Gluconeogenesis

Question 1

Gluconeogenesis

Answer 1

• needed to be able to synthesize glucose when glycogen stores are depleted
• mainly occurs in liver
• bypasses the 3 irreversible reactions of glycolysis

Question 2

3 Irreversible Reactions of Glycolysis

Answer 2

1. glucose to glc-6-P conversion
   - hexokinase
2. fructose 1,6 P to fructose 1,6 bisphosphate
   - phosphofructokinase-1
3. phosphoenolpyruvate to pyruvate
   - pyruvate kinase

Question 3

Gluconeogeneis Replacement Reactions

Answer 3

1. pyruvate converted to oxaloacetate
   - pyruvate carboxylase
2. oxaloacetate to phosphoenolpyruvate
   - PEP carboxykinase
3. fructose 1,6 bisphosphate to fructose 1,6 phosphate
   - fructose 1,6 bisphosphatase
4. glucose 6 - P to free glucose
   - glucose 6 phosphatase

Question 4

Oxaloacetate Synthesis

Answer 4

• exergonic reaction
• bicarbonate + pyruvate using ATP
• pyruvate carboxylase
• synthesises the 4C compound oxaloacetate

Question 5

Phosphoenolpyruvate Synthesis

Answer 5

• exergonic reaction
• oxaloacetate reacts with GTP to form phosphoenolpyruvate
• gives GDP and carbon dioxide

Question 6

Pyruvate Carboxylase

Answer 6

• carboxylates pyruvate using biotin cofactor
• biotin attached via amide linkage to lysine
• biotin carboxylated using ATP
• carbon dioxide then attached to pyruvate

Question 7

Net Gluconeogenesis Reaction

Answer 7

2 pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 4H20 == glucose + 4ADP + 2GDP + 2Pi + 2NAD+

• energetically costly but essential in the absence of carbs

Question 8

Cori Cycle

Answer 8

• metabolic pathway in which lactate produced by anaerobic glycolysis in muscles is transported to the liver and converted to glucose, which then returns to the muscles and is cyclically metabolized back to lactate
• muscular activity uses ATP provided by glycogen breakdown
• releases G1P which is converted to G6P by phosphoglucomutase
• G6P feeds into glycolysis or pentose phosphate pathway to form ATP
• If oxygen is not enough to undergo oxidative phosphorylation, lactate is formed from pyruvate using lactate dehydrogenase
• NAD+ is regnerated to keep glycolysis going
• lactate produced is taken up by the liver, initiating the other half of the Cori cycle
• converts lactate into pyruvate and then glucose
• glucose supplied to muscle

Overall, the glycolysis steps of the cycle produce 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis steps. Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules in the Cori cycle shifts the metabolic burden from the muscles to the liver.

Question 9

Futile Cycle

Answer 9

• substrate cycling
• glycolysis goes from fructose 6P into fructose 1,6 BPG
• gluconeogenesis goes from fructose 1,6 BPG to fructose 6P
• if both run at the same time, you just have a net reaction of ATP and water reacting to give off heat and ADP
• this is stupid
• therefore, the cycles are regulated reciprocally to avoid waste

Question 10

Glycolysis Regulation

Answer 10

Phosphofructokinase
- activated by AMP/F 2,6 BP
- inactivated by ATP/citrate/protons
Pyruvate kinase
- activated by F 1,6 BP
- inactivated by ATP/alanine

Question 11

Gluconeogenesis Regulation

Answer 11

Fructose 1,6 BP
- inactivated by F 2,6 BP/AMP
- activated by citrate
PEP Carboxykinase
- inactivated by ADP
Pyruvate carboxylase
- inactivated by ADP
- activated by acetyl CoA

Question 12

Fructose 2,6 BP

Answer 12

• regulator of PFK1/FBPase1

- synthesized from fructose 6P using ATP by PFK2 or hydrolyzed back to fructose 6P using water by FBPase 2

Question 13

Reciprocal Regulation of PFK/FBPase

Answer 13

• activities combined into one bifunctional enzyme
• cAMP dependent kinase (activated by PKA/glucagon) phosphorylates enzyme and activates FBPase/inactivates PFK2
• phosphatase (insulin activated) dephosphorylates enzyme and activates PFK2/inactivates FBPase

concentration of F 2,6 BP:

• high concentration: inhibits gluconeogenesis, stimulates glycolysis
• low concentration: inhibits glycolysis, stimulates gluconeogenesis

Question 14

Reducing Equivalents

Answer 14

• NAD+ and NADP+
• similar structures but NADP+ has an extra phosphate on the 2’OH of the ribose ring
• both carry 2 electrons but only retain one proton

Question 15

NADH

Answer 15

Activated electron carrier for fuel oxidation
Reduced in oxidation of fuel molecules
Electrons passed to oxygen to generate ATP
- concentration high for catabolism (oxidation)

Question 16

NAPH

Answer 16

Acts as an electron donor in reductive biosynthesis
HIgh potential electrons needed because precursors are more oxidised than the products like lipid synthesis
- concentration high for biosynthesis (reduction)

Question 17

Pentose Phosphate

Answer 17

• produces NADPH
• 2 phases: oxidative and non-oxidative
• produces 2 NADPH molecules

Question 18

Oxidative Phase of Pentose Phosphate Pathway

Answer 18

• produces NADPH
• ribulose 5 phosphate end product: isomerized to ribose
  1. G6P oxidised (inihibited by high NADPH)
  2. hydrolysis
  3. oxidative decarboxylation
• rate controlled by NADP+ concentrations: low levels limit regulatory first step because NADP+ is needed as an electron acceptor

Question 19

Net Reaction of oxidative phase

Answer 19

G6P + 2NADP + 2H20 = 2NADPH + ribose-5-P + CO2 + 2H2

Question 20

Glutathione

Answer 20

• prevents oxidative damage
• antioxidant in cytoplasm and reduces ROS like peroxides
• keeps SH groups in reduced form in the cytoplasm
• NADPH used to regenerate reduced form so that it can continue to react with ROS

Question 21

Non-oxidative phase

Answer 21

• if nucleotides not needed, ribulose-5-P converted back into intermediates of glycolysis/gluconeogenesis
• 2 x fructose 6P
• glyceraldehyde 3P
• rate controlled by substrate availability

Question 22

Fates of Glucose 6 Phosphate

Answer 22

1. pentose phosphate pathway: ribose + NADPH
2. glycolysis: pyruvate
3. conversion to G1P: glycogen metabolism
4. conversion to glucose: use by other tissues