Eicosanoids- Exam 4
What is the immediate precursor of the prostaglandins and how is it formed?
*Arachidonic acid*- formed from the *essential fatty acid linoleic acid* Linoleic acid ----> linoleoyl-CoA Linoleoyl CoA ----> Arachidonyl-CoA 3 Steps: d6-desaturase, elongation with malonyl CoA, and d5-desaturase The desaturases use NADH and O2
What is the function of 5- or 12 HETE?
(hydroxy) Aggregates human leukocytes Promotoes leukocyte chemotaxis
What is the function of 5- or 12-HPETE?
(peroxy) Vasodilation of rat and rabbit gastric circulation - inhibits platelet aggregation
How does aspirin work?
*Acetylates a Ser hydroxyl group* in the active site of CO component of prostaglandin endoperoxide synthase - In simpler terms: blocks the first step in the synthesis of prostaglandins
Describe the structure of COX and the differences between type 1 and 2
*Heme-containing* membrane proteins that exist as *dimers* - COX1: 599 amino acids - COX2: 604 amino acids - COX2 has 18 aa near the c-terminus - COX1 has 17 aa near n-terminus - COX 1 has *Ile* residues at 434 and 523 - COX2 has *Val* residues at 434 and 523
How do NSAIDS work?
*Inhibit COX* (most inhibit cox 1 and 2, but some show selectivity for cox2) Inhibition of the synthesis of PGs (which are involved in inflammatory response) provides relief from pain and inflammation
What are the enzymes in the 5-lipoxygenase pathway and how do they work?
*Lipoxygenases* - Iron containing enzymes that oxidize polyunsaturated fatty acids Substrate: FA with cis,cis-1,4-pentadiene Product: *hydroperoxides*
____ size of ____ in COX2 allows what? How does COX 1 differ?
*Smaller* size of *Valine* in COX2 allows access to a side pocket, while the longer Isoleucine in COX1 sterically blocks inhibitor access
What else does aspirin affect?
*Thromboxane A2* (because synthesis of PG and TXA is the same) Therefore, it *minimizes platelet aggregation*
Where is arachidonic acid (AA) found and how is it obtained?
- *Phospholipid bilayer* of plasma membrane is made up of esters of AA - *Phospholipase A2* hydrolyzes the phospholipids to release AA from position 2 of the phosphoglycerol esters - *Phospholipase C* hydrolyzes PIP2 to form DAG and IP3 - DAG is hydrolyzed by other lipases to form AA
Where are leukotrienes derived from and what are the 2 classes?
- Product of the 5-lipoxygenase pathway 1. Hydroxylated Eicosatetraenoic acids (LTs): LTB4 2. Peptidoleukotrienes (pLTs): LTC4, LTD4, LTE4
Describe the metabolism of prostaglandins
- Prostaglandins and thromboxanes have very short half-lives that range from seconds to minutes - *Oxidation* of the essential C-15 hydroxy group to a *ketone* Or - *Reduction* of the C13 double bond - b and w-oxidation of the side chains affords *dicarboxylic acids* that are excreted in the urine
How are eicosandoids unlike hormones?
- They are extremely *potent* - They are produced in *very small quantities* in almost *all tissues* - Their action is *local* rather than being carried through blood to distant sites - They are *rapidly metabolized* to inactive and have *short half lives* - Their actions are *mediated through GPCRs*
Describe the mechanism of how leukotrienes are made
Arachidonic acid -----> 5-HPETE 5-lipoxygenase 5-HPETE ----> LTA4 (unstable) LTA synthetase LTA4 (unstable)-----> LTC4, LCD4 or LTE4 Glutathione-s-transferase LTA4 (unstable)----> LTB4 LTA hydrolase
Selective COX2 inhibitors do not bind the ____ that is used by arachidonic acid. How do they bind?
Arg 120 residue They bind to the major pocket AND open the "branch" pocket
What is the role of leukotrienes in asthma?
Cause - bronchoconstriction - Increased mucus secretion - Inflammatory cell infiltration into airway wall --> airflow obstruction
What is the enzyme for biosynthesis of prostaglandins and thromboxanes?
Enzyme = *Prostaglandin endoperoxide synthase* which has - *cyclooxygenase* activity - *peroxidase* acitivy
Which eicosanoids are produced from the cytochrome P450 pathway?
Epoxides, HETEs and diHETEs
What are thromboxanes? Where are they derived from? What are their functions?
Fatty acid derivatives derived from prostaglandin endoperoxides (PGH2) PGH2 -----> TXA2 by enzyme TXA synthase - Produced by platelets - Vasoconstriction - Platelet aggregation - Lymphocyte proliferation - Bronchoconstriction
Describe the metabolism of thromboxanes
Half life is really short (about 30 seconds) - Rapidly converted to *TXB2* via *hydrolysis*- cleavage of the oxygen bridge between C9 and C11
PGF2a function
Increase: - IP3 and DAG - Vasoconstriction - Bronchoconstriction - Smooth muscle contraction (induces labor)
PGI2, PGE2 and PGD2 function
Increase: - Vasodilation - cAMP Decrease: - Platelet aggregation - Leukocyte aggregation - IL-1 and IL-2 - Lymphocyte migration
LTC4, LTD4 and LTE4
LTC4 has glutamine and glycine residues LTD4 does not have glutamine, only glycine LTE4 loses both glu and gly
Which leukotrienes are SRSAs?
