What are the key characteristics of lipids?
They are a heterogeneous group of structures.
They are hydrophobic.
Their main functions are to make membranes and store energy.
Types of lipids
Fatty acids
Triacylglycerol
Phospholipids
Glycolipids
Steroids
Terpenes
Steroids don’t have what
fatty acids
Fatty acids vary in what?
Length
What is a key property of phospholipids?
They are amphipathic, meaning they have both hydrophilic (water-loving) and hydrophobic (water-fearing) parts. This allows them to form bilayers and make up cellular membranes. They also have two fatty acids (one of them is unsaturated).
What are some examples of cells that specialize in storing lipids?
Adipocytes in mammals: The cytoplasm is occupied by a very large lipid droplet.
Fat body cells in insects: Also store large amounts of lipids.
What is a Triacylglycerol (TAG)?
It is a lipid molecule composed of a glycerol backbone with three fatty acids esterified (attached) to it. It is a primary form of energy storage.
Glycerol makes an ester bond with three fatty acids, and loses three waters to make TAG.
What is the first pathway for making TAG?
Step 1: MGAT (monoacylglycerol acyltransferase) adds a fatty acid to a monoacylglycerol to form a diacylglycerol.
Step 2: DGAT (diacylglycerol acyltransferase) adds another fatty acid to the diacylglycerol to form a TAG.
Key Point: Acyltransferases are the enzymes that move fatty acids from a fatty acid-CoA donor.
What is the second, major pathway for making TAG?
The Glycerol-3-Phosphate (Kennedy) Pathway.
Step 1: GPAT adds a fatty acid to glycerol-3-phosphate to form lysophosphatidic acid (LPA).
Step 2: AGPAT adds a second fatty acid to form phosphatidic acid (PA).
Step 3: PAP removes the phosphate group from PA to form diacylglycerol (DAG).
Step 4: DGAT adds a third fatty acid to DAG to form TAG.
Key Point: This pathway occurs in membranes and uses acyltransferases and fatty acid-CoA.
Cells can make tag from what?
monoacylglycerols and also glycerol-3-phosphate
Where in the cell is TAG made?
TAG is synthesized in the Smooth Endoplasmic Reticulum (SER), which is the part of the ER without associated ribosomes.
Whats the smooth er made of?
Flatten sacs.
Where does TAG accumulate?
TAG does not accumulate in the ER membrane. Instead, it is stored in organelles called Lipid Droplets (LDs), which originate by budding out from the ER.
What is the critical structural feature of a Lipid Droplet?
Lipid Droplets are unique organelles with a single-layer (monolayer) membrane of phospholipids, unlike most organelles which have a double-layer (bilayer) membrane.
What is inside a Lipid Droplet?
Lipid Droplets have a hydrophobic core of neutral lipids, primarily TAGs and sterol esters.
What assists lipid droplets with budding from the ER
Seipin
Does the cytoplasm have lipids?
No
Class 1 protein on lipid droplet
It moves the lipid droplet around. Has hydrophobic (inside membrane) and hydrophilic parts (outside membrane) .
Class ll protein on lipid droplet
Fully ouside because its hydrophilic.
What proteins coat Lipid Droplets?
Perilipins are a family of proteins that associate with the cytoplasmic side of the Lipid Droplet’s monolayer membrane, helping to regulate lipid storage and breakdown.
What is the evidence that TAG synthesis is required for Lipid Droplets?
Yeast cells that are genetically engineered to lack TAG synthesis enzymes (so no TAG synthesis) do not form Lipid Droplets at all.
Saccharomyces
eukaryotic, single-celled microorganism. Very powerful genetic tools.
Three steps of lipid droplet synthesis
TAG synthesis and lens formation.
Emergence and nascent LD formation.
Budding and growth
What is the first step in making a Lipid Droplet?
triacylglycerol (TAG) synthesis and cholesterol ester synthesis enzymes deposit neutral lipids between the two leaflets of the ER bilayer, forming an oil “lens.”
