BIOC 460 DR. TISCHLER LECTURE 34

 SYNTHESIS & PROCESSING OF FATS

OBJECTIVES

1. Sequence leading from glucose to fatty acids via lipogenesis including roles of pyruvate carboxylase and pyruvate dehydrogenase.

2. Malic enzyme and acetyl CoA carboxylase

3. For fatty acid synthase: a) substrates/key products; b) sources of NADPH; c) general mechanism

4. Relationship: regulation of carnitine-palmitoyl transferase-I and preventing oxidation of synthesized palmitoyl CoA

5. Eicosanoids: a) fatty acid from which they are derived; b) specific functions of each eicosanoid; c) general pathway of production; effects of glucocorticoids

(cortisol) and aspirin

principally in adipose tissue and liver

lipogenesis – cytoplasm; requires acetyl CoA

adipose: FA stored as triacylglycerols via esterification

liver: produces TAG packaged into VLDL and exported

compounds metabolized to acetyl CoA can serve as a fat precursor

glucose = primary source of carbons for fat synthesis.

LIPOGENESIS

CYTOPLASM MITOCHONDRIAL MATRIX

Pyruvate

Citrate

CSOxaloacetate

PCATP, CO2

ADP, Pi

PPP

Pyruvate

Glucose

Glycolysis

FAS

FattyAcids

Citrate

Acetyl CoA

CLATP, CoA

ADP+Pi

Oxaloacetate ACCADP, Pi

CO2, ATP

Malonyl CoA

Acetyl CoA

NAD, CoA NADH, CO2

PDH

MDH

NADH

NAD+Malate

MENADP+

NADPH

CO2

Figure 1. Export of acetyl CoA as citrate for fatty acid biosynthesis, generation of NADPH and pathway of lipogenesis.

KEY MITOCHONDRIAL REACTIONS

PYRUVATE CARBOXYLASE

pyruvate + CO2 + ATP oxaloacetate + ADP + Pi

PYRUVATE DEHYDROGENASE

pyruvate + NAD + coenzyme A (CoA) acetyl CoA + CO2 + NADH

Citrate Lyasecitrate + CoA + ATP acetyl CoA + oxaloacetate + ADP + Pi

Malate dehydrogenaseoxaloacetate + NADH malate + NAD+

Malic Enzymemalate + NADP+ pyruvate + NADPH

KEY CYTOPLASMIC REACTIONS INDIRECTLY NEEDED FOR LIPOGENESIS

KEY CYTOPLASMIC REACTIONS DIRECTLY NEEDED FOR LIPOGENESIS AND FATTY ACID ACTIVATION

Acetyl CoA Carboxylase: acetyl CoA + HCO3

- + ATP malonyl CoA + ADP + Pi

Fatty Acid Synthase: acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+ palmitate + 7 CO2 + 8 CoA + 14 NADP+

Acyl CoA Synthetase: (also used for fatty acids other than palmitate) palmitate + ATP + CoA palmitoyl CoA + AMP + PPi

condensation

reductiondehydration

reduction

2 NADPH 2 NADP+

ACP

CEacp

ACP

CEacp

ACP

CEacp

Figure 2. General mechanism for the fatty acid synthase reaction. CE is condensing enzyme. ACP is acyl carrier protein. This row represents the initial steps for priming the reaction with acetyl CoA and the addition of two carbons from malonyl CoA.

COO-

C=O

CH2

C=O

CH2

C=O

CH3

C=O

CH2

CH2

CH3malonyl CoA

CH3

C=O

acetyl CoA

CO2

CO2

CO2

CO2

CH3

C=O

CH3

C=OC=O

CH2

CH3

C=O

C=O

CH2

CH3

C=O

C=O

CH2

CH3

C=O

4-Cunit

Figure 2. General mechanism for the fatty acid synthase reaction. CE is condensing enzyme. ACP is acyl carrier protein. This row depicts a typical cycle of adding two more carbons to the fatty acid chain.

malonyl CoA

condensation

CO2

reductiondehydration

reduction

2 NADPH 2 NADP+

ACP

CEacp

6-Cunit

ACP

CEacp

6-Cunit

ACP

CEacp

4-Cunit

Figure 2. General mechanism for the fatty acid synthase reaction. CE is condensing enzyme. ACP is acyl carrier protein. This row shows the release of the finished product, palmitate, through cleavage by thioesterase.

malonyl CoA

ACP

CEacp

16-Cunit

palmitate

ACP

CEacp

6-Cunit

5 more cycles adding 10 more carbons

5CO2

10NADP+

5malonyl CoA10NADPH

ACP

CEacp

thioesterasecleavage

palmitate

malic enzyme:

Malate + NADP+ Pyruvate + CO2 + NADPH

pentose phosphate pathway:

Glucose-6-P + 2 NADP+ Ribulose-5-P + 2 NADPH + CO2

Sources of NADPH for the Biosynthesis of Fatty Acids.

Figure 3. Formation of phosphatidic acid from glycerol-3-P or DHAP, and its conversion to triacylglycerol

Lysophosphatidic acid

Phosphatidic acid

Triacylglycerol

NADPH

NADP+

Diacylglycerolphosphatase

CoA

Acyldihydroxyacetone phosphate

fatty acyl CoA

Dihydroxyacetone phosphate

fatty acyl CoA

CoA

ADP

ATP glycerolkinase

Glycerol-3-P

Glycerol

CoA

fatty acyl CoA

CoA

fatty acyl CoA

Pi

EICOSANOIDS

hormones localized to tissues where they are produced. prostaglandins, thromboxanes and leukotrienes. derived from arachidonic acid arachidonic acid from linoleic acid an essential fatty acid

Table 1. Physiological functions of eicosanoids.

Eicosanoid Functions

prostaglandins inflammation, fever production, prevent platelet aggregation (prevent clotting); induce labor

thromboxanes produced by platelets to promote their aggregation (blood clotting)

leukotrienes allergic reactions

Membrane Phospholipid

Phospholipase A2

Arachidonic acid

Lipoxygenase

Leuokotrienes

Cyclooxygenase

PGH2 Thromboxanes in platelets

Prostaglandins in many cells

Figure 4. Conversion of arachidonic acid to eicosanoids.

inhibited by glucocorticoids

inhibited by aspirin, ibuprofen