Biochem Lect 27: Gluconeogenesis

gluconeogenesis

“creation of new glucose”

____ and _____ can be recycled to glucose

pyruvate and lactate

______ and ______ are major sites of gluconeogenesis

liver and kidney

_____ and ______ are major sites of glucose consumption

brain and muscle

-

(glucose is transported from liver and kidney via bloodstream)

Why is gluconeognesis NOT simply the reverse of glycolysis? (2)

1) glycolysis is exergonic → reverse would not be spontaneous

2) need different enzymes for different regulation

What is reciprocal regulation?

one pathway ON → other pathway OFF

Which of the following CANNOT be converted to glucose?

• pyruvate

• lactic acid

• most amino acids

• fatty acids

• glycerol

• citric acid cycle intermediates

fatty acids

(can only be converted to acetyl-CoA

Overall Summary of Gluconeogenesis enzymes (4)

1a) Pyruvate carboxylase (replaces step 10)

1b) PEP Carboxykinase (replaces step 10)

2) Fructose 1,6 Bisphosphatase (replaces step 3)

3) Glucose 6 Phosphatase (replaces step 1)

Name which glycogenesis enzymes each gluconeogenesis enzyme is replacing (3)

1) Pyruvate kinase (step 10)

2) PFK (step 3)

3) Hexokinase NOT glucokinase (step 1)

Summary of Gluconeogenesis reactions

1a) pyruvate → oxaloacetate

1b) oxaloacetate → PEP

2) F1,6 BP → F6P 

3) G6P → Glucose

Gluconeogenesis overall energetics

1a) -2 ATP (doubled)

1b) -2 GTP (doubled)

2) 

3) 

…

-2 ATP from other steps

-

*** -4 ATP and -2 GTP

Reaction 1a (replace step 10)

Pyruvate → Oxaloacetate

Enzyme = Pyruvate carboxylase

-

-1 ATP (-ΔG)

_____ and ______ are components of pyruvate carboxylase

biotin (coenzyme), lysine (in active site)

What does biotin do?

carries carboxyl groups

-

always in reactions with bicarbonate

regulation (2)

1) Acetyl-CoA activates

2) ATP activates

mechanism (not detailed)

biotin attacks phosphate while carrying lysine

structure of enzyme

homotetramer, 3 domains (1 for biotin carboxylase?)

oxaloacetate exits mitochondrial membrane by turning into ______

malate (then back to oxaloacetate)

oxaloacetate → malate enzyme

NADH linked malate dehydrogenase

Reaction 1b (replace step 10)

Decarboxylation

Oxaloacetate → PEP

Enzyme = PEP carboxykinase

-

-1 GTP (-ΔG)

Is this reaction favorable?

YES

decarboxylation is always favorable

GTP compared to ATP

energetically equivalent

Reaction 2 (replace step 3)

Remove one phosphate

F 1,6 BP → F6P

Enzyme = F1,6 Bisphosphatase

regulation

• citrate _______

• F2,6BisP _______

• AMP _______

• citrate activates

• F2,6BisP inhibits

• AMP inhibits

* opposite of PFK regulation (reciprocal regulation)

_____ and ______ are part of the tandem bifunctional enzyme

PFK2 and F26 BPase

List the reactions that each enzyme catalyzes:

1) PFK 1

2) PFK 2

3) F1,6 BPase

4) F2,6 BPase

1) F6P → F 1,6 BP

2) F6P → F 2,6 BP

3) F 1,6 BP → F6P

4) F 2,6 BP → F6P

Which of the following are products or regulators: 

1) F6P

2) F 1,6 BP

3) F 2,6 BP

1) product of gluconeogenesis

2) product of glycolysis

3) regulator

high F2,6BPase

• activates/inhibits PFK 1

• activates/inhibits F1,6BPase

activates, inhibits

-

(indicates that we must finish glycolysis, too much F6P)

low blood sugar → glucagon → ____ → (enzymes)

gluconeogenesis, F1,6BPase and F2,6BPase

-

(make F6P)

high blood sugar → insulin → ____ → (enzymes)

glycolysis, PFK 1 and PFK 2

-

(make F1,6 BP)

Reaction 3 (replace step 1)

Remove one phosphate

G6P → Glucose

Enzyme = Glucose 6 Phosphatase

-

(-ΔG)

Glucose 6 Phosphatase is absent from _____ and _____

muscle and brain

(too busy burning glucose to make it)

Reaction 3 involves _____ intermediate

phosphohistidine

-

(nucleophilic from His N → intermediate)

Cori cycle is what?

how glucose, lactate, and NADH are recycled between liver and muscle

Summary of Cori cycle

Liver: lactate → pyruvate → glucose

Muscle: glucose → pyruvate → lactate

muscle → lactate + NADH

-

What happens to NADH?

used in pyruvate → lactate

muscle → lactate + NADH

-

What happens to lactate?

→ liver (LDH → pyruvate → glucose) → muscle

futile cycle

both cycles ON at same time (what happens when no reciprocal control)

reciprocal control depends on energy status

• low energy status → activate ____

• high energy states → activate ____

glycolysis, gluconeogenesis

G6 Phosphatase activity increases linearly with _____

substrate concentration

Do these indicate high/low energy status?

• Acetyl-CoA → ____

• AMP → ____ / ATP → ____

• F 2,6 BP → ____

• high

• low / high

• low