Chapter 16 (Part 3) Fatty acid Synthesis. Fatty Acid Synthesis In mammals fatty acid synthesis...

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