Lecture 20 - Lipids and membranes Flashcards
Name the groups of complex lipids.
Complex Lipids
* Neutral lipids
○ Triacylglycerols
* Polar Lipids
○ Phospholipids (Glycerophospholipids, Sphingophospholipid)
○ Glycolipids (Glyceroglycolipid, Sphingoglycolipid)
* Steroids
○ Cholesterol
Both steroids and polar lipids contribute to cell membranes
Describe the structure of glycerophospholipids
Contain two fatty acid chains attached to the glycerol as well as a phosphate which has an alcohol group attached. The most simple example of this is phosphatidic acid.
A range of alcoholic groups can be attached and this determines the name of the glycerophospholipid however the length, saturation and orientation of the fatty acids are needed to define the lipid.
The C1 fatty acid is often saturated whereas the C2 fatty acid is often polyunsaturated.
Describe the structure of ether lipids
Ether lipids/Plasmogen
Contain and ether linkage to the glycerol
They have either choline or ethanolamine attached to the phosphate group
Describe the structure of sphingophospholipids.
Sphingophospholipid
Instead of having a glycerol back bone they instead have a sphingosine backbone. They only have one fatty acid group and one phosphate group with an alcohol group attached to it. (part of the sphingosine backbone takes up the role of the second fatty acid. The fatty acid attaches through an ammino group.
Describe the structure of glyceroglycolipids
They have two fatty acid chains attached to the glycerol backbone as well as one or more sugar groups (in plants normally galactose).
Describe the structure of sphingoglycolipids
The lipid has a sphingosine backbone with one fatty acid attaches as well as one or more sugar groups. Part of the sphingosine and the fatty acid make up the two tails. Sphingoglycolipids normally have more sugar groups attached (up to 7 in comparison to the 1 or 2 on glyceroglycolipids)
Cerebroside
Describe why lipids form membranes.
All have hydrophilic head and 2 hydrophobic tails meaning that they form the same structures
* Micelle
* Bilayer
The majority of lipids found in membranes are phosphoglyceride
Synthesis of membrane glycerophospholipids
Conversion of Dihydroxyacetone to Glycerol 3-phosphate:
Catalyzed by glycerol 3-phosphate dehydrogenase.
Converts dihydroxyacetone into glycerol 3-phosphate.
Formation of Lysophosphatidate:
Catalyzed by glycerol phosphate acyltransferase.
Addition of a fatty acid group from acyl CoA to C1 of glycerol 3-phosphate.
Forms lysophosphatidate.
Formation of Phosphatidate:
Catalyzed by glycerol phosphate acyltransferase.
Addition of a second fatty acid to C2 of lysophosphatidate.
Forms phosphatidate.
Fate of Phosphatidate:
Can be converted into two intermediates:
Diacylglycerol: Formed by phosphatidic acid phosphatase removing the phosphate.
CDP-diacylglycerol: Precursor for phosphatidylglycerol synthesis.
Formation of Triacylglycerol:
Catalyzed by diacylglycerol acetyltransferase.
Addition of a third fatty acid to diacylglycerol.
Forms triacylglycerol, a neutral lipid storage form.
Synthesis of Phosphatidylethanolamine (or Choline):
Ethanolamine kinase catalyzes the phosphorylation of ethanolamine into phosphorylethanolamine.
CTP-phosphoethanolamine cytidyltransferase adds cytidine and a phosphate, forming CDP-ethanolamine.
CDP-ethanolamine:1,2-diacylglycerol phosphoethanolamine transferase removes CMP, forming phosphatidylethanolamine.
Synthesis of Phosphatidylserine:
Phosphatidylethanolamine + serine or phosphatidylcholine + serine.
Phosphatidylethanolamine serine transferase
Formation catalyzed by enzymes, leading to the production of phosphatidylserine.
