Berg J.M., Tymoczko J.L., Stryer L. Biochemistry

Подождите немного. Документ загружается.

II. Transducing and Storing Energy 22. Fatty Acid Metabolism 22.5. Acetyl Coenzyme A Carboxylase Plays a Key Role in Controlling Fatty Acid Metabolism

Figure 22.27. Dependence of the Catalytic Activity of Acetyl CoA Carboxylase on the Concentration of Citrate.

The dephosphorylated form of the carboxylase is highly active even when citrate is absent. Citrate partly overcomes the

inhibition produced by phosphorylation. [After G. M. Mabrouk, I. M. Helmy, K. G. Thampy, and S. J. Wakil. J. Biol.

Chem. 265(1990):6330.]

II. Transducing and Storing Energy 22. Fatty Acid Metabolism 22.5. Acetyl Coenzyme A Carboxylase Plays a Key Role in Controlling Fatty Acid Metabolism

Figure 22.28. Filaments of Acetyl CoA Carboxylase. The electron micrograph shows the enzymatically active

filamentous form of acetyl CoA carboxylase from chicken liver. The inactive form is an octamer of 265-kd subunits.

[Courtesy of Dr. M. Daniel Lane.]

II. Transducing and Storing Energy 22. Fatty Acid Metabolism

22.6. Elongation and Unsaturation of Fatty Acids Are Accomplished by Accessory

Enzyme Systems

The major product of the fatty acid synthase is palmitate. In eukaryotes, longer fatty acids are formed by elongation

reactions catalyzed by enzymes on the cytosolic face of the endoplasmic reticulum membrane. These reactions add two-

carbon units sequentially to the carboxyl ends of both saturated and unsaturated fatty acyl CoA substrates. Malonyl CoA

is the two-carbon donor in the elongation of fatty acyl CoAs. Again, condensation is driven by the decarboxylation of

malonyl CoA.

22.6.1. Membrane-Bound Enzymes Generate Unsaturated Fatty Acids

Endoplasmic reticulum systems also introduce double bonds into long-chain acyl CoAs. For example, in the conversion

of stearoyl CoA into oleoyl CoA, a cis-∆

double bond is inserted by an oxidase that employs molecular oxygen and

NADH (or NADPH).

This reaction is catalyzed by a complex of three membrane-bound enzymes: NADH-cytochrome b

reductase,

cytochrome b

, and a desaturase (Figure 22.29). First, electrons are transferred from NADH to the FAD moiety of

NADH-cytochrome b

reductase.

The heme iron atom of cytochrome b

is then reduced to the Fe

state. The nonheme iron atom of the desaturase is

subsequently converted into the Fe

state, which enables it to interact with O

and the saturated fatty acyl CoA

substrate. A double bond is formed and two molecules of H

O are released. Two electrons come from NADH and two

from the single bond of the fatty acyl substrate.

A variety of unsaturated fatty acids can be formed from oleate by a combination of elongation and desaturation reactions.

For example, oleate can be elongated to a 20:1 cis-∆

fatty acid. Alternatively, a second double bond can be inserted to

yield an 18:2 cis-∆

,∆

fatty acid. Similarly, palmitate (16:0) can be oxidized to palmitoleate (16:1 cis-∆

), which can

then be elongated to cis-vaccenate (18:1 cis-∆

Unsaturated fatty acids in mammals are derived from either palmitoleate (16:1), oleate (18:1), linoleate (18:2), or

linolenate (18:3). The number of carbon atoms from the ω end of a derived unsaturated fatty acid to the nearest double

bond identifies its precursor.

Mammals lack the enzymes to introduce double bonds at carbon atoms beyond C-9 in the fatty acid chain. Hence,

mammals cannot synthesize linoleate (18:2 cis-∆

, ∆

) and linolenate (18:3 cis-∆

, ∆

). Linoleate and

linolenate are the two essential fatty acids. The term essential means that they must be supplied in the diet because they

are required by an organism and cannot be endogenously synthesized. Linoleate and linolenate furnished by the diet are

the starting points for the synthesis of a variety of other unsaturated fatty acids.

