M2C Unit 2 Lesson 15,16, 17

Acetyl CoA can be converted to fatty acids which then increases

• triglycerides (triacylglycerols)

• phospholipids

• eicosanoids (e.g., prostaglandins)

• ketone bodies

fatty acids are cholesterol precursors and can be converted intp

• steroid hormones

• bile acids

Function of CoA

commonly used carrier for activated acyl groups (acetyl, fatty acyl and others)

What is Fatty acid synthesis?

process of combining eight two-carbon fragments (acetyl groups from acetyl CoA) to form a 16-carbon saturated fatty acid, palmitate.

Fatty acid synthesis occurs primarily in the cytoplasm of which tissues

• liver

• adipose (fat)

• central nervous system

• lactating mammary gland

Fatty Acid Synthesis Sum of reactions

8 acetyl CoA + 7 ATP + 14 (NADPH + H+) ----> palmitate (16:0) + 8 CoA + 7 (ADP + Pi) + 14 NADP+ + 6 H2O

Citrate, in TCA, can be transported out of the mitochondria to the cytoplasm (where fatty acid synthesis occurs), and there split to generate

cytoplasmic acetyl CoA for fatty acid synthesis. Citrate can also be oxidized by the TCA cycle in the mitochondria to yield energy

What enzyme catalyzes the reaction: acetyl CoA + HCO3 - + ATP ---> malonyl CoA + ADP + Pi?

Acetyl CoA carboxylase

Six molecules of malonyl CoA and one molecule of acetyl CoA then interact sequentially with fatty acid synthase to yield the final product?

palmitate

Acetyl CoA carboxylase has three important features

1. It contains the prosthetic group, biotin

2. The carboxylation reaction is driven to completion by hydrolysis of ATP.

3. The enzyme catalyzes the rate-limiting reaction for fatty acid synthesis, and is under tight short-term control

Fatty acid Synthesis is down-regulated by

◦ palmitoyl CoA (endproduct regulation).

◦ phosphorylation of the enzyme (through a glucagon-cAMP cascade).

Fatty acid Synthesis is up-regulated by

◦ citrate (allosteric)

◦ dephosphorylation of the enzyme (influenced by the

In the first reaction acetyl CoA is added to a cysteine - SH group of the condensing enzyme (CE) domain:

acetyl CoA + CE-cys-SH ----> acetyl-cys-CE + CoASH

In the second reaction malonyl CoA is added to the ACP sulfhydryl group:

malonyl CoA + ACP-SH ---> malonyl ACP + CoASH

• this -SH group is part of a phosphopantethenic acid prosthetic group of the ACP.

In the third reaction the acetyl group is transferred to the malonyl group with the release of carbon dioxide:

malonyl ACP + acetyl-cys-CE --> beta-ketobutyryl-ACP + CO2

In the fourth reaction the keto group is reduced to a hydroxyl group by the beta-ketoacyl reductase activity

beta-ketobutyryl-ACP + NADPH + H+ ------> beta-hydroxybutyryl-ACP + NAD+

In the fifth reaction the beta-hydroxybutyryl-ACP is dehydrated to form a trans- monounsaturated fatty acyl group by the beta-hydroxyacyl dehydratase activity:

beta-hydroxybutyryl-ACP ----> 2-butenoyl-ACP + H2O

In the sixth reaction the double bond is reduced by NADPH, yielding a saturated fatty acyl group two carbons longer than the initial one (an acetyl group was converted to a butyryl group in this case

2-butenoyl-ACP + NADPH + H+ ----> butyryl-ACP + NADP+

The butyryl group is then transferred from the ACP sulfhydryl group to the CE sulfhydryl

butyryl-ACP + CE-cys-SH ----> ACP-SH + butyryl-cys-CE

When the fatty acyl group becomes 16 carbons long, a thioesterase activity hydrolyses it, forming free palmitate:

palmitoyl-ACP + H2O ----> palmitate + ACP-SH

Fatty acyl synthase has three important characteristics

1. It is essential, but not rate-limiting, for fatty acid synthesis. It is not

subject to short term control.

2. ACP and the catalytic activities are on a single continuous protein

(257 kDa).

3. In animals the synthase is active only as a dimer. The

malonyl/acetyl transferase, condensing enzyme and dehydratase

activities from the first subunit and all the other activities from the

second subunit form one functional unit. A second functional unit

forms from the remainder of the two subunits.

