Section 7. Lipid Metabolism Fats: fatty acid biosynthesis 11/04/05.

Section 7. Section 7. Lipid MetabolismLipid Metabolism

Section 7. Section 7. Lipid MetabolismLipid Metabolism

Fats: fatty acid biosynthesisFats: fatty acid biosynthesisFats: fatty acid biosynthesisFats: fatty acid biosynthesis

11/04/0511/04/05

Oxidation of Fatty Acids Other Than PalmitateOxidation of Fatty Acids Other Than PalmitateOxidation of Fatty Acids Other Than PalmitateOxidation of Fatty Acids Other Than Palmitate

• Any even number of saturated carbons does not require any Any even number of saturated carbons does not require any additional enzymes. Products are as for palmitate.additional enzymes. Products are as for palmitate.

• An odd number of saturated carbons does not require any An odd number of saturated carbons does not require any additional enzymes. Same products plus one propionyl CoA.additional enzymes. Same products plus one propionyl CoA.

• Unsaturated fatty acids require additional enzymes. Same Unsaturated fatty acids require additional enzymes. Same products, but less energy, compared toproducts, but less energy, compared to

saturated fatty acid the same length.saturated fatty acid the same length.• See table 12.1 for a list of common See table 12.1 for a list of common

fatty acids.fatty acids.

• Any even number of saturated carbons does not require any Any even number of saturated carbons does not require any additional enzymes. Products are as for palmitate.additional enzymes. Products are as for palmitate.

• An odd number of saturated carbons does not require any An odd number of saturated carbons does not require any additional enzymes. Same products plus one propionyl CoA.additional enzymes. Same products plus one propionyl CoA.

• Unsaturated fatty acids require additional enzymes. Same Unsaturated fatty acids require additional enzymes. Same products, but less energy, compared toproducts, but less energy, compared to

saturated fatty acid the same length.saturated fatty acid the same length.• See table 12.1 for a list of common See table 12.1 for a list of common

fatty acids.fatty acids.propionyl CoA

SCoACH3CH2 C

O

propionyl CoA

SCoACH3CH2 C

O

Arachidonate (eicosatetraenoate) 20:4 all cis-5,8,11,14

–CH2

CH2

CH2

CH2CH3 CH2

CHCH

CH2

CH2

CH2

CCHCH

CH2

CH CH CH

CH2

O

O

CH

Arachidonate (eicosatetraenoate) 20:4 all cis-5,8,11,14

–CH2

CH2

CH2

CH2CH3 CH2

CHCH

CH2

CH2

CH2

CCHCH

CH2

CH CH CH

CH2

O

O

CH

1

Double Bonds in Odd-Numbered PositionsDouble Bonds in Odd-Numbered Positions

• An odd-numbered double bond An odd-numbered double bond moves into the 3-position, which moves into the 3-position, which is not a substrate for the principal is not a substrate for the principal enzymes of the cycle.enzymes of the cycle.

• An isomerase moves it to the 2-An isomerase moves it to the 2-position. position.

• Moving the double bond is Moving the double bond is energy neutral, but one less energy neutral, but one less FADHFADH22 is made because one is made because one

acyl CoA dehydrogenase step is acyl CoA dehydrogenase step is “skipped.”“skipped.”

• As acetyl CoA’s are removed As acetyl CoA’s are removed from an unsaturated fatty acid, from an unsaturated fatty acid, double bonds move into or near double bonds move into or near the active sites of the the active sites of the --oxidation cycle enzymes.oxidation cycle enzymes.

2(p 610)

16:1 cis-9

Double Bonds in Even- Numbered Positions.Double Bonds in Even- Numbered Positions.

• An even-numbered double bond An even-numbered double bond moves to the 4-position.moves to the 4-position.

• Acyl CoA dehydrogenase oxidizes Acyl CoA dehydrogenase oxidizes it producing FADHit producing FADH22 and the non- and the non-substrate dienoyl CoA.substrate dienoyl CoA.

