Fatty Acid Handling
• Beta-oxidation
• FA transport
• Integration of metabolic signaling
Fatty Acid/-oxidation Cycle• Acyl(n)-CoA + NAD+ + FAD
Acyl(n-2)-CoA + Acetyl-CoA + NADH +FADH2
FAD
FADH2
NAD+NADH
CoA-SH
Acyl-CoA dehydrogenase
Acyl-CoA hydrase
3-hydroxyacyl-CoA dehydrogenase
acetyl-CoA acyltransferase
Carnitine palmitoyltransferase
Fatty acid elongation
Acyl-CoA synthase Acyl-CoA
Didehydroacyl-CoA
Hydroxyacyl-CoAOxoacyl-CoA
Acetyl-CoA
Acyl-CoA
1x FADH21x NADHAcetyl-CoA– 3x NADH+–1xFADH2
Fatty acid/carbohydrate oxidation
• Oxygen– CnH2n + 3/2 n O2 n CO2+ n H2O
– CnH2nOn +n O2 n CO2 + n H2O
– Respiratory Quotient CO2/O2• 0.67 Fatty acids• 1.00 Carbohydrates
• Adenine electron transporters– 6-C glucose6 NADH + 2 FADH2 (3:1)
– 16-C FA 32 NADH + 16 FADH2 (2:1)
Reactive oxygen
Acyl-CoA
Didehydroacyl-CoA
FAD
FADH2
Acyl-CoA dehydrogenase
Acyl-CoA
Didehydroacyl-CoA
O2
H2O2
Acyl-CoA oxidase
UQ
UQH2
• FADH2 oxidative stress– Succinate; saturated FA
– FADH2 + Fe3+ FADH • + H+ + Fe2+
– Fe2+ + H2O2Fe3+ + OH- + OH•
• FADH2 more completely reduces UQ than does NADH
FADH2
FAD
ETF:QO oxidoreductase
Lipogenesis
• De novo synthesis of fatty acid– Mostly liver (human; diet)– Cytoplasmic – ACC expression
• Malonyl-CoA– Carboxylation of Acetyl-CoA– Working substrate for FAS
• Fatty acid synthase– Sequentially transfers 2x C of Malonyl-CoA
to fatty acid chain– 16-C palmitoyl-CoA
Acetyl-CoA
Malonyl-CoA
Fatty acid
Acetyl-CoACarboxylase
Fatty AcidSynthase
Fatty AcidSynthase
Free fatty acids from triglycerides
• FFA cleavage from circulating lipoproteins– Protein/cholesterol carriers: Lipoprotein
• Density inversely correlates with lipid• Correlates with cholesterol/FA (except HDL)• VLDL & LDL to IDL
– Lipoprotein lipase (LPL)– HDL scavenges cholesterol & facilitates IDL
breakdown
• Triglycerides are retained in intracellular droplets– Don’t fit in membrane (no phosphate)– Not water soluble
Mitochondrial import of fatty acids
• FAAcyl-CoA Acyl-Carnitine Acyl-CoACytoplasm Intermembrane Matrix Working substrate
Boron & Boulpaep
Mitochondrial Transport
• Carrier protein (FABP)
• Long chain acyl-CoA synthetase (LCAS)
• Cross outer membrane via porin
• Convert to acylcarnitine in intermembrane
• Cross inner membrane via carnitine:acylcarnitine transferase
• Convert back to acyl-CoA in matrix
Regulation of lipid metabolism
• Substrate/Allosteric– Palmitate inhibits insulin signaling– Malonyl-CoA inhibits FA transport to mt
• Esp muscle, where ACC is mitochondrial
– Citrate activates acetyl-CoA carboxylase– Palmitoyl-CoA inhibits ACC
• Phosphorylation– AMP dependent kinase inhibits ACC– FAT/CD36 translocation
Allosteric regulation of FA metabolism
Citric Acid Cycle
CitrateNADH
Fatty Acid Synthetase
ACC Palmitoyl-CoAMalonyl-CoA
Acetyl-CoA
B-oxidation
Carnitine Palmitoyltransferase
Citric acid cycle products promote FA synthesisFA synthesis intermediaries inhibit FA import
Metabolic substrate selection
• Fatty acids– Energy dense (37
kJ/g)– Abundant (95%)– O2 delivery limit– Diffusion limit
• CHO– Low density (17 kJ/g)– Limited supply (5%)– Hydrated (67 wt%)– Lower O2 requirement– Readily available
McClelland, 2004Total metabolic rate (%VO2 max)
Lip
oly
sis
rate
(%
VO
2)
Glc
oly
sis
rate
(%
VO
2)
Substrate selection is an issue for fed/fasted state and for overall activity
Signaling integration
• Insulin/IGF-1– GLUT4 translocation– InsRIRS-1PI3KPDKPKB
--|GSK--|GS
mTORp70S6k/4EBP1
• Fatty acids– Inhibit pyruvate dehydrogenase– Inhibit insulin signaling
• FADAGPKC--|IRS-1
– Activate glycogen synthase
AMP kinase
• Allosterically activated by AMP– Adenylate kinase: 2 ADP AMP + ATP– ADP levels insensitive to energy state
PFKglycolysis
--|GSGlyconeogenesis
--|ACCMalonyl CoA--|CPTFA oxidation
--|ACClipogenesis
TSC2--|mTOR…protein synthesis
--|HMGCoAcholesterol synthesis
Hormonal Regulation
• Insulin• Glucagon• Thyroid hormone (Triiodothyronine )
– Steroid– T3TR
?CaMKKβ AMPK
Complex III/IV proteins, PGC-1
GLUT4
• Epinepherine (adrenaline)– Tissue specific: muscle/liver– AdbARGsACcAMPPKA
GPglycogenolysislipolysis
Peroxisome Proliferator
• PPAR-// (nuclear hormone, fatty acid sensor)
• Transcriptional complex– PPAR(//), PGC-1(/), NRF-1/2, CREB– Subunits of Complex I-V; FAT; GLUT4– Mitochondrial biogenesis
• Transcriptional Pathways– InsulinAkt--|FOXO--|PGC-1– Ca2+CaMKCREBPGC-1– Glucagon/stressPKACREBPGC-1– AMPK?PGC-1
• Post-Translational, tooUQ
UQH2
FADH2
FAD
Mitochondria
O2
H2O2
FADH2
FAD
Peroxisome
PPAR/PGC signaling
Fernandez-Marcos & Auwerx, 2011
Visceral FA subcutaneous
↑ insulin sensitivity
↓ plasma NEFA ↑ mitochondria
↑ fluid retention↑ congestive heart failure
Close relation between nutritive status and growth
– Insulin/IGF via PI3K• Glycolysis/lipolysis• Protein synthesis• Differentiation
– AMP-Kinase• Glycolysis/lipolysis• Protein synthesis• IGF inhibition
– AcetylCoA/CoA• Inhibit pyruvate dehydrogenase
– NADH converts oxaloacetate to malate, reduces TCA intermediaries, including citrate
• Growth hormone release• NAD+/NADH increases with cell confluence