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FA & TG Synthesis-1 Fatty Acids and Triglycerides—Synthesizing Fuel Stores Dianzheng Zhang, Ph.D. Objectives: To understand how fatty acids are synthesized To realize the differences between fatty acids and triglycerides To appreciate the benefits of fuel storage Concept Map of Fuel Synthesis when insulin, glucagon References: Champe & Harvey, Biochemistry, 3 rd Ed., Chapter 16 (pp. 179-187) Marks’ Basic Medical Biochemistry, 2 nd Ed., Chapter 33 (pp. 594ff) (Blood) Liver Lipogenesis Carbohydrates Glucose Fatty Acids Triglycerides VLDL VLDL breakdown to Fatty acids for: Storage in adipose Use by muscle, etc. Conditions: insulin glucagon
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Fatty Acid Synthesis

Apr 12, 2016

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Page 1: Fatty Acid Synthesis

FA & TG Synthesis-1

Fatty Acids and Triglycerides—Synthesizing Fuel Stores Dianzheng Zhang, Ph.D.

Objectives:

To understand how fatty acids are synthesized

To realize the differences between fatty acids and triglycerides

To appreciate the benefits of fuel storage

Concept Map of Fuel Synthesis when insulin, glucagon

References: Champe & Harvey, Biochemistry, 3

rd Ed., Chapter 16 (pp. 179-187)

Marks’ Basic Medical Biochemistry, 2nd

Ed., Chapter 33 (pp. 594ff)

(Blood)

Liver Lipogenesis Carbohydrates

Glucose Fatty Acids Triglycerides VLDL

VLDL breakdown to Fatty acids for:

Storage in adipose

Use by muscle, etc.

Conditions:

insulin

glucagon

Page 2: Fatty Acid Synthesis

FA & TG Synthesis-2

I. Fatty Acid Synthesis

A. OVERVIEW of LIPOGENESIS / FATTY ACID SYNTHESIS: Why does eating excess carbohydrates result in fat formation??

Converting excess dietary glucose adipose tissue triglycerides

1. Accumulation of reactants for fatty acid synthesis

What is needed for fat synthesis:

Substrate: Acetyl CoA (in the cytosol)

Energy: ATP

Reducing power: NADPH From (1) the pentose pathway (see CHO lecture)

(2) the NADP+-linked malic enzyme Glycerol backbone

Figure 1: Overview of

Lipogenesis [Marks. Basic Med Biochem. Fig 33.1]

FA synthesis occurs mainly in the liver

GlucosePyruvateAcetyl CoA

FA synthesisTGs

1A

Figure 2: 1ST

STEP:

Conversion of glucose to acetyl CoA. Acetyl CoA cannot move through mitochondrial

membrane. It must be converted to citrate, which can be transported to the cytosol by a membrane transporter. In the cytosol citrate is converted back to acetyl CoA and OAA by citrate lyase (also called citrate cleavage enzyme). [Marks. Basic Med Biochem. Fig 33.7]

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C

Figure 3: Obtaining NADPH for lipogenesis [Marks. Basic Med Biochem. 2nd Ed. Fig 36.4, p. 671]

Provided by (1) the pentose pathway and (2) the malic enzyme. Cytosolic malate dehydrogenase and the malic enzyme provide a transhydrogenase mechanism in the cytosol to transfer hydrogen from NADH to NADPH.

B. Mechanics and enzyme action in Fatty acid synthesis Stage 1: Acetyl CoA carboxylase A biotin containing enzyme The regulated step (see later)

Fig. 4. Acetyl CoA Carboxylase reaction [Marks’ Basic Medical Biochemistry, 2nd Ed. Fig. 33.15A, p. 601]

Stage 1: Formation of substrate for Fatty Acid Synthase (malonyl CoA) Carboxylases always use biotin as a cofactor.

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Stage 2: Fatty acid synthase: Is a dimer—2 identical subunits.

Each subunit is a multifunctional protein Each subunit contains seven enzyme activities and an acyl carrier group (often called "acyl carrier protein").

The acyl carrier group contains a phosphopantetheine prosthetic group.

