If fatty acid synthesis occurs in the cytosol, where does the acetyl-CoA come from? Acetyl-CoA (not...

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 b5, and cytochrome b5 reductase

• Most common unsaturated fatty acids are oleic acid (18:19) and palmitoleic acid (16:19) – 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