Phosphatidylcholine serine transferase
Fate of Phosphatidate
The conversion of phosphatidate (PA) to CDP-diacylglycerol (CDP-DAG) and phosphatidylinositol (PI) involves a series of enzymatic reactions. Here’s a description of these processes:
- Conversion of Phosphatidate to CDP-diacylglycerol (CDP-DAG):
a. Formation of CDP-diacylglycerol (CDP-DAG):
Phosphatidate undergoes a reaction catalyzed by cytidine triphosphate (CTP) to form CDP-diacylglycerol (CDP-DAG).
This reaction is catalyzed by the enzyme CDP-diacylglycerol synthase.
In this reaction, CTP donates a cytidine monophosphate (CMP) group to phosphatidate, resulting in the formation of CDP-DAG and inorganic pyrophosphate (PPi).
b. Further Utilization of CDP-DAG:
CDP-DAG serves as an important precursor molecule in the synthesis of various complex lipids, including phosphatidylinositol (PI) and phosphatidylglycerol (PG).
CDP-DAG can also be converted into phosphatidylserine (PS) through the addition of serine.
2. Formation of Phosphatidylinositol (PI):
a. Incorporation of Inositol into CDP-DAG:
CDP-DAG undergoes a reaction with inositol to form phosphatidylinositol (PI).
This reaction is catalyzed by the enzyme CDP-diacylglycerol-inositol 3-phosphatidyltransferase.
In this reaction, inositol is attached to the phosphate group of CDP-DAG, resulting in the formation of PI and CMP.
b. Biological Functions of Phosphatidylinositol (PI):
Phosphatidylinositol is a precursor molecule for the synthesis of various phosphoinositides, which play crucial roles in cell signaling and membrane trafficking.
Phosphoinositides are involved in regulating processes such as cell proliferation, apoptosis, cytoskeletal organization, and vesicular trafficking.
Breakdown of membrane glycerophospholipids
Breakdown of Membrane Glycerophospholipids:
Phospholipase enzymes hydrolyze glycerophospholipids to release fatty acids and other products.
Various phospholipases target different bonds in the glycerophospholipid molecule, leading to the release of specific components.
Breakdown products such as fatty acids can be utilized for energy production or membrane remodeling.
Describe the different phospholipases
Phospholipases:
Phospholipases are enzymes that catalyze the hydrolysis of glycerophospholipids, leading to the release of fatty acids and other products.
Different phospholipases target specific bonds in the glycerophospholipid molecule, resulting in the selective release of components.
Types of Phospholipases:
a. Phospholipase A1 (PLA1):
Catalyzes the hydrolysis of the fatty acid ester bond at the sn-1 position of glycerophospholipids.
Releases one fatty acid molecule, leaving a lysophospholipid as a product.
b. Phospholipase A2 (PLA2):
Catalyzes the hydrolysis of the fatty acid ester bond at the sn-2 position of glycerophospholipids.
Releases one fatty acid molecule, leaving a lysophospholipid as a product.
c. Phospholipase B (PLB):
Catalyzes the sequential hydrolysis of both fatty acid ester bonds at the sn-1 and sn-2 positions of glycerophospholipids.
Releases both fatty acid molecules, leaving a monoacylglycerol as a product.
d. Phospholipase C (PLC):
Catalyzes the hydrolysis of the phosphodiester bond between the glycerol backbone and the phosphate group of glycerophospholipids.
Produces diacylglycerol (DAG) and a water-soluble head group containing the phosphate.
e. Phospholipase D (PLD):
Catalyzes the hydrolysis of the phosphodiester bond between the phosphate group and the head group of glycerophospholipids.
Produces phosphatidic acid (PA) and a water-soluble head group containing the phosphate.
Biological Functions:
Phospholipases play important roles in cell signaling, membrane remodeling, and lipid metabolism.
They are involved in the production of signaling molecules such as diacylglycerol (DAG) and phosphatidic acid (PA), which regulate various cellular processes.
Dysregulation of phospholipase activity has been implicated in various diseases, including inflammation, cancer, and neurodegenerative disorders.
