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Coordinated regulation of glycolysis/gluconeo genesis

Coordinated regulation of glycolysis/gluconeogenesis

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Page 1: Coordinated regulation of glycolysis/gluconeogenesis

Coordinated regulation of glycolysis/gluconeogenesis

Page 2: Coordinated regulation of glycolysis/gluconeogenesis

Regulation of glycolysis

Hexokinase

Phosphofructokinase-1

Pyruvate kinase

Page 3: Coordinated regulation of glycolysis/gluconeogenesis

Hexokinase

• There are four isozymes (I, II, III and IV) of hexokinase encoded by four different genes.

• Hexokinase I and II are allosterically inhibited by their product, glucose 6-phosphate. Hexokinase IV is not inhibited by G-6-P.

Page 4: Coordinated regulation of glycolysis/gluconeogenesis

Hexokinase

• Hexokinase I and II are the predominant forms existing in muscle. Hexokinase IV is the predominant form in liver.

• Hexokinase I and II will be half-saturated at about 0.1mM, but hexokinase IV will not be half-saturated until 10mM.

Page 5: Coordinated regulation of glycolysis/gluconeogenesis

Hexokinase

• Hexokinase has different functions in liver and muscle.

• Muscle consumes glucose, using it for energy production.

• Liver maintains blood glucose homeostasis by removing or producing glucose.

Page 6: Coordinated regulation of glycolysis/gluconeogenesis

Muscle hexokinase

• Because blood glucose concentration is about 4 to 5 mM, hexokinase in the muscle (which will be half saturated at 0.1mM) is always working at or near its maximal rate.

Page 7: Coordinated regulation of glycolysis/gluconeogenesis

Liver hexokinase

• However, liver hexokinase (half-saturated at 10mM) will not ever reach its maximal rate even after meal.

Page 8: Coordinated regulation of glycolysis/gluconeogenesis

Liver hexokinase

• Liver hexokinase (glucokinase) is also regulated by a regulatory protein. When glucokinase is bound by it (which is enhanced by F-6-P), it become nuclear-localized and inactive.

Page 9: Coordinated regulation of glycolysis/gluconeogenesis

Liver hexokinase

• After meal, glucose enter hepatocytes by GLUT2 transporter and is converted to G-6-P. G-6-P competes with F-6-P for glucokinase, which relieves its inhibition by regulatory protein.

Page 10: Coordinated regulation of glycolysis/gluconeogenesis

Phosphofructokinase-1

• PFK-1 catalyze the committing step of glycolysis.

• This enzyme is regulated by ATP, AMP, ADP, citrate and fructose 2,6-bisphosphate.

Page 11: Coordinated regulation of glycolysis/gluconeogenesis

ATP regulate the affinity of PFK-1 towards its substrate F-6-P

• Not only as a substrate, ATP is also one of the end product of the glycolytic pathway.

• ATP inhibit PFK-1 by binding to an allosteric site and lowering the affinity of the enzyme for F-6-P.

Page 12: Coordinated regulation of glycolysis/gluconeogenesis

Other molecules regulate PFK-1

• ADP and AMP relieve the inhibition by ATP.• Citrate increases the inhibitory effect of ATP.• F-2,6-BP is the strongest activator of PFK-1.

Page 13: Coordinated regulation of glycolysis/gluconeogenesis

Pyruvate kinase

• Pyruvate kinase has at least three isozymes and one of them is liver-specific.

• The liver pyruvate kinase is being regulated differently than other tissue type.

Page 14: Coordinated regulation of glycolysis/gluconeogenesis

Regulation of pyruvate kinase

cAMP dependent

Page 15: Coordinated regulation of glycolysis/gluconeogenesis

Regulation of gluconeogenesis

Pyruvate carboxylase

FBPase-1

Page 16: Coordinated regulation of glycolysis/gluconeogenesis

Pyruvate carboxylase

• Pyruvate carboxylase is being positively regulated by acetyl-CoA.

• The accumulation of acetyl-CoA signals that cell’s energy demands are met.

• Acetyl-CoA also indirectly inhibit pyruvate dehydrogenase complex.

Page 17: Coordinated regulation of glycolysis/gluconeogenesis

E2

How acetyl-CoA regulate PDC

• Acetyl-CoA indirectly inhibit PDC by stimulating a protein kinase that inactivates the dehydrogenase.

