Organic Chemistry

012 CHAPTER 26 Lipids

stabilized by electron delocalization. The radical reacts with oxygen to form a peroxy

radical. Notice that these two steps are the same as the first two steps in the reaction

that causes fats to become rancid (Section 26.3). The peroxy radical rearranges and re-

acts with a second molecule of oxygen. The enzyme then uses its hydroperoxidase ac-

tivity to convert the OOH group into an OH group, forming which rearranges

to form a prostaglandin.

In addition to serving as a precursor for the synthesis of prostaglandins, is a

precursor for the synthesis of thromboxanesand prostacyclins. Thromboxanes con-

strict blood vessels and stimulate the aggregation of platelets, the first step in blood

clotting. Prostacyclins have the opposite effect, dilating blood vessels and inhibiting

PGH 2

PGE 2 ,

PGH 2 ,

COOH

arachidonic acid

HH

COOH

H

O

O

COOH

COOH

O

O

O

OH

O

H

O

O

COOH

H

O

O

OH

COOH

COOH

OH

OH

COOH

O

HO

HO

OH

OOH

HO H

H

H

H

H

H

H

H

H

H

O

O

cyclooxygenase

biosynthesis of prostaglandins, thromboxanes, and prostacyclins

a peroxy radical

severalsteps hydroperoxidase

several

steps

PGH 2

a thromboxane

PGE 2

a prostaglandin

a prostacyclin