Fatty Acid Metabolism

FA

TT

Y A

CID

SWhere & when are fatty acids

synthesized?

• Synthesis of Fatty Acids (FA) occurs primarily in the liver and lactating mammary gland, less so in adipose tissue

• FA are synthesized from acetyl CoA derived from excess protein and carbohydrate

• FA synthesis uses ATP and NADPH as energy sources

FA

TT

Y A

CID

SFA synthesis requires

lots of acetyl CoA• Transfer of acetyl CoA from mitochondria to cytosol

involves the citrate shuttle

• Occurs when citrate concentration in mitrochondria is high due to inhibition of isocitrate dehydrogenase by high levels of ATP. (Note: High ATP levels are also required for FA synthesis.)

FA

TT

Y A

CID

SFirst step in FA synthesis is synthesis of malonyl CoA

• Energy to form C-C bonds is supplied indirectly by synthesizing malonyl CoA from acetyl CoA using ATP and CO2

• The reaction is catalyzed by Acetyl CoA carboxylase

FA

TT

Y A

CID

SAcetyl CoA carboxylase

• A key regulatory enzyme activated by citrate to produce active polymers, and deactivated (depolymerized) by fatty acyl CoA

• Phosphorylation deactivates the enzyme (in response to epinephrine, c-AMP, protein kinase cascade)

• Dephosphorylation (due to insulin) activates the enzyme

FA

TT

Y A

CID

SFatty acid synthase

• Catalyzes reactions of FA synthesis

– It is a multienzyme complex in bacteria

– It is a dimer with multiple (7) activities in animals

• Growing FA chain is tethered by a 4'-phospho-pantetheine group (a component of CoA) to the acyl carrier protein (ACP) subunit

FA

TT

Y A

CID

SSteps in FA synthesis

The multifunctional fatty acyl synthase molecule has multiple enzymic domains that carry out the various catalytic reactions

FA

TT

Y A

CID

SSteps in FA synthesis

1) Acetyl CoA + ACP-SH Acetyl-S-ACP + CoA

(primes the system)

FA

TT

Y A

CID

SSteps in FA synthesis

2) Acetyl-S-ACP + Enzyme-SH Acetyl-S-Enzyme + ACP-SH

FA

TT

Y A

CID

SSteps in FA synthesis

3) ACP-SH + Malonyl CoA Malonyl-S-ACP + CoA

FA

TT

Y A

CID

SSteps in FA synthesis

4) Malonyl-S-ACP + Acetyl-S-Enz Acetoacetyl-S-ACP + CO2

FA

TT

Y A

CID

SSteps in FA synthesis

5) Acetoacetyl-S-ACP + NADPH + H+ β-hydroxybutyryl-S-ACP + NADP+

FA

TT

Y A

CID

SSteps in FA synthesis

6) β-hydroxybutyryl-S-ACP crotonyl-S-ACP + H2O

FA

TT

Y A

CID

SSteps in FA synthesis

7) crotonyl-S-ACP + NADPH + H+ butyryl-S-ACP + NADP+

FA

TT

Y A

CID

SSteps in FA synthesis

Repeat steps 3 through 7. . .

for seven cycles, ultimately yielding palmitate

FA

TT

Y A

CID

S

FA synthesis

• After 7 cycles, palmitoyl-S-ACP is produced and palmitate is released by palmitoyl thioesterase

• Overall reaction is:

8 acetyl CoA + 14 NADPH + 14H+ + 7ATP

palmitate + 8CoA + 14 NADP+ + 7ADP + 7 Pi + 7H2O

FA

TT

Y A

CID

S

FA synthesis

•Further elongation and desaturation of palmitate and dietary FAs (if required) occurs in mitrochondria and ER by diverse enzymes

FA

TT

Y A

CID

SFA synthesis

• Sources of NADPH for FA synthesis are the hexose monophosphate pathway and the malic enzyme reaction that converts malate to pyruvate + NADPH in the cytosol

FA

TT

Y A

CID

S

Fatty Acid Oxidation

FA

TT

Y A

CID

SBeta-oxidation of fatty acids

• β-oxidation of FA produces acetyl CoA and NADH and FADH2, which are sources of energy (ATP)

• First, FA are converted to acyl CoA in the cytoplasm:

FA

TT

Y A

CID

S

• Where does beta-oxidation of fatty acids take place?

FA

TT

Y A

CID

SCarnitine shuttle

• For transport into mitochondria, CoA is replaced with carnitine by acylcarnitine transferase I

• Inside mitochondria a corresponding enzyme (II) forms acyl CoA

• Malonyl CoA inhibits acylcarnitine transferase I

• So, when FA synthesis is active, FA are not transported into mitochondria

• Defects in FA transport (including carnitine deficiency) are known

FA

TT

Y A

CID

SReactions of beta-

oxidation

• The cycle of reactions is repeated until the fatty acid is converted to acetyl CoA

FA

TT

Y A

CID

SEnergy yield from beta-oxidation

of fatty acids

• For palmitate (16:0) the overall reaction is:

Palmitate + 8CoA + 7NAD+ + 7FAD + 7H2O

8 Acetyl CoA + 7NADH + 7FADH2 + 7 H+

• Energy yield as ATP for palmitate:

7 FADH2 = 1.5 x 7 = 10.5 ATP

7 NADH = 2.5 x 7 = 17.5 ATP

8 Acetyl CoA = 10 x 8 = 80 ATP

Total: 108 ATP• But, two high energy bonds used in acyl CoA formation, so

overall yield is 106 ATP. Why do we subtract two ATPs?