Figure 5: Stage 2: formation of the fatty acyl chain (the fatty acid synthase reactions) (enzymatic reactions of fatty acid synthase) [Marks. Basic Med Biochemistry 2nd Ed. Fig 33.15, p. 601]

Priming step: Covalent attachment of 1st acyl (from acetyl CoA) to the enzyme

(initially onto the phosphopantetheine-SH, then transferred over to the cysteine-SH)

Rxn Covalent attachment of malonyl group to the phosphopantetheine of the enzyme

(loading step).

Rxn Condensation of the carboxyl carbon of the acetyl group with the methylene carbon of the

malonyl group; CO2 is released; a 4-carbon keto chain is formed.

Reduction, dehydration and reduction steps—formation of a 4-carbon saturated chain.

Shift of the completely reduced chain to the cysteine-SH.

Attachment of another malonyl group to the enzyme. Another condensation occurs and

the cycle repeats to form a 6-carbon saturated acyl chain. The cycle continues to repeat

until a 16-carbon acyl chain (palmitic acid) is formed and is released from the enzyme complex.

Stage 1: production of Malonyl CoA

FA Synthase has 2 arms—the phosphopantetheine and a cysteine amino acid. Both attach to FAs via –SH groups.

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Stage 3: ELONGATION of palmitate and

DESATURATION of fatty acids

Fig. 6: Elongation of long chain fatty acids [Marks. Basic Med Biochem. Fig 33.17]

Acyl CoA Synthetase attaches the CoA to palmitate. (lipid lecture—requires ATP).

Occurs in the ENDOPLASMIC RETICULUM.

Similar reactions to fatty acid synthesis except that

A fatty acyl CoA is the substrate which condenses with malonyl CoA.

Fig. 7: Desaturation of fatty acids: creating double bonds [Marks. Basic Med Biohcem. Fig 33.18]

Occurs in the ENDOPLASMIC RETICULUM.

Uses molecular oxygen.

Two things are oxidized—the fatty acid and the NADH.

Human desaturases cannot introduce double bonds between carbon 9 and the methyl end.

When we need other double bonds, we use essential fatty acids as precursors (linolenic and linoleic).

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Humans cannot introduce double-bonds between carbon 9 and the methyl end. However, we can add double bonds to the essential fatty acids that have double bonds beyond carbon 9.

Fig. 8. Conversion of linoleic acid to arachidonic acid. [Marks’ Basic Medical Biochemistry, 2nd Ed. Fig. 33-19 p. 603]

Linoleic acid (18:2, 9,12) is one of the essential fatty acids. It serves as a precursor of arachidonic acid, which is the source eicosanoids, prostaglandins and leukotrienes. Linolenic acid (18:3, 9,12,15) also forms eicosanoids.

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C. Regulation of fatty acid synthesis

Conditions for FA synthesis: when there are lots of reactants:

ATP, Acetyl CoA, NADPH (lots of citrate in TCA cycle)

Figure 8: Regulation of Acetyl CoA carboxylase

(1) Short term regulation (minutes to hours) via Acetyl CoA carboxylase:

The enzyme exists as inactive monomers that polymerize when active. citrate Inactive Monomers Active Polymer

fatty acyl CoA

Covalent Modification o Glucagon activates cAMP dependent protein kinase A to phosphorylate, inactivate. o Insulin activates a phosphatase to remove phosphate group, activate.

(2) Long term regulation (days):

Induction (changes in cellular content) of key enzymes: acetyl CoA carboxylase, fatty acid synthase, citrate lyase, malic enzyme, G6PDH, and others Enzyme synthesis is increased (due to insulin and glucagon levels)

If an individual has a good diet over time:

If an individual has a high carbohydrate OR fat free diet: Enzyme synthesis is decreased

If an individual is fasting OR on a high fat diet:

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(3). Regulation to prevent a futile cycle between

FA synthesis and breakdown:

A futile cycle between fatty acid synthesis and fatty acid breakdown is avoided because

malonyl CoA (the product of the 1st stage of FA synthesis) inhibits fatty acids from being transferred to carnitine and entering the mitochondria.

Fig. 9. Inhibition of transport of fatty acids (FA) into mitochondria by malonyl CoA. [Marks’ Basic Medical Biochemistry 2nd Ed. Fig. 36.6, p. 672]

Specifically, in the fed state, when malonyl CoA is being formed for FA synthesis, malonyl CoA inhibits CPTI (carnitine palmitoyltransferase I, or carnitine:acyltransferase I). [See Lipids lecture for more on CPTI.]