22.6.2. Eicosanoid Hormones Are Derived from Polyunsaturated Fatty Acids

Arachidonate, a 20:4 fatty acid derived from linoleate, is the major precursor of several classes of signal molecules:

prostaglandins, prostacyclins, thromboxanes, and leukotrienes (Figure 22.30).

A prostaglandin is a 20-carbon fatty acid containing a 5-carbon ring (Figure 22.31). A series of prostaglandins is

fashioned by reductases and isomerases. The major classes are designated PGA through PGI; a subscript denotes the

number of carbon-carbon double bonds outside the ring. Prostaglandins with two double bonds, such as PGE

, are

derived from arachidonate; the other two double bonds of this precursor are lost in forming a five-membered ring.

Prostacyclin and thromboxanes are related compounds that arise from a nascent prostaglandin. They are generated by

prostacylin synthase and thromboxane synthase respectively. Alternatively, arachidonate can be converted into

leukotrienes by the action of lipoxygenase. These compounds, first found in leukocytes, contain three conjugated double

bonds

hence, the name. Prostaglandins, prostacyclin, thromboxanes, and leukotrienes are called eicosanoids (from the

Greek eikosi, "twenty") because they contain 20 carbon atoms.

Prostaglandins and other eicosanoids are local hormones because they are short-lived. They alter the activities both of

the cells in which they are synthesized and of adjoining cells by binding to 7TM receptors. The nature of these effects

may vary from one type of cell to another, in contrast with the more uniform actions of global hormones such as insulin

and glucagon. Prostaglandins stimulate inflammation, regulate blood flow to particular organs, control ion transport

across membranes, modulate synaptic transmission, and induce sleep.

Recall that aspirin blocks access to the active site of the enzyme that converts arachidonate into prostaglandin H

(Section 12.5.2). Because arachidonate is the precursor of other prostaglandins, prostacyclin, and thromboxanes,

blocking this step affects many signaling pathways. It accounts for the wide-ranging effects that aspirin and related

compounds have on inflammation, fever, pain, and blood clotting.

II. Transducing and Storing Energy 22. Fatty Acid Metabolism 22.6. Elongation and Unsaturation of Fatty Acids Are Accomplished by Accessory Enzyme Systems

Figure 22.29. Electron-Transport Chain in the Desaturation of Fatty Acids.

II. Transducing and Storing Energy 22. Fatty Acid Metabolism 22.6. Elongation and Unsaturation of Fatty Acids Are Accomplished by Accessory Enzyme Systems

Figure 22.30. Arachidonate Is the Major Precursor of Eicosanoid Hormones. Prostaglandin synthase catalyzes the

first step in a pathway leading to prostaglandins, prostacyclins, and thromboxanes. Lipoxygenase catalyzes the initial

step in a pathway leading to leukotrienes.

II. Transducing and Storing Energy 22. Fatty Acid Metabolism 22.6. Elongation and Unsaturation of Fatty Acids Are Accomplished by Accessory Enzyme Systems

Figure 22.31. Structures of Several Eicosanoids.

II. Transducing and Storing Energy 22. Fatty Acid Metabolism

Summary

Triacylglycerols Are Highly Concentrated Energy Stores

Fatty acids are physiologically important as (1) components of phospholipids and glycolipids, (2) hydrophilic modifiers

of proteins, (3) fuel molecules, and (4) hormones and intracellular messengers. They are stored in adipose tissue as

triacylglycerols (neutral fat).

