Chapter 16 (Part 3)

Fatty acid Synthesis

Fatty Acid Synthesis• In mammals fatty acid synthesis

occurs primarily in the liver and adipose tissues

• Also occurs in mammary glands during lactation.

• Fatty acid synthesis and degradation go by different routes

• There are four major differences between fatty acid breakdown and biosynthesis

The differences between fatty acid biosynthesis and

breakdown • Intermediates in synthesis are linked

to -SH groups of acyl carrier proteins (as compared to -SH groups of CoA)

• Synthesis in cytosol; breakdown in mitochondria

• Enzymes of synthesis are one polypeptide

• Biosynthesis uses NADPH/NADP+; breakdown uses NADH/NAD+

ACP vs. Coenzyme A

•Intermediates in synthesis are linked to -SH groups of acyl carrier proteins (as compared to -SH groups of CoA)

Fatty Acid Synthesis Occurs in the Cytosol

• Must have source of acetyl-CoA• Most acetyl-CoA in mitochondria• Citrate-malate-pyruvate shuttle provides

cytosolic acetate units and reducing equivalents for fatty acid synthesis

Citrate synthaseCitrate Lyase

Malate dehydrogenase

Malate EnzymePyruvate

carboxylase

Fatty Acid Synthesis• Fatty acids are built from 2-C units

derived from acetyl-CoA• Acetate units are activated for transfer

to growing FA chain by conversion to malonyl-CoA

• Decarboxylation of malonyl-CoA and reducing power of NADPH drive chain growth

• Chain grows to 16-carbons (eight acetyl-CoAs)

• Other enzymes add double bonds and more Cs

Acetyl-CoA Carboxylase

• The "ACC enzyme" commits acetate to fatty acid synthesis

• Carboxylation of acetyl-CoA to form malonyl-CoA is the irreversible, committed step in fatty acid biosynthesis

Acetyl-CoA + HCO3- + ATP malonyl-CoA + ADP

Acetyl-CoA

Carboxylase

Regulation of Acetyl-CoA Carboxylase

(ACCase)• ACCase forms long, active

filamentous polymers from inactive protomers

• Accumulation of palmitoyl-CoA (product) leads to the formation of inactive polymers

• Accumulation of citrate leads to the formation of the active polymeric form

• Phosphorylation modulates citrate activation and palmitoyl-CoA inhibition

• Unphosphorylated ACCase has low Km for citrate and is active at low citrate

• Unphosphorylated ACCase has high Ki for palmitoyl-CoA and needs high palmitoyl-CoA to inhibit

• Phosphorylated E has high Km for citrate and needs high citrate to activate

• Phosphorylated E has low Ki for palmitoyl-CoA and is inhibited at low palmitoyl-CoA

Regulation of Acetyl-CoA Carboxylase (ACCase)

Fatty Acid Synthesis

• Step 1: Loading – transferring acetyl- and malonyl- groups from CoA to ACP

• Step 2: Condensation – transferring 2 carbon unit from malonyl-ACP to acetyl-ACP to form 2 carbon keto-acyl-ACP

• Step 3: Reduction – conversion of keto-acyl-ACP to hydroxyacyl-ACP (uses NADPH)

• Step 4: Dehydration – Elimination of H2O to form Enoyl-ACP

• Step 5: Reduction – Reduce double bond to form 4 carbon fully saturated acyl-ACP

Step 1: Loading Reactions

H3C C

O

S CoA C C

O

S CoACO

O

H

H HS-ACPHS-ACP

HS-CoAHS-CoA

H3C C

O

S ACP C C

O

S ACPCO

O

H

H

acetyl-CoA

acetyl-ACP

malonyl-CoA

malonyl-ACP

acetyl-CoA:ACPtransacylase

malonyl-CoA:ACPtransacylase

Step 2: Condensation Rxn

H3C C

O

S ACP

HS-Ketoacyl-ACP Synthase

HS-ACP

H3C C

O

S ketoacyl-ACP SynthaseC C

O

S ACPCO

O

H

H

CO2

C C

O

S ACPC

H

H

O

H3C

acetyl-ACP

malonyl-ACP

+

keto-ACP synthase

acetoacetyl-ACP

Step 3: Reduction

C C

O

S ACPC

H

H

O

H3C

NADP+

C C

O

S ACPC

H

H

OH

H3C

H

acetoacetyl-ACP

-hydroxybutyryl-ACP

NADPH + H+

Ketoacyl-ACP Reductase

Step 4: Dehydration

C C

O

S ACPC

H

trans-enoyl-ACP

H3C

H

H20

-hydroxyacyl-ACPdehydrase

C C

O

S ACPC

H

H -hydroxyacyl-ACP

OH

H3C

H

Step 5: Reduction

C C

O

S ACPC

H

H3C

H

NADP+

C C

O

S ACPC

H

H3C

H

H

H

trans-enoyl-ACP

enoyl-ACP reductase

NADPH + H+

trans-enoyl-ACP

Step 6: next condensation

C C

O

S ACPC

H

H

H3C

H

HHS-Ketoacyl-ACP Synthase

HS-ACP

C C

O

S KASC

H

H

H3C

H

H

C C

O

S ACPCO

O

H

H

CO2

C C

O

S ACP

H

H

C C

O

C

H

H

H3C

H

H

butyryl-ACP

malonyl-ACP

+

keto-ACP synthase

ketoacyl-ACP

Termination of

Fatty Acid Synthesis

C C

O

S ACPH3C

H

H

HS-ACP

C C

O

OH3C

H

H

AMP + PPi

C C

O

SH3C

H

H

CoA

14

Palmitoyl-ACP

14

Palmitic Acid

14

Thioesterase

ATP + HS-CoA

Palmitoyl-CoA

Acyl-CoA synthetase

Organization of Fatty Acid Synthesis Enzymes• In bacteria and plants, the fatty acid

synthesis reactions are catalyzed individual soluble enzymes.

• In animals, the fatty acid synthesis reactions are all present on multifunctional polypeptide.

• The animal fatty acid synthase is a homodimer of two identical 250 kD polypeptides.

Animal Fatty Acid Synthase

Further Processing of Fatty acids: Desaturation and Elongation

Regulation of FA Synthesis

• Allosteric modifiers, phosphorylation and hormones

• Malonyl-CoA blocks the carnitine acyltransferase and thus inhibits beta-oxidation

• Citrate activates acetyl-CoA carboxylase

• Fatty acyl-CoAs inhibit acetyl-CoA carboxylase

• Hormones regulate ACC• Glucagon activates lipases/inhibits ACC• Insulin inhibits lipases/activates ACC

Allosteric regulation of fatty acid synthesis occurs at ACCase and the carnitine acyltransferase

Glucagon inhibits fatty acid synthesis while increasing lipid breakdown and fatty acid -oxidation

Insulin prevents action of glucagon