Fatty Acid Handling

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