33. Biochemistry | Gluconeogenesis and Pentose Phosphate Pathway

Which enzymes in gluconeogenesis bypass the irreversible step catalyzed by pyruvate kinase in glycolysis?

Pyruvate Carboxylase (PC) and PEP Carboxykinase (PEPCK).

What are the requirements and activators of pyruvate carboxylase (PC)?

Requires biotin (B7) and ATP; activated by acetyl-CoA; mitochondrial enzyme.

What reaction does pyruvate carboxylase catalyze?

Pyruvate + CO2 + ATP → Oxaloacetate (OAA).

What reaction does PEP carboxykinase (PEPCK) catalyze?

OAA + GTP → PEP + CO2.

Which enzyme bypasses phosphofructokinase-1 (PFK-1) in gluconeogenesis?

Fructose-1,6-bisphosphatase (F-1,6-BPase).

Which enzyme bypasses hexokinase/glucokinase in gluconeogenesis?

Glucose-6-phosphatase.

Where is glucose-6-phosphatase located and what is its significance?

Located in ER membrane of liver and kidney; releases free glucose into blood.

Which reactions are shared between glycolysis and gluconeogenesis?

Seven reversible glycolysis reactions (steps 2, 4–9).

What is the entry point of lactate into gluconeogenesis?

Converted to pyruvate via lactate dehydrogenase (Cori cycle).

What is the entry point of alanine into gluconeogenesis?

Converted to pyruvate via alanine aminotransferase (Glucose-Alanine cycle).

How do α-ketoacids from glucogenic amino acids enter gluconeogenesis?

Enter TCA cycle → oxaloacetate → PEP.

Why can't amino acids that form acetyl-CoA be used in gluconeogenesis?

Because pyruvate dehydrogenase is irreversible; these are ketogenic, not glucogenic.

How does glycerol enter gluconeogenesis?

Converted to glycerol-3-P by glycerol kinase (ATP), then to DHAP by glycerol-3-P dehydrogenase (NAD+).

What feature do all gluconeogenic precursors share?

All have a 3-carbon backbone.

What is the net energetic cost of synthesizing one glucose via gluconeogenesis?

Approximately 12 ATP equivalents (6 ATP + 2 NADH).

What signals increase gluconeogenesis during fasting and starvation?

Fasting, prolonged exercise, high protein diet, stress/injury; high acetyl-CoA from β-oxidation.

How does insulin regulate gluconeogenesis?

Insulin increases F-2,6-BP, which stimulates glycolysis (PFK-1) and inhibits gluconeogenesis (F-1,6-BPase).

How does glucagon regulate gluconeogenesis?

Glucagon decreases F-2,6-BP, which inhibits glycolysis and stimulates gluconeogenesis.

How does acetyl-CoA regulate pyruvate carboxylase?

Acetyl-CoA is a positive allosteric activator of PC.

How does AMP regulate gluconeogenesis?

AMP inhibits fructose-1,6-bisphosphatase.

How does alcohol impair gluconeogenesis?

Alcohol metabolism consumes NAD+, impairing lactate → pyruvate conversion, causing hypoglycemia and lactic acidosis.

What ensures glycolysis and gluconeogenesis do not occur simultaneously in the same cell?

Reciprocal regulation via F-2,6-BP and hormonal control by insulin and glucagon.

What are the two main purposes of the pentose phosphate pathway?

Production of NADPH and ribose-5-phosphate.

What is NADPH used for?

Reductive biosynthesis, regeneration of glutathione (antioxidant), and respiratory burst in phagocytes.

What is ribose-5-phosphate used for?

Nucleotide and nucleic acid synthesis.

Does the PPP produce ATP?

No, NADPH does not interact with the ETC.

What is the rate-limiting enzyme of the oxidative phase of the PPP?

Glucose-6-phosphate dehydrogenase (G6PDH).

What reaction does G6PDH catalyze?

G6P → 6-phosphoglucono-δ-lactone + NADPH.

What reaction does 6-phosphogluconate dehydrogenase catalyze?

6-phosphogluconate → ribulose-5-P + NADPH + CO2.

Which enzyme transfers 2C units in the PPP non-oxidative stage and what cofactor does it require?

Transketolase; requires thiamine pyrophosphate (TPP).

Which enzyme transfers 3C units in the PPP non-oxidative stage?

Transaldolase.

What are the two phases of the pentose phosphate pathway?

Oxidative (irreversible) and non-oxidative (reversible).

What does the oxidative phase of PPP produce?

2 NADPH, CO2, and ribulose-5-phosphate.

What does the non-oxidative phase of PPP produce?

Interconverts sugars to form ribose-5-phosphate or glycolytic intermediates (F-6-P, G3P).

What is the biochemical consequence of G6PD deficiency?

Decreased NADPH → decreased reduced glutathione → increased ROS.

Why are RBCs particularly vulnerable in G6PD deficiency?

