Voet

Bio

chem

istry

3e©

2004

Joh

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& So

ns, I

nc.

Last Time

Voet

Bio

chem

istry

3e©

2004

Joh

n W

iley

& So

ns, I

nc.

Lipid Metabolism – Anabolism

1. Fatty Acid Synthesis (comparison with beta-oxidation)

2. Regulation of Lipid and Carbohydrate Metabolism (short term vs. long term)

3. Cholesterol Metabolism

4. Eicosanoid Metabolism

Voet

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2004

Joh

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ns, I

nc.

Synthesis of fatty acidsFatty acids are synthesized from 2 carbon units (although activation passes through a 3 carbon unit)

Synthesis is reductive, and uses NADPH pool

Humans are largely “fat burners” even though carbohydrate is main caloric source. Excess carbohydrate is converted to lipid.

Recall, when cellular energy is high, isocitrate DH in TCA cycle is down regulated. Isocitrate and citrate accumulate. Citrate enters cytoplasm to regulate glycolysis, but also constitutes the raw material for fatty acid biosynthesis.

Voet

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2004

Joh

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Figure 25-28 A comparison of fatty acid βoxidation and fatty acid biosynthesis.

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β oxidation fatty acid biosynthesis

Voet

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Figure 25-29 The phosphopantetheine group in acyl-carrier protein (ACP) and in CoA.

AMP

Voet

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Acetate is shuttled out of the mitochondria to the cytoplasm

8 acetates must be transported for each palmitate made. When energy levels are high, TCA cycle slows at isocitrate DH, so citrate accumulates in mitochondria.

Note malic enzyme can create NADPH needed for synthesis

Voet

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Joh

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AcetylCoA carboxylase (ACC), the activation of acetate units by carboxylation

AcCoA + HCO3- +ATP MalCoA + ADP + Pi

7 of the 8 acetates used to make palmitate are activated in this way. We will see ACC is the main regulatory point for fatty acid biosynthesis

Voet

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Figure 25-31 Reaction cycle for the biosynthesis of fatty acids.

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KR

DH

ER

TE

6x1x

KS

MAT

Fatty Acid Synthase- Generate palmitate in 6 cycles.(Multifunctional Enzyme – 7 activities)

1 acetyl-CoA primer /7 malonyl-CoA extenders

Voet

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2004

Joh

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Chemistry of fatty acid synthase.

An AcCoAis used to prime, and subsequent elongation uses malCoA.

Note: loss of CO2 drives reaction, and carbanioncharacter is nucleophile for attack on thiol ester.

From Lehninger by Nelson and Cox; Freeman publishers

Voet

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2004

Joh

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Fatty acid synthase is a complex with 7 active sites.

In bacteria the complex is formed from separate chains, but in animals FAS is a single multifunctional protein (270 KDa) which dimerizes.

Voet

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Fatty Acid Synthesis:

Stage 2: FA Synthase

Figure 25-32 The mechanism of carbon–carbon bond formationin fatty acid biosynthesis.

Step 2b – (condensing enzyme) β-ketoacyl-ACP-synthase (KS)

Voet

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2004

Joh

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& So

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A simple overview of carbon flow in fatty acid biosynthesis

Voet

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Joh

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Figure 25-37 Mitochondrial fatty acid elongation.

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Elongation:

Reversal of β-Oxid.;

NADPH vs FADH2

C16 C18, C20

Fatty acids can be ELONGATED in the mitochondria

Voet

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∆ ∆ ∆ ∆ ∆ ∆4 5 6 9 12 15

Essential Fatty Acids: Linoleic 18:2n-6α - Linolenic 18:3n-3

γ - Linolenic 18:3n-6

Arachidonic 20:4n-6

mammalian plants

Fatty acids can be desaturated

Mammals cannot unsaturate beyond C9 of a fatty acid; poly unsaturated fatty acids must be ingested, often from vegetable oils.

This is a “mixed function oxidase”. Two substrates, FA and NADPH, are oxidized by O2

Voet

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Synthesis of triacylglycerides and glycerophospholipids, part aPa

ge 9

39

Voet

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Synthesis of triacylglycerides and glycerophospholipids, part b.Pa

ge 9

39

Voet

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The biosynthesis of phosphatidylethanolamineand phosphatidylcholine.

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DAG

This is one example of how diacyl glycerides can be modified into a vast array of glycerol phospholipids for membrane synthesis.

Voet

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Figure 25-40 Sites of regulation of fatty acid metabolism.

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The regulation of lipid metabolism does not currently appear to be as complex as that for carbohydrates. Fatty acid regulation is largely controlled by hormone signals that integrate information from the whole animal.

Some signals are relatively short term, like the insulin triggered dephosphorylation of ACC that up-regulates fatty acid biosynthesis, while glucagon signals the destruction of storage fat.

