ADVANCED NUTRITION MACRONUTRIENTS EXAM 2: CHOLESTEROL

Cholesterol

A 27-carbon compound w/ a hydrocarbon tail , central sterol nucleus made of 4 hydrocarbon rings and hydroxyl group

• central sterol nucleus or ring is a feature of all sterol molecules

• structural component of cell membranes = impacting permability/stability

• Precursor of steroid hormones => Estrogen , Testerone, aldosterone, cortisol

• Isoprenoids- biosyn. precursor

Estrogen , Testerone, aldosterone, cortisol

Precursor of steroid hormones

Isoprenoids

biosyn. precursor

- Important for synthesis of dolichol required for glycoprotein synthesis & coenzyme Q (ETC)

- For post-translation modifications => farnesylation & prenylation

- cholesterol is also used in pTMs through covalent attachments

- Helps anchor protein to specific membrane locations

Oxysterols

Regulates lipid metabolic pathways

Bile acids

- required for lipid absorption

- Increases energy expenditure in BAT

- Activates signal transduction pathways-Increase release olincnetin hormone from intestine (glucagon-like peptide - /GLP-1)

Liver & Ileum

The main sites/organs of synthesis for Cholesterol?

All nucleated cells of body , skin , gonads, adrenal cortex

Other sites of cholesterol?

in cytosol close to smooth ER w a substrate is acetyl CoA

Where in the cell does cholesterol synthesis happen?

• Secretion of HDL & VLDL

• Free cholesterol secreted in bile

• Cholesterol can be converted too bile acids/salts

What are the major ways cholesterol can leave the liver?

Thiolase

Enzyme that aids in the breakdown of acetyl CoA to acetoacetyl coA

Citrate

This can come from ATPCL in cytosol that can be converted into 2 acetyl CoA

HMG-CoA synthase

converts acetoacetyl CoA to HMG-CoA

HMGCR-ER membrane protein

has an active site in cytosol in de novo lipogenesis

Mevalonate synthesis from acetyl CoA

Stage that makes Mevalonate & requires NADPH in cytosol & highly regulated

Synthesis ol isophenoid units from mevalonate

Stage that has 3 sequential phosphorylations = 3 mol of ATP needed

squalene to cholesterol

Stage that can be inhibited by statins with key intermediate used for multiple sterol synthesis (happens in cytosol with a substrate of acetyl CoA with most regulated enzyme: HMG-CR)

Lanosterol

key intermediate used for multiple sterol synthesis

7-dehydrocholesterol

Last product before cholesterol synthesis and acts as a precursor to Vitamin D3

building blocks for squalene (sterol) synthesis

5C isopentenyl pyrophosate and its isomer dimethylallyl pyrophosphate

Stage 3A: Synthesis of farnesyl pyrophosphate

first branchpoint in synthesis of steroid and other isoprenoid compounds

Farnesyl pyrophosphate

precursor for cholesterol, dolichol, ubiquinone (CoQ), bile acids, vitamin D

Bisphosphonates

osteoporosis drugs – inhibit FPP synthesis (FPP promotes attachment of osteoclast to bone)

Statin

induced rhabdomyolysis is due to the depletion of FPP and subsequent depletion of CoQ in ETC

HMG-CoA reductase

HMG-CoA converts to mevalonate —> Active site in cytosol/Rate-determining reaction/Target of statin drugs

1. Product inhibition -- mevalonate, cholesterol

2. Phosphorylation ( promoted by glucagon) → Dephosphorylation (promoted by insulin)

3. Enzyme synthesis/degradation via gene transcription and protein stability – downregulated by cholesterol

How can HMG CoA be regulated?

Glucagon and Epinepherine can increase amount of cAMP to make PKA active to make HMGR inactive

Two things that can decrease HMGR activity

Insulin can make Protein Phosphatase active by dephosphorylation and increase HMGR activity

Two things that can increase HMGR activity

Liver

Where is cholesterol synthesis highest in?

Dietary cholesterol

suppresses cholesterol synthesis in liver via inhibition of HMGCR

Bind to active site of HMGCR – prevent HMGCoA binding and mevalonate production

How do statins inhibit HMG-CoAR?