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II. Triglyceride synthesis

Glycerol-3-phosphate synthesis

Figure 10: Synthesis of glycerol-3- phosphate in liver and adipose tissues. [Marks. Basic Med Biochem. Fig 33.20]

Glycerol-P-dehydrogenase is present in both liver and adipose; Glycerol kinase is present only in liver.

Addition of acyl-CoA chains

to form triacylglycerol

Figure 11: Synthesis of triacylglycerol in liver and adipose tissues. [Marks. Basic Med Biochem. Fig 33.20]

Glycerol 3-phosphate is produced from glucose in both liver and adipose.

It can also be produced directly from glycerol in the liver, which has glycerol kinase.

Glycerol 3-phosphate is not produced from glycerol in adipose tissue, which lacks glycerol kinase.

After glycerol 3-phosphate is formed, the steps are the same in both liver and adipose.

A futile cycle between TG synthesis and TG breakdown is avoided in the adipose because the adipose must obtain glycerol-3-phosphate from glycolysis; there is no glycerol kinase found in the adipose.

When HSL (activated by glucagon) cleaves TG to FA + glycerol, the reaction is not reversible without glycerol kinase, so the FA + glycerol leave the adipose to be used for energy.

TGs are only synthesized in times of plenty, when glycolysis is increased.

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3. Formation of VLDL Triglycerol molecules made by the liver are carried through the blood on Very Low Density Lipoproteins (VLDL).

Figure 12: Synthesis, processing and secretion of VLDL. [Marks. Basic Med Biochem. Fig 33.24]

Proteins (Apo B100) are synthesized on the

rough endoplasmic reticulum (RER). Triacylglycerols are synthesized on the smooth endoplasmic reticulum (SER) and/or on FA synthase in the cytosol.

TG and proteins are packaged in the Golgi

complex to form VLDL.

VLDL are transported to the cell membrane in

secretory vesicles and secreted by exocytosis. The dots represent VLDL particles. An enlarged VLDL particle is depicted at the bottom of the figure.

Fig. 13. Composition of a typical VLDL particle. The major component is triacylglycerol (TG). C = cholesterol; CE = cholesterol ester; PE = phospholipid. [Marks. Basic Med Biochem. Fig 33.21]

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4. Transport and Storage of Triglycerides Transport of triacylglycerols from liver (site of synthesis) to

The adipose tissue (site of storage)

Figure 14: Transport of TG in blood: 1. Glucose enters the liver and is

converted to fatty acids and glycerol-3-P.

2. Triglycerides are formed from FA-CoAs and glycerol-3-P.

3. Triglycerides are incorporated into very-low-density lipoproteins (VLDL).

4. VLDL goes into the blood and carried to adipose tissue.

5. Triglycerides in the VLDL are hydrolyzed by lipoprotein lipase (LPL) and the released FAs are taken into adipose tissue.

6. In the adipocytes, the FA’s are resynthesized into triglycerides for storage.

B. Conversion of excess dietary fat to adipose tissue triglycerides

Figure 15: Dietary lipid digestion and

storage: 1. Dietary lipids are digested and

absorbed into intestinal cells.

2. Absorbed lipids are incorporated into chylomicrons in intestinal cells.

3. Chylomicrons enter the blood and carry the dietary lipids to adipose tissue.

4. Triglycerides in the chylomicrons are hydrolyzed by lipoprotein lipase (LPL), and the released fatty acids are taken into adipose tissue.

5. Fatty acids are incorporated into storage triglycerides.

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Accumulation of triglycerides in adipose tissue

TG storage in the adipocyte

Figure 16: Conversion of fatty acids (FA) from triacylglycerols (TG) in

chylomicrons and VLDL to the TG stored in adipose tissue [Marks’ Basic Med Biochemistry 2nd Ed. Fig 36-7, p. 673]

Insulin stimulates the transport of glucose into adipose cells.

Glucose provides the glycerol-3-phosphate for TG synthesis.

Insulin stimulates the synthesis and secretion of lipoprotein lipase (LPL).

ApoCII (obtained from HDL in the blood) activates LPL.

ApoCII is present on both the VLDL and chylomicrons.