This shows the overall reaction of fatty acid elongation in mitochondria. The process is essentially a reversal of beta-oxidation, except that

one NADPH and one NADH are required (beta-oxidation yields two NADH

The desaturation reaction requires an electron transport system involving

1. cytochrome b5

2. Desaturase

3. NADPH-cytochrome b5 reductase This complex system avoids

generating H2O2 in the vicinity of the sensitive double bonds.

The system is associated with the membranes of the endoplasmic

reticulum.

Cholesterol Synthesis location

Occurs in the cytosol and microsomes from acetyl-CoA

5 major steps of Cholesterol Synthesis

1. Acetyl-CoAs are converted to 3-hydroxy-3-methylglutarylCoA (HMG-CoA)

2. HMG-CoA is converted to mevalonate

3. Mevalonate is converted to the isoprene based molecule,

isopentenyl pyrophosphate (IPP)

4. IPP is converted to squalene

5. Squalene is converted to cholesterol.

Hydroxymethylglutaryl-coenzyme A (HMG-CoA

is the precursor for cholesterol synthesis

HMG-CoA

an intermediate on the pathway for synthesis of ketone bodies from acetyl-CoA.

HMG-CoA Reductase

(an integral protein of endoplasmic reticulum membranes) catalyzes production of mevalonate from HMG-CoA

Whay is rate-limiting for cholesterol synthesis?

HMG-CoA Reductase

What is phosphorylated by 2 sequential phosphate transfers from ATP, yielding the pyrophosphate derivative?

Mevalonate

ATP-dependent decarboxylation, with dehydration, yields?

isopentenyl pyrophosphate

Isopentenyl Pyrophosphate Isomerase

inter-converts isopentenyl pyrophosphate and dimethylallyl pyrophosphate.

Prenyl Transferase (Farnesyl Pyrophosphate Synthase)

catalyzes a series of head-to-tail condensation reactions.

Squalene Synthase

catalyzes head-to-head condensation of 2 molecules of farnesyl pyrophosphate, with reduction by NADPH, to yield squalene.

Squalene epoxidase

catalyzes oxidation of squalene to form 2,3-oxidosqualene. This mixed function oxidation requires NADPH as reductant and O2 as oxidant. One atom of oxygen is incorporated into the substrate (as the epoxide) and the other oxygen atom is reduced to water.

Squalene Oxidocyclase

catalyzes a series of electron shifts, initiated by donation of a proton to the epoxide, that lead to cyclization. The product is the sterol lanosterol.

Conversion of lanosterol to cholesterol involves __reactions, catalyzed by enzymes associated with endoplasmic reticulum membranes.

19

HMG-CoA Reductase is inhibited by phosphorylation, catalyzed by

AMP-Dependent Protein Kinase (also called HMG-CoA Reductase Kinase)

AMP-Dependent Protein Kinase is active when

cellular AMP is high, (ATP is low). So, when cellular ATP is low, energy is not spent in synthesizing cholesterol.

Long-term regulation of cholesterol synthesis

by varied formation and degradation of HMG-CoA Reductase and other enzymes of the pathway

Regulated proteolysis of HMG-CoA Reductase

Degradation of HMG-CoA Reductase is stimulated by:

cholesterol,

oxidized derivatives of cholesterol,

mevalonate,

farnesol (dephosphorylated farnesyl pyrophosphate)

HMG-CoA Reductase has a transmembrane sterol-sensing domain that has a role in

HMG-CoA Reductase has a transmembrane sterol-sensing domain that has a role in

The end products of cholesterol utilization are

the bile acids, synthesized in the liver. Used for the excretion of excess cholesterol. However, the excretion of cholesterol in the form of bile acids is insufficient to compensate for an excess dietary intake of cholesterol.

The most abundant bile acids in human bile are:

chenodeoxycholic acid (45%) cholic acid (31%) = primary bile acids

secondary bile acids

deoxycholate and lithocholate.

What happens to both primary and secondary bile acids

are reabsorbed by the intestines and delivered back to the liver via the portal circulation

Synthesis of primary bile acids is catalyzed by

the 7a-hydroxylase is the rate limiting step. Conversion of 7a-hydroxycholesterol to the bile acids requires several steps.

Bile acids inhibit what part of cholesteroo breakdown

Cholesterol → 7a-hydroxycholesterol (enzyme = 7a-hydroxylase)

Bile acids perform four physiologically significant functions:

1. their synthesis---excretion in the feces is the only significant

mechanism for the elimination of excess cholesterol.

2. bile acids and phospholipids solubilize cholesterol in the bile,

preventing the precipitation of cholesterol in the gallbladder.

