LIPID BIOSYNTHESIS

• Fatty acid biosynthesis-basic fundamentals

• Fatty acid biosynthesis-elongation and desaturation

• Triacylglycerols

• Phospholipids

• Cholesterol

• Cholesterol metabolism

Fatty Acid Biosynthesis

• Cytosol• Requires NADPH• Acyl carrier protein• D-isomer

• CO2 activation

• Keto saturated

• Mitochondria

• NADH, FADH2

• CoA • L-isomer

• No CO2

• Saturated keto

Beta OxidationSynthesis

Rule:

Fatty acid biosynthesis is a stepwise assemblyof acetyl-CoA units (mostly as malonyl-CoA) ending with palmitate (C16 saturated)

Fatty acid biosynthesis is a stepwise assemblyof acetyl-CoA units (mostly as malonyl-CoA) ending with palmitate (C16 saturated)

Activation

Elongation

Termination

3 Phases

CH3C~SCoA

O

ACTIVATION

-OOC-CH2C~SCoA

O

HCO3-

NN

O

SCH2CH2CH2CH2CO

HH

LYS

NHCH2CH2CH2CH2 ENZYME

NN

O

SCH2CH2CH2CH2CO

HC

O

O

Carboxybiocytin

Biotin

active carbon

Acetyl-CoA carboxylase

CO2

Biocytin

Cofactor

ATP

ADP + Pi

Acetyl-CoA CarboxylaseThe rate-controlling enzyme of FA synthesis

• In Bacteria -3 proteins (1) Carrier protein with Biotin (2) Biotin carboxylase (3) Transcarboxylase

• In Eukaryotes - 1 protein (1) Single protein, 2 identical polypeptide chains

• (2) Each chain Mwt = 230,000 (230 kDa)(3) Dimer inactive (4)

Activated by citrate which forms filamentous form of protein that can be seen in the electron microscope

Yeast Fatty Acid Synthase Complex

2,500 kDa Multienzyme Complex

6 molecules of 2 peptide chains called A and B

(66)A: (185,000) Acyl Carrier protein -ketoacyl-ACP synthase (condensing enzyme) -ketoacyl-ACP reductase

B: (175,000) -hydroxy-ACP dehydrase enoyl-ACP reductase palmitoyl thioesterase

Fatty AcidSynthaseComplex

Acyl carrier protein10 kDa

Cysteamine

Phosphopantetheine

HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-P-O-CH2

O O O

OH H

H

HO CH3

H

O

OO Adenine

O-P-OO

OH

OHH

Coenzyme A

Acyl Carrier ProteinAcyl Carrier Protein

HS-CH2-CH2-N-C-CH2-CH2-N-C-C-C-CH2-O-P-O-CH2-Ser-O O O

OH H

H

HO CH3

H

ACP

Overall Reaction

CH3C~SCoA

O

CH3C-

O

CH2C~S-

O

ACP

HS-CoACO2

NOTE:

Malonyl-CoA carbons become new COOH end

Nascent chain remains tethered to ACP

Acyl CarrierProtein

CO2, HS-CoA are released at each condensation

Malonyl-CoA + ACP

-OOC-CH2C~S-

O

ACP + HS-CoA

Initiation

CH3C-

O

CH2C~S-

O

ACP

NADPH

CH3CH2CH2C~S-

O

ACP

CH3C- CH2C~S-

O

ACP

HO

H

CH3C- = C- C~S-

O

ACPH

H

-H2O

NADPH

-Carbon Elongation

D isomer

Reduction

Dehydration

Reduction

-Ketoacyl-ACP reductase

-Hydroxyacyl-ACP dehydrase

Enoyl-ACP reductase

-KS

CO2

-S-ACP

TERMINATION Ketoacyl ACPSynthase

Free to bindMalonyl-CoA

Transfer to KS

Split out CO2

Transfer to Malonyl-CoA

-CH2CH2CH2C~S-

O

ACP

When C16 stage is reached, instead of transferring to KS,the transfer is to H2O and the fatty acid is released

ACP

KS -SH

HSAcetyl-CoA

CoA-SH

-C-CH3

OS

KS S-C-CH3

OKS -SH

SH

CoA-SH

Malonyl-CoA

S -C-CH2-COO-

O

CO2C=O

CH2

C=O

CH3

S

O

CH3-CH -CH2-C-S

OH

OCH3-CH=CH-C-S

OCH3-CH2-CH2-C-S

S-C-CH2-CH2-CH3

O

KS

KS

NADP+

NADPH H+

NADPH H+

NADP+

H2O

Initiation or priming

Elongation

Fatty Acid SynthaseFatty Acid Synthase

-Ketoacyl-ACP reductase

-Ketoacyl-ACP reductase

-Hydroxyacyl-ACP dehydrase

-Hydroxyacyl-ACP dehydrase

Enoyl-ACP reductase

Enoyl-ACP reductase

-Keto-ACP synthase (condensing enzyme)