• The additional enzyme 2,4-dienoyl The additional enzyme 2,4-dienoyl CoA reductase uses NADPH to CoA reductase uses NADPH to produce a produce a position double bond.position double bond.

• The additional enzyme cis The additional enzyme cis 33-enoyl -enoyl CoA isomerase (see above) moves CoA isomerase (see above) moves the double bond from the 3- to the the double bond from the 3- to the 2-position.2-position.

• NADH is produced after hydration, NADH is produced after hydration, and then acetyl CoA, as usual (not and then acetyl CoA, as usual (not shown).shown).

• The net effect for the cycle is the The net effect for the cycle is the equivalent of one less NADH.equivalent of one less NADH.

acyl CoA dehydrogenase

2,4-dienoyl CoA reductase

cis 3-enoyl CoA isomerase

FAD

FADH2

acyl CoA

NADPH + H+

NADP+

2,4-dienoyl CoA

cis -enoyl CoA

4

R CoA

R CoACH2 CH C

O

SCH CH2

CH2CH2

C

O

SCH CH

R CoACH CH C

O

SCH CH

R CoACH CH C

O

SCH2CH2

3

4

Fig. 22.10modified

+ acetyl CoA + NADH (no FADH2)

+ 3 acetyl CoA + 3 FADH2 + 3 NADH

Example: Linoleoyl CoA

C18:2 cis-9,cis-12

Ketone Ketone BodiesBodies

• Production is enhanced by low carbohydrate (diabetes, starvation) and/or Production is enhanced by low carbohydrate (diabetes, starvation) and/or low Olow O22 (hypoventilation) (hypoventilation)

• General anesthesia: COGeneral anesthesia: CO22 up, pH down, ketone bodies up. up, pH down, ketone bodies up.• Volatile acetone formation is non-enzymatic.Volatile acetone formation is non-enzymatic.

• The liver normally The liver normally converts acetyl CoA to converts acetyl CoA to ketone bodies that are ketone bodies that are used by peripheral used by peripheral tissues.tissues.

• See fig. 22.19See fig. 22.19

CH2

C

C S

O

O

CH3

CoA

CH3 C S

O

CoA

–

CH2

C

C S

O

OH CH3

CH2

COO

CoA

CH2

C

CH3

O

C

OO –

2

CoACoA

acetyl CoA+ H2O

acetyl CoAacetoacetyl CoA

3-hydroxy-3-methylglutaryl CoA

acetyl CoA

acetoacetate

acetone

3-D-hydroxybutyrate

H+ + NADHNAD+

–

+ CO2CH2

CH

CH3

OH

C

OO

CH3

C

CH3O

5

Acetoacetate Acetoacetate UtilizationUtilization

Acetoacetate Acetoacetate UtilizationUtilization

• In peripheral tissues, In peripheral tissues, the ketone body the ketone body acetoacetate is acetoacetate is activated, and activated, and converted back to converted back to acetyl CoA.acetyl CoA.

• 3-hydroxybutyrate 3-hydroxybutyrate and acetoacetate are and acetoacetate are favored over favored over glucose by the renal glucose by the renal cortex and cardiac cortex and cardiac muscle. muscle.

• In peripheral tissues, In peripheral tissues, the ketone body the ketone body acetoacetate is acetoacetate is activated, and activated, and converted back to converted back to acetyl CoA.acetyl CoA.

• 3-hydroxybutyrate 3-hydroxybutyrate and acetoacetate are and acetoacetate are favored over favored over glucose by the renal glucose by the renal cortex and cardiac cortex and cardiac muscle. muscle. Fig. 22.20

6

Summary of Fatty Acid BiosynthesisSummary of Fatty Acid BiosynthesisSummary of Fatty Acid BiosynthesisSummary of Fatty Acid Biosynthesis

• When the cell energy level is high, rather than When the cell energy level is high, rather than being used by the Krebs cycle, acetyl CoA is being used by the Krebs cycle, acetyl CoA is transferred from the mitochondrial matrix to the transferred from the mitochondrial matrix to the cytosol.cytosol.