LTC4, D4 and E4 - potent and prolonged contraction found in guinea pig
What are the cysteinyl leukotrienes and how are they made?
LTD4, LTC4, and LTE4 Made from unstable LTA4 with enzyme *glutathione-s-transferase* GSH- (*gamma-glutamylcystenylglycine*) attacks the epoxide
What can treat asthma?
Leukotriene receptor antagonists (zafirlukast and montelukast)
Which eicosanoids are produced from the lipoxygenase pathway?
Leukotrienes, HETEs and lipoxins
PGD structure
OH group is alpha
PGE structure
OH group is alpha
How are eicosanoids formed from arachidonic acid (3 pathways/enzymes)? Where does this occur?
Once in the *cytosol* 1. Cyclooxygenase pathway 2. Lipoxygenase pathway 3. Cytochromes P450
What does a lipoxygenase do to arachidonic acid?
Oxidizes it to afford a number of *hydroperoxy-eicosatetraenoic acid derivatives (HPETEs)*
PGH2 can form what other things?
PGD2 by PGD synthase PGE2 by PGE synthase PGF2a by endoperoxide reductase *PGI2 (prostacyclin)* by PGI synthase *Thromboxane (TXA2)* by TXA synthase
What are leukotrienes?
Produced by cells of the immune system and leukocytes - Regulate immune responses - Histamine and prostaglandins are usually produced when leukotrienes are produced
Where are cysteinyl leukotrienes found and what are their function?
Their receptors are found on endothelial cells, eosinophils, and mast cells - Activation stimulates *proinflammatory activities* such as endothelial adherence and *chemokine production* - Induce *inflammatory disorders and asthma* by bronchoconstriction
What types of lipoxygenases do leukocytes contain?
both *5- and 12-lipoxygenase*
PGFalpha structure
both hydroxyls are alpha, making them cis
What is the function of LTD4?
bronchiole constriction (smooth muscle contraction)
What is the function of LTB4
increases leukocyte chemotaxis and aggregation
PGFbeta structure
one hydroxyl is alpha, the other is beta, making them trans
What types of lipoxygenases do platelets contain?
only *12-lipoxygenase*
Aspirin is shown to inhibit
prostaglandin endoperoxide synthase (the cyclooxygenase component)
Endothelial produces which eicosanoids?
prostaglandins E2 and I2 (PGE2 and PGI2)
Platelets produce which eicosanoids?
thromboxane A2 (TXA2) and 12-hydroxyeicosatetraenoic acid (12-HETE)
Step one of prostaglandin formation from AA
1. Arachidonic acid -----> PGG2 (very unstable) Enzyme= Cyclooxygenase component of PG endoperoxide synthase - Forms endoperoxide between C9 and C11 - Creates 5 member ring by creating bond between C8 and C12 - Oxidizes C15 to a hydroperoxide using *O2*
Step 2 of prostaglandin formation from AA
2. PGG2 ----> PGH2 Enzyme= peroxidase component of PG endoperoxide synthase - Oxidizes *glutathione (GSH) to GSSG* - Reduces the C15 hydroperoxide to an alcohol
What are prostaglandins?
20 carbon fatty acids containing a *5 member ring* - Discovered in seminal fluid and accessory genital glands (hence the name)
What is the result of overproduction of leukotrienes?
Allergic rhinitis and asthma (LTD4 causes bronchiole constriction) (because histamine and prostaglandins produced when leukotrienes produced)
What is the result of an excess of leukotrienes?
Anaphylactic shock Slow reacting substance of anaphylaxis is LTC4, D4 and E4 (SRSAs)
What are eicosanoids? Name the 3 types
Biologically active *lipids* with *local* hormone-like activity 20 Carbons 1. Prostaglandins 2. Thromboxanes 3. Leukotrienes
What is the difference between COX-1 and COX-2?
COX-1 is *constitutive*- widely expressed in almost *all tissues* - produces PGs and TXs for normal physiological functions COX-2 is *inducible*- regulated by *cytokines and GFs* - Found in inflamed tissue
Which COX is more specific for fatty acid substrates?
COX1
Which COX primarily metabolizes arachidonic acid?
COX1
What is the difference with inhibition of COX1 vs COX2?
COX1 inhibition is *undesirable* because it is important in *homeostatic functions* such as - GI tract, renal tract, platelet function, macrophage differentiation COX2 inhibition is *desirable* and has more *GI safety and less antiplatelet side effects* - This is why *Selective COX 2 inhibitors are better*
Which COX acts on C18 and C20 fatty acid substrates?
COX2
What are inhibitors of Phospholipase A2?
Corticosteroids
Why are antihistamines not good for asthma treatment?
Cysteinyl leukotrienes induce the inflammatory response
What are allosteric activators of COX2?
Cytokines, IL-1, TNF, Growth Factors
Where are the single nonmamalian prostaglandin intermediates found?
Gorgonian sea whip or sea fan, Plexaura homomalla
How do you name prostaglandins?
Prostaglandin (PG) Nomenclature is based on the *prostanoic acid structure* (pictured) - bond 8/7 is alpha, bond 12-13 is beta, making the compound *trans* - The letter after PG designates the substitution and stereochemistry about the 5 member ring (positions 8-12) and at C15 - A subscripted numeral indicates number of double bonds outside of the ring - subscripted a or b indicates the OH group at C9
Which eicosanoids are produced from the cyclooxygenase pathway?
Prostaglandins and thromboxanes
Prostaglandins are referred to as "____ _____"
local hormones