Other effects, primarily at the level of enzyme expression, are more long term and reflect major dietary trends.

Voet

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chem

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3e©

2004

Joh

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& So

ns, I

nc.

Lipid Metabolism – Part II – Anabolism

1. Fatty Acid Synthesis (comparison with beta-oxidation)

2. Regulation of Lipid and Carbohydrate Metabolism (short term vs. long term)

3. Cholesterol Metabolism

4. Eicosanoid Metabolism

What is the starting material for the synthesis of non-fatty acid lipids - sterols?

Where does the synthesis of sterols take place?

What is unique about the pathway for the synthesis of sterols?

What are the roles of HMG-CoA / Mevalonate / Farnesyl pyro-P and Squalene?

How is the pathway controlled?

What are acceptable levels of cholesterol?

How does Lipitor and other statins lower cholesterol levels?

Voet

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Figure 25-41 All of cholesterol’s carbon atoms are derived from acetate.

Voet

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Sterol SynthesisSterol synthesis can be broken into 4 stages:

Creation of mevalonate, formation of active isoprene, condensation of 6 units to squalene, and folding into the steroid

Lanosterol

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Stage 1: Synthesis of Mevalonate

Note: HMG-CoA reductase is the target for drugs aimed at hypercholesterolemia

Voet

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Competitive inhibitors of HMG-CoA reductase used for the treatment of hypercholesterolemia.

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Voet

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Figure 25-42 Squalene. (a) Extended conformation. Each box contains one isoprene unit. (b) Folded in preparation for cyclization as predicted by Bloch and Woodward.

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C5 C5C5C5C5C5

2 NADPH / 3 ATP

C5C6

X 6

Voet

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Figure 25-56 The 19-reaction conversion of lanosterol to cholesterol.

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Loss of 3 C

19 steps in ER!

(17 NAD(P)H)

30C 27C

Voet

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Figure 25-61 “Simplified” scheme of steroid biosynthesis.

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Cholesterol as the

starting material for

the synthesis of :

Cortisol

Progesterone

Estradiol

Testosterone

Voet

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The LIVER synthesizes large amounts of sterols, particularly cholesterol

Cholesterol made in the liver can be converted to bile salts to aid in digestion, or ESTERIFIED with fatty acids by AcylyCoA:cholesterol acyltransferase (ACAT). These cholesterol esters are packaged into VLDL lipoprotein bodies for subsequent routing and metabolism throughout the body where they are converted to IDL and LDL bodies. LDL delivers cholesterol to peripheral tissues.

Voet

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Lipoprotein body routingCholesterol and cholesterol esters are exported from the liver as VLDL bodies. They are processed to IDL and LDL bodies. The latter are the main source of extrahepaticcholesterol, and bind to peripheral cells via the LDL receptor. Genetic defects in the receptor retard uptake and produce high serum cholesterol levels associated with plaque formation and heart problems. Brown and Goldstein (UTSW) won the Nobel prize for their work in this field.

Voet

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Joh

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Figure 25-57 LDL receptor-mediated endocytosis in mammalian cells.

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nc.Eicosanoids, local mediators

Eicosanoids are produced from the C20 compound arachidonic acid, cleaved from membranes by phospholipase A2.

The eicosanoids, like protoglandin, thromboxane, and leukotriene have powerful physiologic effects, like hormones. They influence inflammation, fever, blood pressure and other phenomenon.

Like hormones, they often have membrane receptors with associated G protein amplification systems.

They are not hormones, however, as they do not travel through blood. They are instead referred to as local mediators, or paracrine hormones.

Voet

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Figure 25-67 The cyclic pathway of arachidonic acid metabolism.

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Figure 25-68 The reactions catalyzed by PGH synthase(PGHS).

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Prostaglandin H synthase

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Figure 25-69a X-Ray structure of PGH synthase (PGHS) from sheep seminal vesicles in complex with the NSAID flurbiprofen. (a) Homodimeric monotopic membrane protein.

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Figure 25-71 Inactivation of PGH synthase by aspirin.

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Figure 25-70 Some nonsteroidal anti-inflammatory drugs (NSAIDs).

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Figure 25-72 COX-2 inhibitors.

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COX-1 is found in a variety of tissues, including the stomach where its prostoglandins are involved in maintenance of gastric mucosa. Inhibition can lead to stomach and intestinal problems, including ulcers. COX-2 is found in tissues associated with inflammation, like synovial tissue (associated with rheumatoid arthritis ), but not the stomach. COX-2 specific inhibitors are supposed to give relief from inflammation from osteoarthritis and rheumatoid arthritis, and from acute pain, without stomach problems. Vioxx was withdrawn from the market in 2004 over concerns about increased risk of heart attack. Celebrex put itself on a shorter leash as well.