GLYCOLYSIS
What enzyme catalyzes the rate-limiting step of glycolysis, and how is it allosterically regulated by ATP and fructose 2,6-bisphosphate?
Rate-limiting enzyme: Phosphofructokinase-1 (PFK-1)
Activated by: AMP, fructose 2,6-bisphosphate
Inhibited by: ATP, citrate
→ Controls glycolytic flux according to energy state.
Which three irreversible glycolytic reactions are bypassed in gluconeogenesis, and what enzymes replace them?
Bypassed glycolytic steps:
Glycolysis Enzyme Gluconeogenesis Enzyme
Hexokinase Glucose-6-phosphatase
PFK-1 Fructose-1,6-bisphosphatase
Pyruvate kinase Pyruvate carboxylase & PEP carboxykinase
What are the two main functions of the pentose phosphate pathway?
Functions: Generate NADPH (for fatty acid & glutathione reduction).
Produce ribose-5-phosphate (for nucleotide synthesis).
What is the fate of NADH produced during anaerobic glycolysis in muscle cells?
Anaerobic glycolysis (muscle):
NADH donates electrons to pyruvate → lactate, regenerating NAD⁺ via lactate dehydrogenase.
Which steps of glycolysis involve substrate-level phosphorylation, and which enzymes catalyze these reactions?
Substrate-level phosphorylation steps:
1,3-bisphosphoglycerate → 3-phosphoglycerate (via phosphoglycerate kinase)
Phosphoenolpyruvate → pyruvate (via pyruvate kinase)
Explain why acetyl-CoA cannot serve as a substrate for gluconeogenesis.
Acetyl-CoA not used:
Pyruvate dehydrogenase reaction is irreversible, so acetyl-CoA cannot yield oxaloacetate.
Which enzyme catalyzes the rate-limiting step of the oxidative phase, and what are its regulators?
Rate-limiting enzyme:
Glucose-6-phosphate dehydrogenase (G6PD)
Activated: NADP⁺
Inhibited: NADPH
Explain the mechanism by which pyruvate kinase is regulated in the liver and how this regulation contributes to gluconeogenesis.
Pyruvate kinase regulation:
In liver: inhibited by phosphorylation (via glucagon → PKA).
Prevents futile cycle during gluconeogenesis (when liver makes, not burns, glucose).
Why is hexokinase inhibited by glucose-6-phosphate, while glucokinase is not?
Hexokinase vs Glucokinase:
Hexokinase (in muscle): Inhibited by glucose-6-phosphate.
Glucokinase (in liver): Not inhibited, allowing hepatic glucose uptake after meals.
What is the role of biotin in gluconeogenesis, and in which step is it required?
Role of biotin:
Cofactor for pyruvate carboxylase, which converts pyruvate → oxaloacetate in mitochondria.
Why is NADPH, not NADH, used in reductive biosynthesis?
NADPH vs NADH:
NADPH used in reductive biosynthesis & antioxidant defense, not energy production.
How does glucagon promote gluconeogenesis at the molecular level in hepatocytes?
Glucagon action:
↑ cAMP → activates PKA → Phosphorylates PFK-2/FBPase-2 → ↓ fructose 2,6-bisphosphate → inhibits glycolysis, promotes gluconeogenesis.
Compare and contrast the isoenzymes of lactate dehydrogenase found in the liver and heart, and explain their physiological significance.
LDH isoenzymes:
LDH-H (heart): Converts lactate → pyruvate (aerobic).
LDH-M (muscle): Converts pyruvate → lactate (anaerobic).
→ Reflect tissue oxygen use and metabolic roles.
Explain how ethanol consumption affects gluconeogenesis and may lead to hypoglycemia.
Ethanol effect:
Ethanol → ↑ NADH → drives pyruvate → lactate → reduces gluconeogenic substrates → hypoglycemia risk.
How is the non-oxidative phase of PPP linked to glycolysis through transketolase and transaldolase reactions?
Non-oxidative phase linkage:
Transketolase transfers 2C units (requires TPP).
Transaldolase transfers 3C units.
→ Convert excess ribose-5-phosphate into glycolytic intermediates (F6P, G3P).
What would be the metabolic consequence of a deficiency in glucose-6-phosphatase?
Glucose-6-phosphatase deficiency (von Gierke’s disease):
Can’t release glucose → hypoglycemia, hepatomegaly, lactic acidosis.
In the presence of fluoride ions, glycolysis is inhibited. Which enzyme is affected and why?
Inhibits enolase, blocking conversion of 2-phosphoglycerate → phosphoenolpyruvate.
Why can’t muscle cells perform gluconeogenesis even though they contain many of the required enzymes?
Muscle lacks glucose-6-phosphatase → cannot dephosphorylate glucose-6-phosphate → no glucose release to blood.
Describe the role of thiamine pyrophosphate (TPP) in the pentose phosphate pathway.
Role of TPP:
Coenzyme for transketolase → stabilizes carbanion intermediate during C–C transfer reactions.
In rapidly dividing cells, how does the direction of the PPP shift to favor nucleotide synthesis?
Rapidly dividing cells:
Non-oxidative phase runs forward → more ribose-5-phosphate than NADPH → supports DNA/RNA synthesis.