Lecture 22: Gluconeogenesis

Question 1

Anaerobic Metabolism

Answer 1

• During limited oxygen, ETC is inhbited and anaerobic metabolism occurs
• Energy production in oxidative phosphorylation is inhibited and PDC and CAC slows down(NADH rise, inhibit PDC and CDC)
• Glycolysis only way to generate ATP
• Lactate dehydrogenase converts NADH to NAD+ so glycolysis can continue
• Lactate a biochemical marker of anaerobic metabolism

Question 2

Cori Cycle

Answer 2

• RBC or muscle cell(low O2): Glucose -> Pyruvate -> Lactate
• Allow for lactate to be used in gluconeogenesis

Question 3

Gluconeogenesis

Answer 3

• Liver and kidney can make glucose via gluconeogenesis
• Brain cannot use fats for energy, need glucose to generate ATP
• Key substrates include lactate, glycerol, and certain amino acids that can be turned into glucose
• Carbon containing molecule readily available in large quantities from the bloodstream

Question 4

GNG is not reverse of glycolysis

Answer 4

• Glycolysis is exergonic and require a lot of energy to go in reverse to make glucose(+84 kJ/mol)
• Gluconeogenesis is an anabolic pathway that uses high energy intermediates and 4 different enzymes to bypass 3 steps in glycolysis that have negative G and are essentially irreversible

Question 5

1- Use of pyruvate for GNG

Answer 5

• Step 1: Addition of CO2 from bicarbonate onto pyruvate to form oxaloacetate
• Step 2: Decarboxylation to help add a phosphate onto oxaloacetate using PEP carboxykinase to form PEP

Question 6

Fructose 6 Phosphate Formation

Answer 6

• 2 PEP goes through reverse sequence of enzymes in glycolysis to form DHAP and GAP, which join to form Fructose 1,6 Bisphosphate
• A phosphatase reverses step 3 of glycolysis(PFK-1)
• Fructose 6P isomerized to Glucose 6P

Question 7

Glucose Formation

Answer 7

• Glucose 6 phosphatase hydrolyzes off phosphate to form free glucose
• Same enzyme acts on Glc 6P made from glycogen breakdown(glycogen phosphorylase + phosphoglucomutase)

Question 8

Glucose Production from Pyruvate

Answer 8

• 2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 6 H2O + 2 H -> Glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 P
• Gluconeogenesis uses 11 enzymes to make single glucose, but is overall favorable when energy available

Question 9

Glycerol to Glucose Production

Answer 9

• Fatty acids cannot be used to generate glucose because production of pyruvate from acetyl-coA is unfavorable
• Glycerol released during fat mobilization in fat cells can be used and released in larhe amounts into the blood
• Glycerol has 3 carbons: 2 glycerol molecules can be used to make 6 carbon glucose entering the pathway as dihydroxyacetone phosphate(DHAP)
• 2 ATPs needed to drive the anabolic pathway, but 2 NADH are also produced. 2 H2O help remove phosphatases to make 1 glucose molecule

Question 10

Glycolysis vs. GNG

Answer 10

• Glycolysis: Makes ATP/NADH, makes pyruvate/lactate, in all cells, important for generating energy, occurs regularly and when increase glucose
• GNG: Requires ATP, GTP, NADH, makes glucose, in liver, important for supplying blood glucose, occurs in exercise or fasting state

Question 11

Metabolic integration 2

Answer 11

• Alanine, Aspartate, Glutamate used to make glucose
• Alanine used to make pyruvate -> GNG
• Asp make oxaloacetate
• Glu make alpha-ketoglutarate -> used in CAC to make OAA

Question 12

Allosteric Regulation at PFK-1 and FBP-1

Answer 12

• F 2,6 BP and AMP activate glycolysis(Fru 6P -> Fru 1,6 BP) and ATP inhibit glycolysis
• F 2,6 BP and AMP inhibit gluconeogenesis

Question 13

Fructose 2,6-bisphosphate: Allosteric regulator

Answer 13

• F 2,6 BP activates PFK-1 -> Glycolysis
• F 2,6 BP inhibit FBP-1 -> Gluconeogenesis
• Made by PFK-2/FBPase-2 : Controls formation and hydrolysis of F 2,6 BP. Single polypeptide with two active sites that can be regulated via phosphorylation

Question 14

Glycolysis vs. GNG

Answer 14

• Unphosphorylated: PFK-2 activated/FBPase-2 inactivated -> glycolysis
• Phosphorylated: PFK-2 inactivated/FBPase-2 activated -> Gluconeogenesis

Question 15

Reciprocal Regulation

Answer 15

• Goal of reciprocal control in liver is to allow either glycolysis or gluconeogenesis to run
• Regulation achieved by hormone signaling through enzyme dephosphorylation and availability of energy
• Tandem PFK-2/FBPase-2 enzyme controls production and breakdown of Fructose 2,6 BP for allosteric regulation
• Low energy(AMP/ADP) stimulates glycolysis(PFK-1, pyruvate kinase) and slows down GNG(Pyruvate carboxylase, PEP, carboxykinase, F 1,6 BP)
• High energy(ATP) slows down glycolysis(PFK-1, Pyruvate Kinase) and stimulates GNG(pyruvate carboxylase, F 1,6 BP)

Question 16

Hyperglycemia and Glycation

Answer 16

• Glucose highly reactive and can react with N-terminal valine residues of beta chains in hemoglobin
• Forms hemoglobin A1c as marker for high blood glucose
• Other molecules in body can be glycated, resulting in modification which can affect their function

Question 17

Synthetic Insulin and Glucose Homeostasis

Answer 17

• Blood glucose high -> secrete insulin(inhibit alpha cells) -> glucose entry, glycolysis, fat, protein, glycogen synthesis
• Blood glucose low -> synthetic insulin inhibit alpha cell -> increase glucose entry and glycolysis, decrease glycogenolysis and gluconeogenesis -> hypoglycemia

Question 18

Hormonal Regulation and Diabetes

Answer 18

• People with type 1 diabetes need to balance insulin injections with carbohydrate intake and exercise
• Insulin injections necessary for glucose uptake and can inhibit glucagon secretion
• Insulin insensitivity(Type 2) leads to poor glucose uptake and hyperglycemia. Glucagon signaling also turns on GNG and can worsen hyperglycemia
• When blood glucose drops in presence of insulin, glycogenolysis and gluconeogenesis cannot maintain blood glucose levels
• Low blood glucose = hypoglycemia
• Fat breakdown and ketogenesis assist with ATP production