• In the cytosol, acetyl CoA is converted to malonyl In the cytosol, acetyl CoA is converted to malonyl CoA, which is used by fatty acyl synthase (FAS) CoA, which is used by fatty acyl synthase (FAS) for the synthesis of palmitate.for the synthesis of palmitate.

• Palmitate is transported to adipose tissue and Palmitate is transported to adipose tissue and used to synthesize triacylglycerol.used to synthesize triacylglycerol.

• The palmitate synthetic reactions are reversals of The palmitate synthetic reactions are reversals of the degradative reactions, but the enzymes, the degradative reactions, but the enzymes, cofactors and locations are different.cofactors and locations are different.

• When the cell energy level is high, rather than When the cell energy level is high, rather than being used by the Krebs cycle, acetyl CoA is being used by the Krebs cycle, acetyl CoA is transferred from the mitochondrial matrix to the transferred from the mitochondrial matrix to the cytosol.cytosol.

• In the cytosol, acetyl CoA is converted to malonyl In the cytosol, acetyl CoA is converted to malonyl CoA, which is used by fatty acyl synthase (FAS) CoA, which is used by fatty acyl synthase (FAS) for the synthesis of palmitate.for the synthesis of palmitate.

• Palmitate is transported to adipose tissue and Palmitate is transported to adipose tissue and used to synthesize triacylglycerol.used to synthesize triacylglycerol.

• The palmitate synthetic reactions are reversals of The palmitate synthetic reactions are reversals of the degradative reactions, but the enzymes, the degradative reactions, but the enzymes, cofactors and locations are different.cofactors and locations are different.

7

Reactions Reactions on the on the right are right are catalyzed catalyzed by FASby FASin the in the cytosol.cytosol.

CompareComparedegradationdegradationandandsynthesissynthesisstructures.structures.

8

Fig. 22.2

Citrate Shuttle Transfers Acetyls to CytosolCitrate Shuttle Transfers Acetyls to Cytosol

• High [ATP] inhibits the Krebs cycle; [citrate] increases.High [ATP] inhibits the Krebs cycle; [citrate] increases.• Citrate translocase enables citrate and pyruvate to cross the Citrate translocase enables citrate and pyruvate to cross the

mitochondrial inner membrane. CoA does not cross (remember mitochondrial inner membrane. CoA does not cross (remember acyl CoA / acyl carnitine).acyl CoA / acyl carnitine).

• NADPH is made at the expense of NADH in the cytosol.NADPH is made at the expense of NADH in the cytosol.

Fig. 22.25

9

+CO2

ATP + CO2

Activation by Acetyl CoA CarboxylaseActivation by Acetyl CoA Carboxylase

• In the cytosol, acetyl CoA is carboxylated to make the activated precursor, In the cytosol, acetyl CoA is carboxylated to make the activated precursor, malonyl CoA.malonyl CoA.

• This is the committed step in fatty acid biosynthesisThis is the committed step in fatty acid biosynthesis..

• ATP provides energy.ATP provides energy.• Biotin is a cofactor.Biotin is a cofactor.• Two sequential reactions occur in the active site.Two sequential reactions occur in the active site.

biotin-Enz + ATP + HCObiotin-Enz + ATP + HCO33-- CO CO22~biotin-Enz + ADP + Pi~biotin-Enz + ADP + Pi

COCO22~biotin-Enz + acetyl CoA ~biotin-Enz + acetyl CoA malonyl CoA + biotin-Enz malonyl CoA + biotin-Enz

+ ATP + HCO3–

CH3 C SO

CoA CH2 C SO

CO

O

CoA

–

+ ADP + Pi

acetyl CoA malonyl CoA

10

(p 617)

(p 618)

Biotin: a COBiotin: a CO22 Carrier CarrierBiotin: a COBiotin: a CO22 Carrier Carrier• ATP reacts first ATP reacts first

providing energy providing energy to bind and to bind and activate HCOactivate HCO33

--..