FA

TT

Y A

CID

SEnergy yield from beta-oxidation

of fatty acids

• Energy yield as ATP for palmitic acid:

7 FADH2 = 1.5 x 7 = 10.5 ATP

7 NADH = 2.5 x 7 = 17.5 ATP

8 Acetyl CoA = 10 x 8 = 80 ATP

Total: 108 ATP• Two high energy bonds used in acyl CoA formation, so overall

yield is 106 ATP

FA

TT

Y A

CID

SWhy do the Lippincott and Garrett & Grisham texts give different ATP yields for complete

oxidation of palmitate?• Beta oxidation occurs in mitochondria, so NADH and FADH2 can be used directly

in electron transport, and acetyl CoA can also be used directly for production of energy via TCA cycle.

• Theoretical yield of ATP from NADH or FADH2:

2 ATP per FADH2

3 ATP per NADH

• Energy yield as ATP for palmitic acid:

7 FADH2 = 2 x 7 = 14 ATP

7 NADH = 3 x 7 = 21 ATP

8 Acetyl CoA = 12 x 8 = 96 ATP

Total: 131 ATP

• Two high energy bonds used in fatty acyl CoA (palmitoyl CoA) formation, so overall yield is 129 ATP (according to the Lippincott book)

FA

TT

Y A

CID

SActual yield of ATP from NADH or FADH2 is thought to be lower than the theoretical yield because:

– Membranes leak some H+ without forming ATP

– Some of the proton gradient drives other mitochondrial processes

• So, actual yield is thought to be closer to:

1.5 ATP per FADH2

2.5 ATP per NADH

• Actual energy yield as ATP for palmitic acid is therefore:

7 FADH2 = 1.5 x 7 = 10.5 ATP

7 NADH = 2.5 x 7 = 17.5 ATP

8 Acetyl CoA = 10 x 8 = 80 ATP

Total: = 108 ATP

Minus the two high energy bonds used in fatty acyl CoA formation

= 106 ATP

FA

TT

Y A

CID

SBeta-oxidation of odd-chain fatty acids

• Odd-chain FA degradation yields acetyl CoAs and one propionyl CoA

• Propionyl CoA is metabolized by carboxylation to methylmalonyl CoA (carboxylase is a biotin enzyme)

• Methyl carbon is moved within the molecule by methylmalonyl CoA mutase (one of only two Vitamin B12 cofactor enzymes) to form succinyl CoA

FA

TT

Y A

CID

SBeta-oxidation of unsaturated fatty acids

• Unsaturated FA yield a bit less energy than saturated FA because they are already partially oxidized

• Less FADH2 is produced

FA

TT

Y A

CID

SOxidation of FA in peroxisomes

• Very-long-chain FA (VLCA; > 20 carbons) are initially oxidized in peroxisomes

• This process does not generate energy

• Shortened FA-CoA can subsequently be imported into mitochondria for energy production

FA

TT

Y A

CID

SBeta-oxidation of FA

in peroxisomes

• Reaction requires FAD• FADH2 that is generated is

oxidized by molecular oxygen (generating H2O2)

• Diseases:– Deficiency in peroxisomes

(Zellweger syndrome)– Defect in peroxisomal

activation of VLCFA (X-linked adrenoleuko-dystrophy)

– Lead to accumulation of VLCFA

FA

TT

Y A

CID

SAlpha-oxidation of FA

• Branched-chain FA like phytanic acid cannot be oxidized by beta-oxidation

• Instead, hydroxylated on alpha carbon

• Genetic deficiency (Refsum disease)

FA

TT

Y A

CID

S

Are fatty acids glucogenic?

• Fatty acids are not glucogenic in animals

• Why can’t we make glucose from fatty acids?

• Why are the statements above only ~99% true?

FA

TT

Y A

CID

S

Pyruvate dehydrogenase reaction is irreversible

FA

TT

Y A

CID

S

Pyruvate dehydrogenase reaction is irreversible

FA

TT

Y A

CID

SKetone bodies

• Excess acetyl CoA (from FA or carbohydrate degradation) is converted in liver to ketone bodies: acetoacetate, acetone, and β-hydroxybutyrate

• Ketone bodies are soluble in blood and can be taken up and used by various tissues (muscle, heart, renal cortex) to regenerate acetyl CoA for energy production via the TCA cycle

• Even brain can use ketone bodies as their concentrations in blood rise enough

FA

TT

Y A

CID

SKetone bodies

• Acetoacetyl CoA is formed by incomplete FA degradation or by condensation of two acetyl CoAs by thiolase

• Acetoacetyl CoA condenses with a third acetyl CoA to form hydroxymethylglutaryl CoA (HMG-CoA)

• HMG-CoA is cleaved to produce acetoacetate + acetyl CoA

• Reduction of acetoacetate to β-hydroxybutyrate, or spontaneous decarboxylation to acetone, produces the other two ketone bodies

FA

TT

Y A

CID

S• In tissues that use ketone bodies, acetoacetate is

condensed with CoA by transfer from succinyl CoA• acetoacetyl CoA can then be converted to two

acetyl CoAs

FA

TT

Y A

CID

S

Ketone bodies

• Excessive ketone bodies can be produced in diabetes mellitus or starvation (a lot of acetyl CoA in liver)

• When rate of production exceeds utilization, ketonemia, ketonuria, and acidemia can result