L17: Gluconeogenesis/CHO metabolism III

Question 1

Describe the synthesis of lactose.

Answer 1

Question 1

Describe/Draw the oxidative/non-oxidative phase reactions of the pentose phosphate pathway.

Answer 1

Question 1

What pentose phosphate pathway deficiency(ies) seen in human? What is/are the clinical presentation(s)?

Answer 1

• Beriberi: result of thiamine deficiency that in addition to affecting PDH and alphaKGDH, also affects the transketolase reactions of the non-oxidative phase of the PPPathway preventing rearrangement of carbon structures and leading to an imbalance in the CHO pool. Prevalent in low income communities, but most especially in alcoholic patients. Alcoholism reduces thiamine uptake and storage.

Question 2

Why is gluconeogenesis not a reversal of glycolysis? Explain. Explain what reactions occur to circumvent this issue.

Answer 2

• Glycolysis has 3 irreversible reactions: hexokinase, phosphofructokinase 1 and pyruvate kinase. In order for glucose to be synthesized, ie. gluconeogenesis, 4 new enzymes not seen in glycolysis are seen: Pyruvate carboxylase, PEP carboxykinase, Fructose-1,6-bisphosphatase and glucose-6-phosphatase.

Question 2

How can oxaloacetate initiate gluconeogenesis?

Answer 2

• Malate-oxaloacetate shuttle. Oxaloacetate can be moved from the mitochondria to the cytoplasm via this shuttle and gluconeogenesis can occur.

Question 3

How do high levels of NADH prevent gluconeogenesis from proceeding?

Answer 3

• Malate-oxaloacetate shuttle. When levels of NADH are high, it will donate its electrons to oxaloacetate to form malate, which moves into mitochondria and reconverts to oxaloacetate and NADH in the matrix. This ensures low oxaloacetate in the cytosol and inhibition of gluconeogenesis.

Question 4

How does acetyl-CoA affect regulation of gluconeogenesis?

Answer 4

• Acetyl-CoA stimulates pyruvate carboxylase stimulating gluconeogenesis

Question 4

What pentose phosphate pathway deficiency(ies) are seen in humans? What is/are the clinical presentation(s)?

Answer 4

• G6PD deficiency: defect that lowers the activity of PPPathway and cells have lower NADPH levels. These levels impair glutathione reduction and deprive RBCs of antioxidant protection. Affected pts are very sensitive to hydrogen peroxide, which are produced in infections (by macrophages), by drugs (eg. Primaquine that is used to treat malaria and pneumocystis pneumonia) and during consumption of fava beans (known as favism) as these cause oxidatie stress and trigger hemolytic crisis and hemolytic anemia. Most prevalent in African and Mediterranean Americans. It is x-linked recessive.

Question 6

Which enzymes of gluconeogenesis are regulated? What factors inhibit and stimulate these enzymes? Which is the main regulated step? Which are reversible/irreversible?

Answer 6

• Pyruvate carboxylase: +: acetyl-CoA; -: insulin - PEP carboxykinase: +: glucagon via cAMP; -: insulin, AMP - Fructose-1,6-bisphosphatase: +: citrate; -: F26BP, AMP (main regulated step) - Glucose-6-phosphatase: +: glucagon; -: insulin

Question 7

What are the precursors for gluconeogenesis?

Answer 7

• Lactate, - Amino acids - Glycerol

Question 8

What is the purpose of the pentose phosphate pathway? What are the products from this pathway used for?

Answer 8

• Way for generating pentoses and NADPH - Pentoses are used for synthesis of: DNA, RNA, ATP, NADH, FADH, Coenzyme A - NADPH are used for synthesis of: FAs, cholesterol, NTs, nucleotides and reduction of oxidized glutathione and P450 monooxygenases

Question 9

Describe/draw the synthesis of glucose from non-CHO precursors (gluconeogenesis).

Answer 9

Question 10

Do muscles respond to glucagon?

Answer 10

• No

Question 10

What are the products of the non-oxidative phase of the pentose phosphate pathway? Why are these important?

Answer 10

• 2 F6P and 1 glyceraldehyde-3-phosphate - These are intermediates in the glycolytic / gluconeogenic pathways and can be synthesized from pentoses depending on the need of the cell.

Question 11

How is the pentose phosphate pathway regulated?

Answer 11

• Glucose-6-Phosphate Dehydrogenase is inhibited by NAPDH. When NADPH is high, G6P can be used elsewhere.

