Lecture 4 - Making Lipids Flashcards
What is TAG?
a triacylglycerol
What is TAG made from, and where?
- made from monoacylglycerols using acyltranferases (enzymes that alter lipids - move fatty acids)
- also made from glycerol-3-phosphate
- made in the smooth ER
Where does TAG accumulate?
in organelles called lipid droplets
What are lipid droplets?
storage organelles that originate at the ER
- lipid droplets only have 1 membrane layer b/c only one side of the membrane is in contact with water
What molecule assists the lipid droplet budding from the ER?
seipin
What are perilipins?
a family of proteins that coat lipid droplets
What are the 3 steps in making a lipid droplet?
- TAG synthesis and lens formation
- Emergence and nascent lipid droplet formation
- Lipid droplet budding and growth
Explain step 1 of the process of making a lipid droplet
triacylglycerol synthesis occurs at the smooth ER bilayer
- triacylglycerol synthesis and cholesterol ester enzymes deposit neutral lipids in between the leaflets of the ER bilayer
Explain step 2 of the process of making a lipid droplet
a protein complex forms in the ER side to push the LD out
- seipin and other LD biogenesis factors are recruited to the lens structure and facilitate the growth of the nascent lipid droplet
How does budding happen towards the cytoplasm side?
- there’s asymmetry in phospholipid composition
- saturated phospholipids on ER side are sturdy
- unsaturated phospholipids on cytoplasmic side are wobbly - seisin pushes from ER side
- asymmetry in protein recruitment from cytosol side (i.e., there are more proteins on that side)
- causes membrane to curve outward
Explain step 3 of the process of making a lipid droplet
lipid droplets bud from the ER and grow through fusion or local lipid synthesis
- i.e., make more TAG or fuse with other lipid droplets
How do lipid droplets fuse to other LDs or organelles?
with membrane proteins
- these proteins are always facing the cytoplasmic side
What specific organelle uses lipid droplets? And for what?
mitochondria use lipid droplets to make ATP
What is the branchpoint between TAG and membrane phospholipids? How can it be altered to go between the 2?
Phosphatidic acid
- remove phosphate group = make TAG
- keep phosphate = phospholipids
- phosphatidic acid = smallest phospholipid
Where are enzymes that metabolize phospholipids located?
in ALL membranes - but NOT in cytosol
- these enzymes are hydrophobic
