Lecture 4 - Making Lipids PDF

Making lipids Lecture 4 BIOL2020 Prof. Nicanor González Elizabeth Rideout Announcements Remembering lipids - Heterogeneous structure group Hydrophobic Make membranes and store energy Remember: Phospholipids make membranes • Amphipathic = having both hydrophilic and hydrophobic parts. All...

Making lipids Lecture 4 BIOL2020 Prof. Nicanor González Elizabeth Rideout Announcements Remembering lipids - Heterogeneous structure group Hydrophobic Make membranes and store energy Remember: Phospholipids make membranes • Amphipathic = having both hydrophilic and hydrophobic parts. All cells make lipids, but some make more than others Fat body cells - insects Adipocytes - mammals lipids nuclei lipids the cell cytoplasm is occupied by a very large lipid droplet. Elizabeth Rideout UBC study sex differences in fat metabolism in Drosophila Tag is triacylglycerol Cells make TAG from monoacylglycerols using Acyltransferases MGAT=monoacylglycerol acyltransferase DGAT=diacylglycerol acyltransferase • • Acyltransferases are enzymes that move fatty acids Acyl-CoA is the fatty acid donor Cells also make TAG from glycerol-3-phosphate • • Acyltransferases are enzymes that move fatty acids Acyl-CoA is the fatty acid donor TAG is made in the smooth ER • The smooth ER is the ER without associated ribosomes TAG does not accumulate in the smooth ER but in organelles called lipid droplets Lipid Droplets originate at the ER and are storage organelles ER Cytoplasm • LD have a single layer membrane seipin assists budding from ER lipids This is critical! Lipid Droplets have a single layer membrane ER Cytoplasm seipin assists budding from ER lipids This is critical! a hydrophobic core of neutral lipids a phospholipid monolayer https://www.nature.com/articles/s41580-018-0085-z Perilipins are a family of proteins that coat lipid droplets • Perilipin Perilipin Perilipin https://www.nature.com/articles/s41580-018-0085-z Proteins associate with the LD membrane at the cytoplasmic side Yeast cells lacking the TAG synthesis enzymes donot have lipid droplets • Normal • no TAG synthesis Meet Saccharomyces: eukaryotic, single-celled microorganism Very powerful genetic tools Making a lipid droplet requires 3 steps triacylglycerol synthesis occurs at the smooth ER bilayer triacylglycerol synthesis enzymes deposit neutral lipids in between the leaflets of the ER bilayer Step 1: triacylglycerol (TAG) synthesis (see inset) and cholesterol ester synthesis enzymes deposit neutral lipids in between the leaflets of the ER bilayer AGPAT, acylglycerolphosphate acyltransferase; DAG, diacylglycerol; DGAT, acyl-CoA:diacylglycerol acyltransferase; G3P, glycerol-3-phosphate; GPAT, glycerol-3 phosphate acyltransferase; LPA, lysophosphatidic acid; PA, phosphatidic acid; PAP, phosphatidic acid phosphatase; PLIN, perilipin. A protein complex forms in the ER side to push the LD out Step 2: seipin and other lipid droplet biogenesis factors are recruited to the lens structure and facilitate the growth of the nascent lipid droplet. Budding always happen towards the cytoplasm side • • • Asymmetry in phospholipid composition Seipin push from the ER side Asymmetry in protein recruitment from the cytosol side Grow LD by making more TAG or by fusion Step 3: lipid droplets bud from the ER and grow through fusion or local lipid synthesis. https://commons.wikimedia.org/wiki/File:Quantitative-Analysis-of-Lipid-Droplet-Fus ion-Inefficient-Steady-State-Fusion-but-Rapid-Stimulation-pone.0015030.s010.ogv LD fuse and interact with other organelles via membrane proteins • Proteins are always facing the cytoplasmic side What about phospholipids? • Amphipathic = having both hydrophilic and hydrophobic parts. Phosphatidic acid is the branchpoint between store TAG make membrane lipids All glycerophospholipids (phosphatidylinositol) • • • Remove phosphate group = make TAG Keep phosphate = phospholipids Phosphatidic acid is the smallest phospholipid phosphatidylinositol Enzymes metabolize phospholipids are in ALL membranes but never in the cytosol Additional info: CDS and phosphatidylinositol synthase are in the ER phosphatidylinositol Think of all the things required for Phosphatidylinositol 4,5-bisphosphate (PIP2) to be at the membrane PIP2 is at the cell membrane. two fatty acids glycerol PIP2 the phosphate group (from being a phospholipid) inositol molecule two extra phosphates (specific to PIP2) Summary Lipid Droplets • Bud out of the smooth ER • Have a single-layered membrane • Transport and store TAG and sterols • Seipins and perilipins Phospholipids • Made in membranes • Are the membranes • Phosphatidic acid is the smallest and the starting point