Question 12

What are the products of the oxidative phase of the pentose phosphate pathway?

Answer 12

• Ribose-5-phosphate, 2 NADPH and CO2

Question 13

Can glucose be synthesized from fat?

Answer 13

• No. Fatty acid chains do not contribute to glucose synthesis. Glycerol from triglycerides can, but chains cannot.

Question 14

Explain what reaction(s) the cell can perform when there is a need for NADPH and glucose. ATP is not required.

Answer 14

Question 15

Briefly explain the role of F26BP in gluconeogenesis and glycolysis.

Answer 15

• High F26BP concentrations: glycolysis is stimulated, gluconeogenesis is inhibited - Low F26BP concentrations: glycolysis is inhibited; gluconeogenesis is stimulated

Question 16

How does AMP affect regulation of gluconeogenesis?

Answer 16

• AMP stimulates PFK2, causing increased concentrations of F26BP, which causes inhibition of gluconeogenesis

Question 17

Explain what reaction(s) the cell can perform when there is a need for NADPH and ATP. Ribose is not required.

Answer 17

Question 18

Explain what reaction(s) the cell can perform when there is a need for both NADPH and ribose. ATP is not required.

Answer 18

Question 20

Why is gluconeogenesis important?

Answer 20

• To avoid a coma, blood glucose concentrations must be kept constant by coordinating glucose storage, synthesis and degradation. Hypoglycemia during periods of non-eating is not normal. We don’t have to always be eating.

Question 22

What is the energy difference between the theoretical reversal of glycolysis and gluconeogenesis? Why?

Answer 22

Theoretical reversal of glycolysis would cost 2 ATP, while gluconeogenesis costs 6 ATP. Difference = 4.
Contributing to this difference: Gluconeogenesis requires 4 enzymes not used in glycolysis. The pyruvate carboxylase enzyme uses one ATP – cycles twice = 2. The PEP carboxykinase enzymes uses one GTP – cycles twice = 2. 3PG to 1,3 BPG via phosphoglycerate kinase uses 1 ATP – cycles twice = 2. This accounts for 6 energy equivalents.
Note: F16Bisphosphatase and G6Phosphatase do not give your energy back when they dephosphorylate

Question 24

How do problems in gluconeogenesis cause fasting hypoglycemia and metabolic acidosis?

Answer 24

• Pyruvate can be oxidized to acetyl-CoA and moved into TCA or can be oxidized to lactate by lactate dehydrogenase. - When issues occur with gluconeogenic enzymes, much pyruvate can be oxidized to lactate as opposed to taking gluconeogenic pathways, leading to high levels of serum lactate and metabolic acidosis. - For example, when pyruvate carboxylase functions correctly, pyruvate is converted to oxaloacetate and is prevented from forming lactate and is now dedicate to moving through the gluconeogenesis pathway.

Question 25

What are the factors (or what molecules) regulate gluconeogenesis?

Answer 25

• insulin, glucagon, cAMP, F26BP, AMP, acetyl-CoA

Question 26

How does glucagon via cAMP affect regulation of gluconeogenesis?

Answer 26

• glucagon stimulates PEP carboxykinase via cAMP stimulating gluconeogenesis - glucagon stimulates glucose-6-phosphatase via cAMP stimulating gluconeogenesis

Question 28

How does insulin affect regulation of gluconeogenesis?

Answer 28

• insulin inhibits pyruvate carboxylase inhibiting gluconeogenesis - insulin inhibits PEP carboxykinase inhibiting gluconeogenesis - insulin inhibits glucose-6-phosphatase inhibiting gluconeogenesis

Question 29

Explain how F26BP affects gluconeogenesis. Include details about the enzymes involve, pathways activated and substrates affected.

Answer 29

• Take home message: high concentrations of F26BP inhibit gluconeogenesis, low concentrations of F26BP stimulate gluconeogenesis - F26BP is produced by enzyme PFK2 (produces F26BP from F6P) - Insulin stimulates PFK2 via cAMP leading to increased concentration of F26BP, which causes inhibition of gluconeogenesis via pyruvate carboxylase, PEP carboxykinase and glucose-6-phosphatase - Glucagon inhibits PFK2 via cAMP leading to decreased concentration of F26BP, which causes stimulation of gluconeogenesis via PEP carboxykinase and glucose-6-phosphatase

Question 30

Explain what reaction(s) the cell can perform when there is a need for ribose, but not for NADPH. ATP is not required.

Answer 30