Page 18: Coordinated regulation of glycolysis/gluconeogenesis

FBPase-1 is inhibited by AMP

Page 19: Coordinated regulation of glycolysis/gluconeogenesis

F-2,6-BP: a potent regulator

Page 20: Coordinated regulation of glycolysis/gluconeogenesis

F-2,6-BP reciprocally regulate PFK-1 and FBPase-1

Page 21: Coordinated regulation of glycolysis/gluconeogenesis

F-2,6-BP activates PFK-1

Page 22: Coordinated regulation of glycolysis/gluconeogenesis

F-2,6-BP inhibit FBPase-1

Page 23: Coordinated regulation of glycolysis/gluconeogenesis

The synthesis and breakdown of F-2,6-BP

• F-2,6-BP is synthesized by PFK-2 and brokedown by FBPase-2, which is a single, bifunctional protein. When it is phosphorylated, it is FBPase-2.

Page 24: Coordinated regulation of glycolysis/gluconeogenesis

The bifunctional protein PFK-2/FBPase-2

Page 25: Coordinated regulation of glycolysis/gluconeogenesis

Insulin and glucagon levels affect the balance between PFK-2/FBPase-2

Page 26: Coordinated regulation of glycolysis/gluconeogenesis

A PP2A activated by xylulose 5-phosphate also activate FBPase-2

PP2A

Page 27: Coordinated regulation of glycolysis/gluconeogenesis

Regulation of glycogen metabolism

Glycogen phosphorylase

Glycogen synthase

Page 28: Coordinated regulation of glycolysis/gluconeogenesis

Muscle glycogen phosphorylase

• Muscle glycogen phosphorylase has two forms: the active a form and the less active b form. The active form is phosphorylated.

Page 29: Coordinated regulation of glycolysis/gluconeogenesis

Muscle glycogen phosphorylase

• Glucagon and epinephrine stimulate the kinase that phosphorylate phosphorylase b, therefore active the whole glycogen breakdown process.

Page 30: Coordinated regulation of glycolysis/gluconeogenesis

How glucagon/epinephrine activate phosphorylase b kina

se

• When epinephrine/glucagon is secreted, it started the whole enzyme cascade by activate a GTP-binding protein.

• Enzyme cascade allows for large amplification of the initial signal.

Page 31: Coordinated regulation of glycolysis/gluconeogenesis

Muscle glycogen phosphorylase

• At resting stage, PP1 (phosphorylase a phosphatase) will dephosphorylate phosphorylase a, which will make it returning to the less active form (phosphorylase b).

Page 32: Coordinated regulation of glycolysis/gluconeogenesis

Liver glycogen phosphorylase

• The dephosphorylated form (b) of liver glycogen phosphorylase is essentially inactive. Phosphorylation activates it, but when blood glucose is high, glucose will bind to the inhibitory allosteric site, induces a conformational change that will expose its phosphorylated Ser for PP1 to dephosphorylate (inactivate) this enzyme.

Page 33: Coordinated regulation of glycolysis/gluconeogenesis

Glycogen synthase• The activate form of gl

ycogen synthase is not phosphorylated.

• To inactivate glycogen synthase, it must be phosphorylated by casein kinase II (CKII) first, then glycogen synthase kinase 3 (GSK3) will add phosphoryl groups to three Ser residues near the carboxyl terminus of this protein.

Page 34: Coordinated regulation of glycolysis/gluconeogenesis

GSK3 inactivate glycogen synthase by phosphorylation

Page 35: Coordinated regulation of glycolysis/gluconeogenesis

Glycogen synthase• The activation of gly

cogen synthase requires PP1.

• Glucose 6-phosphate will bind to the allosteric site of glycogen synthase b, making the enzyme a better substrate for PP1.

Page 36: Coordinated regulation of glycolysis/gluconeogenesis

GSK3 can be inactivated by phosphorylation

• Insulin triggers activation of a protein kinase B to phosphorylate GSK3 at a Ser residue near the amino terminus, converting that region of the protein to a pseudosubstrate, preventing GSK3 from binding the real substrate (glycogen synthase).

Page 37: Coordinated regulation of glycolysis/gluconeogenesis

Insulin enhance glycogen synthesis

by inhibiting the kinase that inactivate glycogen synth

ase

Page 38: Coordinated regulation of glycolysis/gluconeogenesis

Phosphoprotein phosphatase 1 (PP1)

• PP1 can remove phosphoryl group from phosphorylase kinase, glycogen phosphorylase (inactivation), and glycogen synthase (activation) in response to glucagon/epinephrine.

• By activating PP1 and inactivating GSK3, insulin stimulates glycogen synthesis.

Page 39: Coordinated regulation of glycolysis/gluconeogenesis

PP1 binds to glycogen-targeting protein (GM) and also other proteins

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Muscle has a different glucose transporter

• GLUT2 is not present in myocyte. Instead, GLUT4 is present in myocyte and its expression is regulated by insulin.

Page 45: Coordinated regulation of glycolysis/gluconeogenesis

Insulin regulated the externalization/internalization of GLUT4