Bempedoic acid

ATPCL inhibitor → Administered as a prodrug – activated only in liver – avoids some muscular side effects associated with statin use

1. ApoE and B100 on LDL bind to LDLR at specialized locations on plasma membrane that contain the protein clathrin

• Clathrin-containing regions form invaginations in PM called clathrin- coated pits

2. Binding of LDL to LDLRs in clathrin-coated pits internalizes the LDL

3. Clathrin is lost and LDL fuses with endosome (cytosolic vesicle for endocytosis)

4. LDL dissociates from LDLR in early endosome - LDLR is recycled back to PM

5. LDL containing endosome-fuses with lysosome (cytosolic organelle containing hydrolytic enzymes)

6. Hydrolytic enzymes in lysosome break down LDL to FC and amino acids

7. FC esterified to CE by ACAT (in liver)

• FC can also be converted to bile acids by the enzyme Cyp7A1 and secreted into bile duct)

LDLR endocytosis steps

ACAT

acylcholesterol acyltransferase (adds a FA onto cholesterol to produce CE (not LCAT)

Cyp7A1

FC can also be converted to bile acids by the enzyme __________ and secreted into bile duct

ApoE and B100

bind to LDLR at specialized locations on plasma membrane that contain the protein clathrin

clathrin-coated pits

Clathrin-containing regions form invaginations in PM

cytosolic vesicle

Clathrin is lost and LDL fuses with endosome

Hydrolytic enzymes in lysosome

Break down LDL to FC and amino acids

Liver

Where does majority of LDL clearence happens in where?

Incoming cholesterol (dietary or de novo synthesized)

• Inhibits HMG-CoAR to reduce chol synthesis

• Increases ACAT activity to increase esterification, since FC is toxic to cell

• Reduces LDLR production to protect liver against excessive cholesterol accumulation

PCSK9* inhibitors

New class of drugs to lower LDL-cholesterol that acts like a protease that degrades the LDL receptor

• maintains

  higher levels of the LDL

  receptor recycling,

  enabling uptake of LDL by

  receptor-mediated

  endocytosis

• ecreasing

  LDL receptor degradation,

  increase LDL-C uptake into

  liver, thereby lowering

  plasma cholesterol

SREBP

transcription factors of cholesterol synthesis:

• Translocates to nucleus

• Increases expression of target genes

ER and 2-step proteolytic cleavage

What are SREBPs regulated by?

SREBP1

primarily regulates DNL

SREPB2

regulates cholesterol metabolism and LDLR upregulated by this

SCAP

Senses sterol levels (when sterols drop, this transports SREBP to Golgi

cholesterol excretion

Increasing __________ to reduce serum cholesterol

• Synthesis of bile acids by liver is the primary

route for cholesterol excretion

• Bile acids are secreted into intestine and could

thus be targeted for excretion

• Dietary cholesterol absorption requires NPC1L1

– blocked by drug Ezetimibe

– blocks cholesterol uptake into enterocyte

Synthesis of bile acids by liver is the primary route or Bile acids are secreted into intestine or using Ezetimibe

What are ways to increase cholesterol excertion

Ezetimibe

Dietary cholesterol absorption requires NPC1L1 that blocks cholesterol uptake into enterocyte

Phytosterols/stanols

Compete with cholesterol for esterification by ACAT and thus decrease cholesterol esterification

– Are themselves poorly esterified and not absorbed

– increase FC pool that can be excreted

• Sources: fruits and vegetables, olive oil, supplements

Soluble fiber

Delays gastric emptying

– May reduce big increases in plasma glucose

and therefore plasma insulin

– Remember insulin promotes cholesterol and

fat synthesis

• Interferes with micelle formation and

reduces lipid absorption

• Binds bile acids in gut → fecal excretion

• Fermentation in colon → propionate – may

inhibit HMGCR

propionate

may inhibit HMGCR in fermentation in colon

fecal excretion

when soluble fibers bind bile acids in gut…

Niemann-Pick Type C disease

accumulation of cholesterol in lysosomes → brain dysfunction and death – rare genetic inheritance

beta-sitosterolemia

defect in ABCG5/8 which excretes sitosterols into int. lumen – increased secretion of beta- sitosterols into circulation → premature atherosclerosis

A-beta lipoproteinemia

rare defect in MTTP gene – no

ApoB containing lipoproteins in blood – cholesterol

absorption defects – treated with fasting, low-fat diets,

and massive amounts of Vitamin E (can help with

lipoprotein assembly and counteract deficiency)