GLUCONEOGENESIS

1. GLUCONEOGENESIS

2. Gluconeogenesis • Continuous supply of Glucose: brain, RBC, kidney medulla, eye-lens/cornea, exercising muscle • Liver glycogen-10-18 hrs • Prolonged fast- glucose is formed from Gluconeogenesis • Primary precursors for gluconeogenesis are lactate, pyruvate, glycerol, TCA intermediates and amino acids (glucogenic) • we cannot make glucose from acetyl CoA • Gluconeogenesis site: 90%-Liver, 10%-Kidneys (minor role but in prolonged starvation-major)

3. Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis. Three Glycolysis reactions have such a large negative DG that they are essentially irreversible. • Hexokinase (or Glucokinase) • Phosphofructokinase • Pyruvate Kinase. These steps must be bypassed in Gluconeogenesis. 4 enzymes in Gluconeogenesis: • Pyruvate Carboxylase (Bypass of Pyruvate Kinase) • PEP Carboxykinase (Bypass of Pyruvate Kinase) • Fructose-1,6-bisphosphatase (Bypass of Phosphofruktokinase) • Glucose-6-Phosphatase (Bypass of Hexokinase)

7. Substrates for Gluconeogenesis: Glucogenic precursors-give rise to glucose synthesis: - all intermediates of Glycolysis & TCA • Glycerol, Lactate, α-keto acids [from deamination of glucogenic amino acids] 1. Glycerol [from lipolysis] ↓+ATP [Glycerokinase] α -Glycero-P ↓+NAD+[α-Glycero-P-dehydrogenase] Dihydoxy acetone-P [Isomerase]→Glycerladehyde-3-P 2. Lactate: released into blood by cells that lack mitochondria [RBC] and by excercising sk muscle → Cori’s cycle M Lactate → Blood → Liver [→Gluconeo →Glucose] →glucose into blood →M [→anaerobic glycolysis] →Lactate

9. Regulation • Glucagon:↑ Gluconeogenesis by 2 mechanisms- • Allosteric: Glucagon→↓F2,6-BP→ ↑ F1,6-BPase & ↓PFK • Covalent modification: Glucagon via cAMP and cAMP-depedant pr kinase →convert pyruvate kinase to phosphorylated [inactive] form →↓PEP to pyruvate [↓Glycolysis] and diverting PEP to ↑ Gluconeogensis Summary of effects of glucagon-cAMP cascade in liver: • Gluconeogenesis is stimulated. • Glycolysis is inhibited. • Glycogen breakdown is stimulated. • Glycogen synthesis is inhibited. • Free glucose is formed for release to the blood.

10. B. Substrate availability: of glucogenic precusors: ↓Insulin →mobilization of aa from muscle protein→ provide C skeleton for gluconeogenesis • Allosteric activation of Pyruvate carboxylase by Acetyl coA: Starvation→↑Lipolysis in adipose tissue →↑FA in liver →↑β-oxidation →↑acetyl coA accumulation →↑ Allosteric activation of Pyruvate carboxylase

11. Glycolysis & Gluconeogenesis: If both pathways were simultaneously active in a cell, it would constitute a "futile cycle" that would waste energy. Glycolysis: glucose + 2 NAD+ + 2 ADP + 2 Pi 2 pyruvate + 2 NADH + 2 ATP Gluconeogenesis: 2 pyruvate + 2 NADH + 4 ATP + 2 GTP glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi To prevent the waste of a futile cycle, Glycolysis & Gluconeogenesis are reciprocally regulated. FBP-1/PFK-1 and FBP-2/PFK-2: In both enzyme complexes- kinase & phosphatase activities are- different domains of one bifunctional polypeptide molecule. Carbohydrate rich meal→↑Insulin /↓Glucagon →↑F2,6 BP →↑Glycolysis

12. Allosteric:Glucagon→ • ↓F2,6-BP→ ↑ F1,6-BPase [FBP-1] & ↓PFK • favors ↑Gluconeogenesis and ↓ Glycolysis • Via cAMP cascade:Glucagon→ • Phosphorylation of FBP-2/PFK-2 complex • [→FBP-2 becomes active and PFK-2 becomes inactive] →↓F2,6-BP→↑Gluconeogenesis & ↓Glycolysis • 2. Phsophorylation of Pyruvate kinase →↓ Glycolysis