• Next acetyl CoA Next acetyl CoA binds and the binds and the

activated COactivated CO22-- is is

transferred to the transferred to the acetyl group.acetyl group.

• ATP reacts first ATP reacts first providing energy providing energy to bind and to bind and activate HCOactivate HCO33

--..

• Next acetyl CoA Next acetyl CoA binds and the binds and the

activated COactivated CO22-- is is

transferred to the transferred to the acetyl group.acetyl group.

Figs. 24-10 and 24-11 (Stryer 4Figs. 24-10 and 24-11 (Stryer 4thth))Figs. 24-10 and 24-11 (Stryer 4Figs. 24-10 and 24-11 (Stryer 4thth))

11

Fatty Acid Synthase ReactionsFatty Acid Synthase Reactions

Condensation forms 4 carbon unit on acyl carrier protein (ACP).Condensation forms 4 carbon unit on acyl carrier protein (ACP).

Reduction of ketone to hydroxyl by NADPH.Reduction of ketone to hydroxyl by NADPH.12

Fig. 22.22Fig. 22.22

CE

CE +

(KR)

Fatty Acid Synthase Reactions, con’tFatty Acid Synthase Reactions, con’t

• Dehydration produces a double bond.Dehydration produces a double bond.• Reduction to a saturated 4 carbon fatty acid chain.Reduction to a saturated 4 carbon fatty acid chain.13

Fig. 22.22Fig. 22.22

(DH) (ER)

FAS is FAS is a a

DimerDimer

FAS is FAS is a a

DimerDimer

• Malonyl transfer (MT), acetyl transfer (ATP and condensation Malonyl transfer (MT), acetyl transfer (ATP and condensation (CE) on one subunit.(CE) on one subunit.

• Reduction (KR), dehydration (DH), reduction (ER) and Reduction (KR), dehydration (DH), reduction (ER) and thiolysis (TE) on the other subunit.thiolysis (TE) on the other subunit.

• The growing FA chain is passed between subunits by ACP.The growing FA chain is passed between subunits by ACP.

• Malonyl transfer (MT), acetyl transfer (ATP and condensation Malonyl transfer (MT), acetyl transfer (ATP and condensation (CE) on one subunit.(CE) on one subunit.

• Reduction (KR), dehydration (DH), reduction (ER) and Reduction (KR), dehydration (DH), reduction (ER) and thiolysis (TE) on the other subunit.thiolysis (TE) on the other subunit.

• The growing FA chain is passed between subunits by ACP.The growing FA chain is passed between subunits by ACP.

Fig. 22.23

14

Acyl carrier protein (ACP)Acyl carrier protein (ACP)

• ACP has a long flexible chain, derived from pantothenic ACP has a long flexible chain, derived from pantothenic acid, to which the growing fatty acid is attached.acid, to which the growing fatty acid is attached.15

Fig. 22.21

CondensationCondensation

• Both subunits of FAS are involved.Both subunits of FAS are involved.• Condensation is catalyzed by CE on upper subunit.Condensation is catalyzed by CE on upper subunit.

16

Fig. 22.24

ReductionReduction

• 2 NADPH are used.2 NADPH are used.• Reduction (reduction, dehydration, reduction see slides 12 Reduction (reduction, dehydration, reduction see slides 12

&13) occur on lower subunit of the dimer.&13) occur on lower subunit of the dimer.17

Fig. 22.24

Translocation and binding of a new Translocation and binding of a new malonyl CoAmalonyl CoA

< The 4 carbon chain is transferred to CE.The 4 carbon chain is transferred to CE.^ A new malonyl CoA binds ACP on other subunit.A new malonyl CoA binds ACP on other subunit.• The cycle (condendation, reduction, The cycle (condendation, reduction,

dehydration, reduction) repeats until 16 carbon dehydration, reduction) repeats until 16 carbon palmitate is formed (not shown).palmitate is formed (not shown).

