Lecture 18

Glycogenesis

Glycogenesis

• pathway for synthesis of glycogen

• occurs when there is excess g-6-Phosphate

• short term energy storage

Glycogenolysis

• pathways for breakdown of glycogen to free glucose

• occurs in muscle cells when there is an immediate need for energy

• occurs in the liver when the blood glucose levels are low

Glucagon

a peptide hormone acting on liver cells

Epinephrine

an amino acid derivative acting on muscle cells

Insulin

a protein hormone acting on most cells, except eyes, nerves, heart, blood vessels, and kidneys.

Which hormones regulate Blood Glucose levels?

Insulin and glucagon

Signaling Process

• when a messenger (hormone) connects the receptor embedded in the cell surface it will create a biological response (cascading effect)

What are the characteristics of actions of insulin?

• replenishes energy stores

• released by pancreatic B cells

• primarily anabolic

  1. Glucose enters the cell

  2. Breakdown of glucose by glycolysis speeds up

  3. Glycogen synthesis increases in the liver

  4. Synthesis of lipids and proteins increase

What are the characteristics of actions of glucagon?

• breakdown energy stores to make new glucose

• released by pancreatic A cells

• primarily catabolic

  1. Glucose entry to the cells slows down

  2. glycogen breakdown in liver speeds up

  3. breakdown of lipids and proteins for glucose synthesis by gluconeogenesis increases

  4. gluconeogenesis accelerates

What does the liver do?

releases or extracts glucose from the blood

What does the pancreas do?

senses blood glucose levels

When Glucagon/Epinephrine hormones that bound to receptors on the surface of the cell what does it activate with the cell?

Adenylate Cyclase

What does epinephrine and glucagon signaling do?

• It causes glycogen to breakdown

• glucose to be synthesized

• blood glucose levels to increase

How is glucagon/epinephrine activated?

by phosphorylating glycogen phsophorylase

How is glucagon/epinephrine deactivated?

by phosphorylating glycogen synthase

glucagon signals the release of glucose from glycogen through _____ of both glycogen phosphorylase and glycogen synthase

covalent modification

What does Insulin signaling do?

• It causes glycogen to be synthesized

• blood glucose levels to decrease

How is insulin activated?

by dephosphorylating glycogen synthase

How is insulin decativated

by dephosphorylating glycogen phosphorylase

What is the non-hormonal control of the regulation of Glycogen Synthase?

glucose-6-P is a positive allosteric regulator

What is the hormonal control of the regulation of Glycogen Synthase?

• when epi/glucagon decrease, glycogen synthase activity inhibits glycogen synthesis

• when insulin increases, glycogen synthase activity stimulates glycogen synthesis.

What is the non-hormonal control of the regulation of Glycogen Phosphorylase?

• (+) 5’ - AMP, increases glycogen phosphorylase

• (-) AT, decreases glycogen phosphorylase

• (-) G-6-P, decreases glycogen phosphorylase

• (-) Glucose, decreases glycogen phosphorylase

What is the hormonal control of the regulation of Glycogen Phosphorylase?

• glucagon and epinephrine increase glycogen phosphorylase

• insulin decreases glycogen phosphorylase

What are the symptoms of Von Gierke’s disease?

• glucose-6-P deficiency

• enlarged liver

• low blood sugar after a few hours

• 1:200,000 autosomal recessive

What are the symptoms of McArdle’s disease?

• muscle glycogen phosphorylase deficiency

• the normal increase in plasma lactate following exercise is absent

• muscles are are damaged because of inadequate energy supplies and glycogen accumulation

• blood analysis shows increased levels of creatine phosphokinase, aldolase, myoglobin

What are the functions of the Pentose Phosphate Pathway?

• provide NADPH for biosynthetic pathways

• provide ribose-5-phosphate for nucleic acid biosynthesis

• occurs when cell is dividing

• metabolism of dietary pentose sugars

Cori Cycle (anaerobic respiration)

glucose is metabolized to pyruvate and then to lactate in muscle, the lactate is released into the blood and carried to the liver, where it is reconverted to pyruvate and used for gluconeogenesis, and the resulting glucose is released and travels back to muscle

Difference between glycolysis vs. Gluconeogenesis?

Glycolysis is activated by low energy molecules while gluconeogenesis is activated by high energy molecule