Dr/ Ragaa Salama1 insulin Objectives Describe the chemical structure of insulin and the steps of its synthesis. Describe the mechanisms of storage, secretion, transport, degradation and excretion of insulin. Explain the mechanism of action of insulin. List the effects of insulin on metabolism and growth..

Dr/ Ragaa Salama2 insulin Objectives Describe the chemical structure of insulin and the steps of its synthesis. Describe the mechanisms of storage, secretion, transport, degradation and excretion of insulin. Explain the mechanism of action of insulin. List the effects of insulin on metabolism and growth..

Dr/ Ragaa Salama3 Chemical structure of insulin -Insulin is a peptide hormone.peptide hormone -composed of 51 amino acid.amino acid -made up of 2 chains of aa, one is 21 aa & one with 30 aa. -3 disulfide bonds. -Zinc forms complex with insulin and proinsulin -It is produced in the islets of Langerhans in the pancreasislets of Langerhanspancreas -effects on both metabolism and several other body systems

Dr/ Ragaa Salama4 Synthesis of Insulin -An RNA transcript is translated → preproinsulin protein, → passes through ER → removal of the signal sequence → formation of 3 disulfide linkages → proinsulin. -it is activated by cleavage and release of the C-peptide, leaving two disulfide linked chains (30 and 21 aa’s) as the mature insulin molecule by specific peptidases and carboxpepidase. -It is stored in granules in the cytoplasm.

Dr/ Ragaa Salama5 Storage as granules in the cytoplasm or secretory vesicle post translation modification

Dr/ Ragaa Salama6 Insulin Secretion The human pancrease secrets units of insulin daily Factors that ↑ insulin secretion 1- glucose : Fasting blood glucose level (60-100mg/dl), the max response at mg/dl. 2- Hormonal factors: GH, cortisol, placental lactogen, estrogen, progesterone ↑ insulin secretion. -Epinephrine → fasting, stress, trauma, and vigorous exercise → release signals energy utilization → ↓ insulin secretion as insulin stimulates energy storage. 3-Drugs: sulfonylurea, tolbutamide. 4-Certain amino acids → stimulate insulin secretion. 5-Gastric inhibitory polypeptide (GIP), a gut hormone released after the ingestion of food → onset of insulin release

Dr/ Ragaa Salama7 Blood Glucose and Insulin Release In  -cells, glucose at elevated levels → Glu- 6-P by glucokinase → glycolysis Increased glycolysis causes calcium entry → release of insulin → reduces blood sugar → slowing insulin release →A perfect feedback system,

Dr/ Ragaa Salama8 Release Insulin is released as blood glucose concentrations rise. Glucose enters beta cells and is metabolized to form ATP. Increased ATP levels → K+ channels to close, → K+ ions build up and depolarize the cell. → activates Ca+2 channels, and Ca+2 ions flow in → triggering the granules to release insulin by exocytosis.

Dr/ Ragaa Salama9 Transport When released, insulin molecules attach to transmembrane receptors outside muscle and fat cells → activate IRS-1 (IRS-1-2,3,4 )proteins inside the cells. IRS-1 proteins stimulates glucose transporter molecules, GLUT4, outside the cells → taking in glucose.

Dr/ Ragaa Salama10 Insulin receptor (tetrameric complex) Insulin Cell membrane Insulin binding site Kinase active site OH HO OP Phosphorylation ATP ADP OP PO Kinase active site opened by induced fit -2 types of subunits, 2α, 2β subunits, - α -subunit insulin binds, -β -subunit, spans the membrane and protrudes into the cytosol -Insulin receptor is tyrosine kinase. -The kinase activity of insulin receptor is essential to its biological activity - Specific amino acids are found to be binding sites. Once insulin binds to the receptor, 2 β-subunits linked by disulfide bonds are sent through the pore to activate other processes.

Dr/ Ragaa Salama11 Effect of insulin on glucose uptake and metabolism. Insulin binds to its receptor (1) which in turn starts many protein activation cascades (2). These include: translocation of Glut-4 transporter to the plasma membrane and influx of glucose (3), glycogen synthesis (4), glycolysis (5) and fatty acid synthesis (6).plasma membraneglycogenglycolysisfatty acid

Dr/ Ragaa Salama12 Insulin and Glucagon are the main players to maintain adequate & normal blood glucose level Insulin ≠ anti-insulin

Dr/ Ragaa Salama13 Degradation Insulin half life is 3-5 min In liver, kidney, placenta By 2 enzymes :- protease (insulinase) enzyme specific for insulin degradion Glutathione-insulin transhydrogenase → ↓ disulfide bond Degradation normally involves endocytosis of the insulin-receptor complex followed by the action of insulin degrading enzyme.endocytosis insulin degrading enzyme Most insulin molecules are degraded by liver cells 50%.liver

Effect of insulin On membrane transport On glucose utilization On glucose production On glucose metabolism On lipid metabolism On protein metabolism On cell replication Dr/ Ragaa Salama14

Dr/ Ragaa Salama15 Effects Control of cellular intake of glucose in muscle and adipose tissue (about ⅔ of body cells).glucoseadipose Increase of DNA replication and Modification of the activity of numerous enzymes.DNA replicationenzymes The actions of insulin on cells include: ↑ glycogen synthesis.glycogen ↑ fatty acid synthesis ↑ lipogenesis in adipose tissue from triglyceridesfatty acidadipose tissue ↑ protein synthesis via control of amino acid uptake.protein synthesis ↓ proteolysis – decreasing the breakdown of protein.proteolysis ↓ lipolysis – forces reduction in conversion of fat cell lipid stores into blood fatty acids; lack of insulin causes the reverse.lipolysis ↓ gluconeogenesis.gluconeogenesis ↑ potassium uptake by absorbing serum potassium relax of arterial muscle wall → increasing blood flow, especially in micro arteries

On membrane transport - Insulin enter the cells by carrier mediated facilitated diffusion. by translocating the glucose transporter from the cytosol (where it resides in the absence of insulin) to the cell surface, in response to insulin. -Glucose transporter (a membrane protein that carries out glucose) 1-4 (GLUT1-4). GLUT1 → brain GLUT2 → liver (gluokinase) GLUT4 → adipose tissue, cardiac ms, skeletal ms Insulin stimulates glucose uptake into muscle and adipose cells,glucose Dr/ Ragaa Salama16