• Palmitate is released by TE (see slide 14). Palmitate is released by TE (see slide 14). 18

Fig. 22.24

Net Reaction for Palmitate SynthesisNet Reaction for Palmitate SynthesisNet Reaction for Palmitate SynthesisNet Reaction for Palmitate Synthesis

8 acetyl CoA + 7 ATP + 14 NADPH + 6 H8 acetyl CoA + 7 ATP + 14 NADPH + 6 H++

palmitate + 14 NADPpalmitate + 14 NADP++ + 8 CoA + 6 H + 8 CoA + 6 H22O + 7 ADP + 7PiO + 7 ADP + 7Pi

For this reaction, there are 7 (for 7 ATP) + 35 (for 14 For this reaction, there are 7 (for 7 ATP) + 35 (for 14 NADPH) = 42 ~P equivalents used. NADPH) = 42 ~P equivalents used.

Compare to 26 obtained from the palmitate conversion Compare to 26 obtained from the palmitate conversion to acetyl CoA by fatty acyl CoA synthetase and the to acetyl CoA by fatty acyl CoA synthetase and the -oxidation cycle.-oxidation cycle.

8 acetyl CoA + 7 ATP + 14 NADPH + 6 H8 acetyl CoA + 7 ATP + 14 NADPH + 6 H++

palmitate + 14 NADPpalmitate + 14 NADP++ + 8 CoA + 6 H + 8 CoA + 6 H22O + 7 ADP + 7PiO + 7 ADP + 7Pi

For this reaction, there are 7 (for 7 ATP) + 35 (for 14 For this reaction, there are 7 (for 7 ATP) + 35 (for 14 NADPH) = 42 ~P equivalents used. NADPH) = 42 ~P equivalents used.

Compare to 26 obtained from the palmitate conversion Compare to 26 obtained from the palmitate conversion to acetyl CoA by fatty acyl CoA synthetase and the to acetyl CoA by fatty acyl CoA synthetase and the -oxidation cycle.-oxidation cycle.

19

(p 622)

Control of Fatty Acid Control of Fatty Acid Synthesis is at the Synthesis is at the Committed StepCommitted Step

• Fig. 24-18 Styer 4thFig. 24-18 Styer 4th

20

Control of Acetyl CoA Carboxylase ActivityControl of Acetyl CoA Carboxylase Activity

• Phosphorylation by kinase inhibits carboxylase (+AMP, Phosphorylation by kinase inhibits carboxylase (+AMP, --ATP). ATP).

• Phosphatase activates carboxylase (+insulin, Phosphatase activates carboxylase (+insulin, --glucagon, & glucagon, & epinephrine).epinephrine).

• Citrate partially activates the inactive phosphorylated acetyl Citrate partially activates the inactive phosphorylated acetyl Co A carboxylase allosterically.Co A carboxylase allosterically.

• Palmitoyl CoA inhibits carboxylase and citrate translocase.Palmitoyl CoA inhibits carboxylase and citrate translocase.

• Phosphorylation by kinase inhibits carboxylase (+AMP, Phosphorylation by kinase inhibits carboxylase (+AMP, --ATP). ATP).

• Phosphatase activates carboxylase (+insulin, Phosphatase activates carboxylase (+insulin, --glucagon, & glucagon, & epinephrine).epinephrine).

• Citrate partially activates the inactive phosphorylated acetyl Citrate partially activates the inactive phosphorylated acetyl Co A carboxylase allosterically.Co A carboxylase allosterically.

• Palmitoyl CoA inhibits carboxylase and citrate translocase.Palmitoyl CoA inhibits carboxylase and citrate translocase.

21

Fig. 22.26

Web linksWeb links

Odd Chain Fatty Acids. The fate of propionyl CoA.

Unsaturated Fatty Acid Oxidation. The role of isomerase.

Next Topic:Next Topic: Membrane lipids. Membrane lipids.