3. they facilitate the digestion of dietary triacylglycerols by acting as

emulsifying agents that render fats accessible to pancreatic lipases.

4. they facilitate the intestinal absorption of fat-soluble vitamins.

Hyperlipoproteinemias

conditions in which the concentration of cholesterol – or TG – carrying lipoproteins in plasma exceeds an arbitrary normal limit, typically defined as the 95th percentile of a random population.

An elevated concentration of lipoproteins can accelerate the development of

atherosclerosis

clinical evidence strongly suggests that reduction of the concentration of lipoproteins in plasma can diminish the increased risk for

atherosclerosis that accompanies hyperlipoproteinemias.

Triacylglycerols (TGs) and glycogen

the two major forms of stored energy in vertebrates

What can supply ATP for muscle contraction for less than an hour?

Glycogen

Sustained work is fueled by metabolism of TGs which are very efficient energy stores because?

(1) They are stored in an anhydrous form

(2) Their fatty acids are more reduced than amino acids or monosaccharides

Fatty acids (FA) and glycerol for metabolic fuels are obtained from triacylglycerols

(1) In the diet (2) Stored in adipocytes (fat storage cells)

In the small intestine, fat particles are coated with bile salts and digested by

pancreatic lipases

Lipases

degrade TGs to free fatty acids and a 2-monoacylglycerol

most abundant bile salts

Taurocholate and glycocholate (cholesterol derivatives)

Free cholesterol is solublized by ___ for adsorption

bile-salt micelles

TGs, cholesterol and cholesterol esters are insoluble in water and cannot be transported in blood or lymph as

free molecules

These lipids assemble with phospholipids and apoproteins (apolipoproteins) to form spherical particles called

lipoproteins

lipoproteins structure

Hydrophobic cores: TGs, cholesteryl esters

Hydrophilic surfaces: cholesterol, phospholipids,

apolipoprotein

Major protein for Chylomicrons

apoB

Major protein for VLDL

aboB

Major protein for IDL

apoB

Major protein for LDL

apoB

Major protein for HDL

apoA-I

Major lipid for Chylomicrons

TG

Major lipid for VLDL

TG

Major lipid for IDL

CE (cholesteryl ester)

Major lipid for LDL

CE (cholesteryl ester)

Major lipid for HDL

CE (cholesteryl ester)

Role of Triglycerides

Major energy source for cells

Role of Cholesterol

Cell growth, cell division, membrane repair, steroid hormone production

Role of Lipids

Transport of fat soluble vitamins

TC/HDLC level for adult female?

3.5

TC/HDLC level for adult male?

4.7

TC/HDLC level for neonate?

2.0

What is TC/HDLC?

Total cholesterol / High density lipoprotein cholesterol

Positive Risk Factors in Atherosclerosis

• Males age 45+ and femailes age 55+

• CHD family history

• smoking

• obesity

• elevates TGs

• Elvated LDL cholesterol

• Diabetes mellitus Hypertension

Negative Risk Factors in Atherosclerosis

• elevated HDL

• Low LDL

• Good genes

• female gender (estrogen)

• exercise

Constellation of several risk factors that increase chance of coronary artery disease, peripheral vascular disease, stroke and type 2 diabetes. Are a combination of 3 or more of the following risks:

• Abdominal obesity

• Triglyceride levels above 150 mg/dL

• Low HDL cholesterol

• Elevated blood pressure (>130/85 mm Hg)

• Fasting blood glucose > 100 mg/dL

Major aproproteins for Chylomicrons

A1, A2, A4, B48

Major aproproteins for Chylomicron remnants

B48 , E

Major aproproteins for VLDL

B100, C, E

Major aproproteins for IDL

B100 and E

Major aproproteins for LDL

B100

Major aproproteins for HDL

A1 and A2

Transport function of Chylomicrons

Dietary TGs

Transport function of Chylomicrons remnants

Dietary cholesterol

Transport function of VLDL

Endogenous TGs

Transport function of IDL

Endogenous cholesterol

Transport function of LDL

Endogenous cholesterol

Transport function of HDL

Remove choles. from extra hepatic tissue

Mechanism of lipid delivery for Chylomicrons

Hydrolysis by LPL

Mechanism of lipid delivery for Chylomicrons remnants

Receptor, endocytoses in liver

Mechanism of lipid delivery for VLDL

Hydrolysis by LPL

Mechanism of lipid delivery for IDL

50% Rec. endo. Liver, 50% LDL

Mechanism of lipid delivery for LDL

Rec. endo. Liver/extra hepatic tissue