They lack mitochondria and depend entirely on PPP for NADPH.

What is the effect of ROS on hemoglobin in G6PD deficiency?

ROS damages Hb, forming Heinz bodies that lead to rigid RBC membranes and hemolysis.

What populations commonly carry G6PD deficiency and why might it be advantageous?

African and Mediterranean males; protective against malaria due to shortened RBC lifespan.

How does alcohol consumption affect gluconeogenesis clinically?

It causes hypoglycemia and lactic acidosis by depleting NAD+.

What are common triggers of hemolytic crisis in G6PD deficiency?

Fava beans, infections, and oxidative drugs (primaquine, sulfa drugs, nitrofurantoin, aspirin).

What are clinical findings of G6PD deficiency?

Jaundice, fatigue, dark urine, hemolytic anemia; blood smear shows Heinz bodies and bite cells.

=== High-Yield Keyword Associations ===

Cori Cycle is associated with which precursor?

Lactate

Which cycle transports both carbon skeletons and ammonia to the liver?

Glucose-Alanine Cycle

Which enzyme requires Biotin (B7)?

Pyruvate Carboxylase

Which enzyme requires GTP?

PEP Carboxykinase (PEPCK)

Which enzyme is inhibited by AMP and F-2,6-BP?

Fructose-1,6-bisphosphatase

Which enzyme is located in the ER membrane and only in liver/kidney?

Glucose-6-phosphatase

Which molecule signals active β-oxidation and activates PC?

Acetyl-CoA

Which regulator links glycolysis and gluconeogenesis reciprocally?

Fructose-2,6-bisphosphate

What is the net ATP cost of gluconeogenesis?

~12 ATP equivalents

Which cofactor is required by transketolase?

Thiamine pyrophosphate (TPP)

What does the oxidative phase of PPP generate?

2 NADPH, CO2, ribulose-5-phosphate

What does the non-oxidative phase of PPP generate?

Ribose-5-phosphate and glycolytic intermediates (F-6-P, G3P)

What is the rate-limiting enzyme of the PPP?

Glucose-6-phosphate dehydrogenase (G6PDH)

What key product of the PPP protects RBCs from ROS?

NADPH (via reduced glutathione)

What pathological RBC finding is associated with G6PD deficiency?

Heinz bodies and bite cells

Which foods can trigger hemolysis in G6PD deficiency?

Fava beans

Which type of drugs can trigger hemolysis in G6PD deficiency?

Oxidative drugs (primaquine, sulfa drugs, nitrofurantoin, aspirin)

What is the protective evolutionary advantage of G6PD deficiency?

Protection against malaria

Which metabolic condition results from alcohol impairing gluconeogenesis?

Hypoglycemia with lactic acidosis

A 45-year-old man with chronic alcoholism presents with confusion, diaphoresis, and hypoglycemia. Lab studies show metabolic acidosis with elevated lactate. Which of the following best explains the biochemical basis of his presentation?

A. Depletion of pyruvate due to pyruvate kinase deficiency
B. Excess NADH from ethanol metabolism impairs lactate to pyruvate conversion
C. Inhibition of pyruvate dehydrogenase by thiamine deficiency
D. Decreased acetyl-CoA from impaired β-oxidation
E. Overactivation of glycolysis by insulin

B

Chronic alcohol metabolism converts NAD⁺ → NADH, lowering NAD⁺ availability. This blocks lactate → pyruvate conversion (LDH requires NAD⁺), leading to impaired gluconeogenesis, hypoglycemia, and lactic acidosis.

A 3-year-old boy is brought in with jaundice and dark urine after eating fava beans. Peripheral smear shows Heinz bodies and bite cells. Which of the following best describes the underlying defect?

A. Impaired generation of NADPH in RBCs due to glucose-6-phosphate dehydrogenase deficiency
B. Impaired ATP production in RBCs due to pyruvate kinase deficiency
C. Overproduction of ROS due to excess NADPH
D. Loss of transketolase activity from riboflavin deficiency
E. Increased glutathione synthesis due to high insulin

In G6PD deficiency, NADPH production falls, reducing glutathione regeneration. Without glutathione, ROS damage hemoglobin, causing Heinz bodies and hemolysis. Triggers include fava beans, sulfa drugs, and infections

A patient with prolonged fasting demonstrates elevated acetyl-CoA levels from β-oxidation. Which of the following gluconeogenic enzymes is directly activated by acetyl-CoA, helping divert pyruvate toward glucose synthesis?

A. Phosphofructokinase-1
B. Pyruvate carboxylase
C. Phosphoenolpyruvate carboxykinase (PEPCK)
D. Fructose-1,6-bisphosphatase
E. Glucose-6-phosphatase

B

Acetyl-CoA is a positive allosteric activator of pyruvate carboxylase. During fasting/β-oxidation, high acetyl-CoA signals the liver to divert pyruvate toward gluconeogenesis rather than the TCA cycle