-Keto-ACP synthase (condensing enzyme)

Malonyl-CoA-ACP transacylase

Malonyl-CoA-ACP transacylase

Acetyl-CoA-ACP transacylase

Acetyl-CoA-ACP transacylase

-Ketoacyl-ACP synthase

-Ketoacyl-ACP synthase

TE

TE

Substrate Entry Reduction Thioesterasepalmitate release

Substrate EntryReductionThioesterasepalmitate release

ACP

ACP

HSSH

AT

AT

CH2

CH2

HSSH

CE

CE

MT

MTER

ERKR

KR DH

DH

Translocation

Translocation

Overall Reactions

Acetyl-CoA + 7 malonyl-CoA + 14NADPH + 14H+

Palmitate + 7CO2 + 14NADP+ + 8 HSCoA + 6H2O

7 Acetyl-CoA + 7CO2 + 7ATP 7 malonyl-CoA +7ADP + 7Pi + 7H+

8 Acetyl-CoA + 14NADPH + 7H+ + 7ATP Palmitate + 14NADP+ + 8 HSCoA + 6H2O + 7ADP +

7Pi

7H+

PROBLEM:

Fatty acid biosynthesis takes place in thecytosol. Acetyl-CoA is mainly in the

Mitochondria

How is acetyl-CoA made available to the cytosolicfatty acyl synthase?

SOLUTION:

Acetyl-CoA is delivered to cytosol from the mitochondria as CITRATE

acetyl-CoA

COO

COO

HO-C-COO

CH2

CH2

COO

COO

HO-C-COO

CH2

CH2

Citrate lyase

Acetyl-CoA

COO

COOCH2

C=O

COO

COOCH2

HO-C-H

NADH

OAA

L-malate

C=OCOO

CH3

NADP+

NADPH + H+

L-malate

mitochondria

CytosolPyruvate

Malic enzymeOAA

Acetyl-CoACO2

PyrCO2

Malatedehydrogenase

HS-CoA

Post-Synthesis Modifications

• C16 satd fatty acid (Palmitate) is the product

• Elongation

• Unsaturation

• Incorporation into triacylglycerols

• Incorporation into acylglycerol phosphates

• C16 satd fatty acid (Palmitate) is the product

• Elongation

• Unsaturation

• Incorporation into triacylglycerols

• Incorporation into acylglycerol phosphates

Elongation of Chain (two systems)

HS-CoA

R-CH2CH2CH2C~SCoAO

OOC-CH2C~SCoA

OCO2

Malonyl-CoA* (cytosol)

R-CH2CH2CH2CCH2C~SCoAO O

O R-CH2CH2CH2CH2CH2C~SCoA

NADPH NADH

1

- H2O2

NADPH3

Elongation systems arefound in smooth ER andmitochondria

CH3C~SCoA

OAcetyl-CoA(mitochondria)

DesaturationRules:The fatty acid desaturation system is in the smooth membranes of the endoplasmicreticulum

There are 4 fatty acyl desaturase enzymes in mammals designated 9 , 6, 5, and 4 fattyacyl-CoA desaturase

Mammals cannot incorporate a double bondbeyond 9; plants can.

Mammals can synthesize long chain unsaturated fatty acids using desaturation and elongation

The Desaturase System requires O2 andresembles an electron transport system

Rule:

NADHCyt b5 reductase

Cyt b5O2

Saturated FA-CoA

(FAD)

NOTE:

1. System is in ER membrane

2. Both NADH and the fatty acid contribute electrons

3. Fatty acyl desaturase is considered a mixed function oxidase

2

2

3

1

Desaturase

Cyt b5

reductase

Cyt b5

C18-stearoly-CoA + O2 + 2H+

C18 9-oleyl-CoA + 2H2O

2 cyt b5 Fe2+ 2 cyt b5 Fe2+

2H+ + cyt b5 reductaseFAD

cyt b5 reductase FADH2

NADH + H+NAD+

Fatty acid desaturation systemFatty acid desaturation system

Desaturase

Palmitate

Stearate

Oleate

Linoleate

-Linolenate-Linolenate

Eicosatrienoate

Arachidonate

18:3(9,12,15)

18:2(9,12)

18:3(6,9,12)

16:0

18:0

Elongase

18:1(9)

Palmitoleate

16:1(9)Desaturase

Desaturase

Desaturase

Desaturase

Desaturase

Desaturase

Elongase

20:3(8,11,14)

20:4(5,8,11,14)

Other lipids

Permittedtransitionsin mammalsEssential

fatty acid