Chapter 16 (Part 4)

Synthesis of Eicosanoids, Glycerolipids and

Isoprenoids

Glycerolipid Biosynthesis• Important for the synthesis of

membrane lipids and triacylglycerol

• Synthesis occurs primarily in ER • Phosphatidic acid (PA) is the

precursor for all other glycerolipids in eukaryotes

• PA is made either into diacylglycerol (DAG) or CDP-DAG

Glycerolipid

Biosynthesis• Phosphatidic

acid is the precursor for all other glycerolipids

NH2

CH CH2C OH

O

OO P

O-

O

H2C

CH

CH2O

O

C

C

O

O

R1

R2

O

N

NH2

ON

O

OHOH

HH

HH

OP

O-

O

O P

O-

O

H2C

CH

CH2O

O

C

C

O

O

R1

R2

NH2

CH CH2C OH

O

HO

OO P

O-

O

H2C

CH

CH2O

O

C

C

O

O

R1

R2

OH

OH

OH OH

OH

H

H H

H

H

CMPCMP

CDP-DAG

phosphatidylserinephosphatidylinositol

Serine

Inositiol

Eicosanoids• Eicosanoids are important regulatory

molecules • Referred to as local regulators. Function

where they are produced.• Two classes:

Prostaglandins/thromboxanes, and Leukotrienes

• Prostaglandins – mediate pains sensitivity, inflammation and swelling

• Thromboxanes – involved in blood clotting, constriction of arteries

• Leukotrienes – attract white cells, involved inflammatory diseases (asthma, arthritis, etc..)

Eicosanoids

Eicosanoid Synthesis

• C20 unsaturated fatty acids (i.e. arachidonic acid (20:4D5,8,11,14) are precursors

• Prostaglandins and Thromboxanes are synthesized by a cyclooxygenase pathway

• Leukotirenes are synthesized by a lipoxygenase pathway

cyclooxygenase

• Arachidonic acid present in membrane lipids are released for eicosanoid synthesis in the cell interior by phospholipase A2

Cyclooxygenase (COX) Inhibitors

• Two COX isozymes: COX-1 and COX-2.• COX-1 – important in regulating mucin

secretion in stomach• COX-2 – promotes pain and inflammation and

fever (involved in prostaglandin synthesis).• Asprin (acetylsalicylate) non-specific COX

inhibitor. Acts by acetylating an essential serine residue in the active site.

• Because asprin inhibits COX-1, causes stomach upset and other side effects.

• New drugs (Vioxx and Celebrex) specifically inhibit COX-2

Isoprenoid Synthesis

• Involves formation of isopentenyl pyrophosphate (IPP) monmers.

• IPP is conjugated in a head to tail manner to generate polyprenyl compounds.

•Formation of the isopentenyl pyrophosphate (IPP) via mevalonate pathway.

•Primary pathway for isprenoid synthesis in animals and cytosolic isoprenoid synthesis in plants

Mevalo

nate

kin

ase

Ph

osp

hom

evalo

nate

kin

ase

pyrohosphomevalonatedecarboxylase

Formation of the isopentenyl pyrophosphate (IPP)

Two Fates of HMG-CoA

Bacteria and Plants Synthesize IPP via Non-

Mevalonate Pathway• In plants and most bacteria, IPP is

synthesized from the condensation of glyceraldehyde-3-phosphate (3 carbons) and pyruvate (3 carbons).

• Forms a 5 carbon intermediate through transketolase type reaction (transfer of 2 carbon aldehyde from pyruvate to G-3-P).

• Occurs in chloroplast of plants. Involved in synthesis of chlorophyll, carotenoids, Vitamins A, E and K.

Very recent discovery (1996)

Pathway still not fully understood.

New pathway provides enzyme targets for new herbicidal and anti-microbial compounds

Condensation of IPP into Polyprenyl

Compounds

Dimethylallylpryophosphate

IPP isomerase

IPP Isomerase

prenyltransferase

prenyltransferase

Squalene synthase

Cholesterol

Synthesis from IPP

Squalenemonooxygenase

2,3-oxidosqualenelanosterol cyclase

cholesterol

20 steps

Regulation of HMG-CoA Reductase

• As rate-limiting step, it is the principal site of regulation in cholesterol synthesis

• 1) Phosphorylation by cAMP-dependent kinases inactivates the reductase

• 2) Degradation of HMG-CoA reductase - half-life is 3 hrs and depends on cholesterol level

• 3) Gene expression (mRNA production) is controlled by cholesterol levels

Inhibiting Cholesterol Synthesis

• HMG-CoA reductase is the key - the rate-limiting step in cholesterol biosynthesis

• Lovastatin (mevinolin) blocks HMG-CoA reductase and prevents synthesis of cholesterol

• Lovastatin is an (inactive) lactone• In the body, the lactone is

hydrolyzed to mevinolinic acid, a competitive (TSA!) inhibitor of the reductase, Ki = 0.6 nM!