with lipoic acid, inhibiting aKG DH, PDH, and branched chain AA DH
