Reaction 5: Reduction 2
6.4.4. SYNTHESIS OF PROSTAGLANDINS AND OTHER EICOSANOIDS
Prostaglandins (PGs) are local, short-lived hormones derived from C20 fatty acids like arachidonic acid. Arachidonate is obtained from the diet, synthesized from essential
unsaturated fatty acids like linoleate (Fig 6.4.11), or released by hydrolysis of membrane phospholipids in response to hormonal and other signals. Prostaglandin synthesis from arachidonate occurs on the smooth endoplasmic reticulum.
Prostaglandins and thromboxanes are synthesized by the “cyclic” pathway. A bifunctional enzyme, prostaglandin H2 synthase (PGH2 synthase) or COX, enables the formation of prostaglandin H2 (PGH2) from arachidonate. The reaction occurs in two steps aided by the two catalytic activities of PGH2 synthase:
i. Cyclooxygenease activity – which adds molecular O2 to arachidonate and produces the cyclopentane-ring needed for prostaglandins. The product is PGG2
ii. Peroxidase activity – which converts PGG2 to PGH2
Prostaglandin H2 (PGH2) is the immediate precursor of the other prostaglandins and thromboxanes.
Fig.6.4.12. Outline of the synthesis of eicosenoids viz. prostaglandins, prostacyclins, thromboxanes and leukotrienes from arachidonate (PLA2 = Phospholipase A2) (Source: Berg et al, 2002, p 933 fig 22.30)
Leukotrienes are formed by a “linear” pathway from arachidonate in which several lipooxygenases catalyze the insertion of molecular O2 into the linear chain of arachidonate.
These enzymes are mixed-function oxidases which use cytochrome P-450.
PGH2-synthase is also referred to as COX owing to its cyclooxygenase activity. Mammals have two isozymes of PGH2-synthase viz. COX-1 and COX-2. COX-1is expressed in most tissues and supports levels of PG synthesis necessary for homeostasis. COX-2 is expressed in certain tissues only in response to inflammatory stimuli and is the cause of inflammation, pain and fever. .
Fig.6.4.13. Site of action of non-steroidal anti-inflammatory drugs (NSAIDs) and COX-2 inhibitors on the cyclooxygenase isozymes COX-1 and COX-2 in prostaglandin synthesis.
(Source: Elliot and Elliot, 2005, p 243 fig 14-9)
Regulation of fatty acid synthesis
The enzymes of fatty acid synthesis are subject to two kinds of regulation:
short-term-regulation – which controls their activity
long-term regulation – which controls the genes that regulate their synthesis
The main target of short-term regulation is acetyl-CoA carboxylase (ACC), the first enzyme in fatty acid synthesis.
The enzyme is activated allosterically by citrate, and is subject to product feedback inhibition by long-chain acyl molecules (e.g. palmitoyl-CoA). Citrate levels are high when [acetyl-CoA] is high and such a situation is conducive to fatty acid synthesis. Citrate has a major role in shifting the metabolism of fuel compounds from oxidation to the synthesis of fatty acids and their storage as triglycerides.
Fig 6.2.12. Allosteric regulation of acetyl-CoA carboxylase (Source: Nelson and Cox, 2005 p 797 fig 21-11)
Long-chain acyl-CoA inhibits fatty acid synthesis in many ways. It has a direct negative feedback effect on ACC. Its inhibitory effect may be enhanced indirectly by suppression of the tricarboxylate transporter so that citrate efflux from the mitochondrion is prevented.
High [Acyl-CoA] inhibits the ATP-ADP exchange transporter in the IMM and also stimulates β-oxidation. This raises the mitochondrial ratio of [ATP]/[ADP] and [NADH]/[NAD+].
Consequently the PDH complex is inhibited and the production of acetyl-CoA falls.
Covalent modification of ACC is by reversible phosphorylation/dephosphorylation. The phosphorylated form of the enzyme exists as protomers and is inactive. The dephosphorylated form is polymerized and active.
Phosphorylation of ACC is caused by an AMP-activated protein kinase (AMPK) which is in turn is activated by another kinase, AMPKK. The activity of AMPKK is increased by cAMP
phosphorylation and hence inactivation of ACC and consequent fall in fatty acid synthesis.
On the other hand a phosphatase which dephosphorylates ACC increases fatty acid synthesis.
Fig 6.2.13. Covalent regulation of the activity of acetyl-CoA carboxylase (Source: Murray et al, 2003 p178 fig 21-6)
The hormones glucagon and insulin affect the activity of ACC. The inhibitory effect of glucagon is via cAMP-dependent PKA. Thus when blood glucose is low, metabolism of the fatty acids can switch from synthesis to oxidation. Insulin probably acts through an “activator protein” and an insulin-dependent protein kinase to stimulate ACC and increase fatty acid synthesis.
Long-term regulation of fatty acid synthesis is by alteration of the activity of the genes that control synthesis of ACC and fatty acid synthase. The total amount of these enzymes is determined by sustained nutritional states. Thus, prolonged fasting, high-fat diet and diabetes result in decrease of the enzymes, while a well-fed state boosts their quantity. Insulin induces enzyme synthesis but glucagon suppresses this action.
Non-steroidal anti-inflammatory drugs (NSAIDs) like aspirin and ibuprofen block the active site of PGH2-synthase and prevent synthesis of prostaglandins and thromboxanes. They are used widely for relief from inflammation, pain and fever. Low doses of aspirin are prescribed for high-risk patients to decrease the production of thromboxanes and hence reduce the chances of heart attacks and strokes. However these drugs have several undesirable side- effects. Cox-2 inhibitors like celecoxib are more specific in action and have lesser side- effects. Anti-inflammatory corticosteroids also inhibit transcription of COX-2 but not COX- 1.
SECTION 5: METABOLISM OF TRIACYLGLYCEROLS