2. By Reem Sallam, MD, MSc, PhD

3.  Phosphatidylinositol (PI)  Structure  Functions  Plospholipases:  Phospholipases A1, A2, C and D  Lysosomal phospholipase: Sphingomyelinase

4. A. Glycerophospholipids Glycerol-containing phospholipids e.g., Phosphatidylinositol B. Sphingo-phospholipids: Sphingosine-containing phospholipids e.g., Sphingolmyelin Background: Phospholipids

5. OH Diacylglycerol (DAG) - Inositol Phosphatidylinositol (PI) Saturated FA Unsaturated FA Parent Phospholipid Phosphatidylinositol (PI)

6. Phosphatidylinositol (PI): Functions 1.Protein anchoring to plasma membranes 2.Reservoir of arachidonic acid in membranes Arachidonic acid is the precursor of prostaglandins 3.Signal transmission across membranes via production of second messengers: Inositol 1,4,5-trisphosphate (IP 3 ) Diacylglycerol (DAG)

7. PI- Protein Anchoring Anchoring of proteins to membranes via Carbohydrate-Phosphatidylinositol Bridge Examples of anchored proteins: 1. Alkaline phosphatase (to the surface of small intestine) 2. Acetylcholine esterase (to postsynaptic membrane) These proteins can be cleaved from their attachment to the membranes by phospholipase C

8. Phosphatidylinositol 4,5 bisphosphate

9. Phospholipase C Diacylglycerol (DAG) Inositol 1,4,5-Trisphosphate (IP 3 )

10. GTP-Dependant Regulatory Proteins (G-Proteins) G q -Proteins: Trimeric membrane proteins ( αβγ) Binds to GTP/GDP Inactive form Trimeric –GDP bound ( αβγ/GDP) Active form α -GTP bound ( α/GTP) Forms of G-Proteins The α-subunit has intrinsic GTP-ase activity, resulting in: Hydrolysis of GTP into GDP  Inactivation of G-proteins

11. Phosphatidylinositol System What is the initial Signal ? Hormones or neurotransmitters e.g., Acetylcholine, antidiuretic hormone (V1-receptor) & catecholamines ( α 1 actions ) What is the Receptor ? G-protein coupled receptor What are the downstream Effects ? Activation of phospholipase C  Hydrolysis of phosphatidylinositol 4,5-bisphosphate  Production of : IP 3 (  release of Ca 2+ from intracellular stores) and DAG Activation of protein kinase C What is the Response ? Phosphorylation of cellular proteins  response to hormones

12. Intracellular Signaling by Inositol 1,4,5-trisphosphate (IP 3 ) Acetylcholine Antidiuretic hormone (ADH) Catecholamines * * with G q protein G q protein GqGq

13. Phospholipases (1) For glycerophospholipids: Phospholipases A1, A2, C and D Present in all tissues and pancreatic juice Present in snake venoms and bacterial toxins (2) For sphingophospholipids: Lysosomal phospholipase: Sphingomyelinase

14. Functions of Phospholipases (1) Degradation of phospholipids Production of second messengers Digestion of phospholipids by pancreatic juice Pathogenic bacteria degrade phospholipids of membranes  spread of infection (2) Remodeling of phospholipids: Specific phospholipase removes fatty acid from phospholipid Replacement of fatty acid by alternative fatty acid using fatty acyl CoA transferase e.g.: Binding of 2 palmitic acids in Dipalmitoylphosphatidylcholine (DPPC; lung surfactant) Binding of arachidonic to carbon # 2 of PI or PC

15. (1) Glycero-phospholipases

16. CH 3 (CH 2 ) 12 CH CH CH CH CH 2 O OH NH C O (CH 2 ) n CH 3 Long Chain Fatty acid Phosphorylcholine

17. CH 3 (CH 2 ) 12 CH CH CH CH CH 2 OH OH NH 2 Long chain, unsaturated amino alcohol

18. CH 3 (CH 2 ) 12 CH CH CH CH CH 2 OH OH NH C O (CH 2 ) n CH 3 Long Chain Fatty acid Ceramide = Sphingosine + Fatty acid

19. CH 3 (CH 2 ) 12 CH CH CH CH CH 2 O OH NH C O (CH 2 ) n CH 3 Long Chain Fatty acid Phosphorylcholine Sphingomyelin = Ceramide + phosphorylcholine

20. (2) Lysosomal Phospholipase: Sphingomyelinase Degradation of sphingomyelin by sphingomyelinase and ceramidase produce intracellular messengers: Ceramide and sphingosine

21. Take Home Message  Phosphatidylinositol (PI) is present in membranes  Functions of PI: Anchoring for proteins to membrane Production of second messengers Reservoir of arachidonic acid, a PG precursor  Phospholipases: Phospholipases A1, A2, C and D Lysosomal Phospholipase: Sphingomyelinase

22. Take Home Message Function of phospholipases: Degradation of phospholipids e.g., production of second messengers Remodeling of phospholipids e.g., production of DPPC (lung surfactant) CONT’D

23. THANK YOU