LECT 22: LIPIDS, MEMBRANES, AND CHOLESTEROL

Lipids and cholesterol in the body derive from food intake and from de novo synthesis.

Phospholipids are principle constituent of cell membranes, and specialized phospholipids participate in signal transduction pathways.

Lipid and cholesterol synthesis intermediate anchors serve to attach certain proteins to membranes.

Cholesterol is a membrane constituent needed in all cells. Bile acids used for intestinal fat uptake and nuclear hormones are synthesized from cholesterol.

Cholesterol is only synthesized in the liver, so that all other tissues must receive cholesterol by trafficking through blood in the form of lipoprotein particles.

Phospholipids Have Glycerol Backbone, Two Fatty Acids, and Phosphate Alcohol

The Four Common Phospholipids

Phosphatidylinositol(PI) has diverse functions. In addition to being a major membrane constituent, PI’s inositol ring can be phosphorylated to generate a signal transduction intermediate. PI can also be further modified to generate the GPI membrane anchor.

Phospholipid Synthesis Uses CDP Intermediates

Sphingosine Is a Fatty Amine Used to Generate the Specialized Lipid Sphingomyelin

Fatty Acids Tend To Form Micelles, While Phospholipids FormBilayer Membranes That Are Water Impermeable

Fatty acids or other detergents cansolubize membranes,

dissolving them into mixed micelles

A Detergent’s “Strength” Reflects Types of Hydrophobic InteractionsIt Can Disrupt

VERY WEAK DETERGENT: Disrupts only very weak hydrophobic interactions. Example: Tween-20. Useful in protein binding reactions (e.g., Western blot) to prevent nonspecific binding of antibody to filter or filter-bound proteins.

MILD NONIONIC DETERGENT: Dissolves plasma membrane, other non-nuclear membranes, and incorporates constituents into detergent micelles. Examples: Triton X-100, Nonidet P40

STRONG IONIC DETERGENT: Solubilizes all membranes, disrupts protein-protein interactions, and denatures proteins. E.g. SDS

Sonicated Phospholipid Forms Small Lipid Vesicles, Liposomes,That Trap Aqueous Materials Inside

Lipophilic Protein Anchors Tether Proteins to Membranes

Some Proteins Contain Signal Motifs for Lipophilic Modification

Lipophilic anchor attachments accompanied by peptide cleavage

-C-A-A-X

SH

-C

S

C15Farnesylation (cytoplasmic)

M-G-(6aa)- G-(6aa)-

NH

C14Myristoylation (cytoplasmic)

GPI Anchoring (ER Lumen)

20aahydrophobic tail

GPI

Membrane Fluidity Demonstrated By Photobleach Recovery

This reaction catalyzed byHMG-CoA reductase

is rate-limiting forcholesterol biosynthesis

HMG-CoA Reductase Initiates the Cholesterol Synthetic Pathway

C5

C10

C15

C30

Geranyl

Isopentyl

Farnesyl

Squalene

Cholesterol Is Built From Six Isopentenes and C30 Cyclization

Cholesterol Is The Precursor For Nuclear Hormones

Cholesterol Biosynthesis in Liver Regulated at the HMGCoAR Gene

HMGCoAR gene transcription controlled by transcription factor SREBP.

When cholesterol is abundant, its interaction with SCAP in the ER keeps SREBP tethered to ER as part of larger precursor.

Decline in cholesterol allows SCAP/SREBP to move to Golgi, where proteases liberate SREBP, which goes to nucleus to activated transcription.

Lipoprotein Particles Employ Specific Apolipoproteins to PackageCholestero Estersl and Triglycerides in a Phospholipid/Cholesterol Shell

Lipoprotein Particles Are Taken Up By Cells By Specific ReceptorsThat Recognize the Apolipoprotein Component

After uptake, lysosomes degrade apolipoprotein, and cholesterols are incorporated into endoplasmic reticular membrane

Lipids and Cholesterol Are Shuttled Through an Organ Network

EXCESS SERUM LDL INCREASES RISK OF CARDIOVASCULAR DISEASE

Genetic Causes of Cardiovascular Disease

Hypercholesterolemia cause by mutations in genes for LDLR or LDL’s ApoB100, which prevent circulating LDL uptake

Mutations in ApoE cause reduction in HDL production, thereby impairing circulating cholesterol trafficking back to liver

Clinical Management of Cardiovascular Disease or Risk

Dietary management, including restricting cholesterol and reducing saturated fat and sugar intake

Cholesterol reuptake inhibitors: Cholestyramines taken orally bind to bile acids in gut and prevent their reabsorption, thereby lowering total cholesterol load

Statins (e.g., atorvistatin, lovastatin) inhibit HMG-CoA reductase, thereby blocking de novo cholesterol synthesis in liver