If fatty acid synthesis occurs in the cytosol, where does the acetyl-CoA come from? Acetyl-CoA (not from fatty acid oxidation) is generated in the mitochondria – hence needs transport mechanism The shuttle involved is interesting as it provides a control mechanism and produces NADPH needed for the process The shuttle involves citrate, which is also produced in the mitochondria, as you should recall from acetyl-CoA and OAA

Involvement of citrate in fatty acid synthesis makes sense Accumulation of citrate indicates too much for citric acid cycle to handle, so what happens? Shut down PFK-1 and glycolysis Divert citrate to fatty acid synthesis since the energy generating pathway is saturated

Palmitate is a precursor for other fatty acids Requires enzymes for elongation and desaturation

Desaturating fatty acids

Regulation of fatty acid biosynthesis When too much food, acetyl CoA diverted to fatty acid biosynthesis instead of citric acid cycle The first committed step is catalyzed by acetyl-CoA carboxylase – the rate limiting step in fatty acid biosynthesis, and the regulatory point

Inhibition and activation

Hormones also regulate this pathway In animal cells: Phosphorylation Respond to NADPH levels also

In eukaryotic cells, elongation occurs in the mitochondria and ER Reactions are analogous to the fatty acid synthase, but use CoA rather than ACP as carrier

Desaturation of fatty acids Involves desaturase, cytochrome b 5, and cytochrome b 5 reductase Most common unsaturated fatty acids are oleic acid (18:1  9 ) and palmitoleic acid (16:1  9 ) – use a desaturase specific for this bond – other desaturases specific for other bond positions in fatty acids However, mammals lack certain desaturases, thus require essential fatty acids from diet

Linoleate and linolenate are essential as precursors for other fatty acids Arachidonic acid is precursor for eicosanoids (cell signaling molecules)

Arachidonic acid is a precursor for prostaglandins Arachidonic acid liberated from phospholipid bilayer in Response to a stimulus, which is either bradykinin, epinephrine, or proteases such as thrombin

Arachidonate is cyclized to PGs Notice use of molecular oxygen here and elsewhere

Arachidonate is also a precursor of leukotrienes Leukotrienes, prostaglandins, and thromboxanes are called eicosanoids, because they share a common origin (arachidonic acid) Biologically active eicosanoids are short- lived, locally acting hormones

Many of these reactions involve molecular oxygen Oxygenases catalyze reactions where oxygen atoms are directly incorporated into substrate –Dioxygenase both oxygen atoms are added to substrate –Monooxygenase – only one oxygen is incorporated, the other is reduced to water

Cytochrome P-450 is an interesting monooxygenase family Heme containing protein that binds oxygen and carbon monoxide (when CO bound, absorbs light at 450 nm) Most vertebrates genomes contain more than 40 genes encoding variants of this protein These proteins act upon thousands of xenobiotics – hydroxylation usually increases their solubility and helps in detox, metabolism and excretion Expressed in liver, brain

Fatty acids  Triacylglycerol and membrane lipids Need glycerol 3-phosphate, which is generated from DHAP (from glycolysis) or by ATP-dependent phosphorylation by glycerol kinase (adipocytes lack glycerol kinase) Regardless, glycerol 3-phosphate undergoes two successive enzymatic esterifications with fatty acyl-CoAs to yield diacylglycerol 3-phosphate (phosphatidic acid)

To form a triacylglyerol: Phosphatidic acid phosphatase cleaves off the phosphate group and another transesterification

Pathways in glycerophospholipid biosynthesis

Divergence between triacylglycerol and glycerolphospholipid Note first steps are shared, however, need attachment of head group Note on previous slide this attachment is mediated by nucleotide activation (CTP) The nucleotide activation can occur either through activation of head group or diacylglycerol

CTP activation of diacylglycerol occurs in prokaryotes

This pathway yields cardiolipin and… Cardiolipin accounts for 5-15% of E. coli membrane Not observed in hepatocyte plasma membrane, but a prominent component of mitochondrial inner membrane

phosphatidylethanolamine Phosphatidylethanolamine is about 75-85% of E. coli membrane and prominent in all animal cell membrane layers

CTP activation of diacylglycerol also is important for eukaryotic plasma membrane

Eukaryotes have a distinct mechanism for other membrane phospholipids Swap head groups between serine and ethanolamine Phosphatidylcholine is generated by three methylation reactions using phosphatidylethanolamine as a substrate OR By activation of headgroup with CTP

CDP activation can generate phosphatidylcholine and phosphatidylethanolamine

Alternative path for phosphatidylserine

Can also go back

Ether-linked lipids Built on DHAP, where ester-linked fatty acyl group is displaced by a long chain alcohol, reduced, then head group attached Still contain ester-linked acyl group at C2 Comprise 50% of choline phospholipids in heart tissue, but virtually undetectable in other tissues Synthesis carried out in peroxisomes

Archaeal membrane phospholipids Ether linkages at both C1 and C2 Lack fatty acids, instead have repeating units of isoprenes Major lipids in membranes are glycerol diethers and tetraethers Tetraethers yield a monolayer, instead of bilayer, which are quite resistant to peeling apart – facilitates growth under hyperthermophilic conditions