The Utilization of Fatty Acids as Fuel Requires Three Stages of Processing

Triacylglycerols can be mobilized by the hydrolytic action of lipases that are under hormonal control. Fatty acids are

activated to acyl CoAs, transported across the inner mitochondrial membrane by carnitine, and degraded in the

mitochondrial matrix by a recurring sequence of four reactions: oxidation by FAD, hydration, oxidation by NAD

, and

thiolysis by CoA. The FADH

and NADH formed in the oxidation steps transfer their electrons to O

by means of the

respiratory chain, whereas the acetyl CoA formed in the thiolysis step normally enters the citric acid cycle by condensing

with oxaloacetate. Mammals are unable to convert fatty acids into glucose, because they lack a pathway for the net

production of oxaloacetate, pyruvate, or other gluconeogenic intermediates from acetyl CoA.

Certain Fatty Acids Require Additional Steps for Degradation

Fatty acids that contain double bonds or odd numbers of carbon atoms require ancillary steps to be degraded. An

isomerase and a reductase are required for the oxidation of unsaturated fatty acids, whereas propionyl CoA derived from

chains with odd numbers of carbons requires a vitamin B

-dependent enzyme to be converted into succinyl CoA.

Fatty Acids Are Synthesized and Degraded by Different Pathways

Fatty acids are synthesized in the cytosol by a different pathway from that of β oxidation. Synthesis starts with the

carboxylation of acetyl CoA to malonyl CoA, the committed step. This ATP-driven reaction is catalyzed by acetyl CoA

carboxylase, a biotin enzyme. The intermediates in fatty acid synthesis are linked to an acyl carrier protein. Acetyl ACP

is formed from acetyl CoA, and malonyl ACP is formed from malonyl CoA. Acetyl ACP and malonyl ACP condense to

form acetoacetyl ACP, a reaction driven by the release of CO

from the activated malonyl unit. A reduction, a

dehydration, and a second reduction follow. NADPH is the reductant in these steps. The butyryl ACP formed in this way

is ready for a second round of elongation, starting with the addition of a two-carbon unit from malonyl ACP. Seven

rounds of elongation yield palmitoyl ACP, which is hydrolyzed to palmitate. In higher organisms, the enzymes carrying

out fatty acid synthesis are covalently linked in a multifunctional enzyme complex. A reaction cycle based on the

formation and cleavage of citrate carries acetyl groups from mitochondria to the cytosol. NADPH needed for synthesis is

generated in the transfer of reducing equivalents from mitochondria by the malate-pyruvate shuttle and by the pentose

phosphate pathway.

Acetyl Coenzyme A Carboxylase Plays a Key Role in Controlling Fatty Acid

Metabolism

Fatty acid synthesis and degradation are reciprocally regulated so that both are not simultaneously active. Acetyl CoA

carboxylase, the essential control site, is stimulated by insulin and inhibited by glucagon and epinephrine. These

hormonal effects are mediated by changes in the amounts of the active dephosphorylated and inactive phosphorylated

forms of the carboxylase. Citrate, which signals an abundance of building blocks and energy, allosterically stimulates the

carboxylase. Glucagon and epinephrine stimulate triacylglycerol breakdown by activating the lipase. Insulin, in contrast,

inhibits lipolysis. In times of plenty, fatty acyl CoAs do not enter the mitochondrial matrix because malonyl CoA inhibits

carnitine acyltransferase I.

Elongation and Unsaturation of Fatty Acids Are Accomplished by Accessory Enzyme

Systems

Fatty acids are elongated and desaturated by enzyme systems in the endoplasmic reticulum membrane. Desaturation

requires NADH and O

and is carried out by a complex consisting of a flavoprotein, a cytochrome, and a nonheme iron

protein. Mammals lack the enzymes to introduce double bonds distal to C-9, and so they require linoleate and linolenate

in their diets.

Arachidonate, an essential precursor of prostaglandins and other signal molecules, is derived from linoleate. This 20:4

polyunsaturated fatty acid is the precursor of several classes of signal molecules

prostaglandins, prostacyclins,

thromboxanes, and leukotrienes that act as messengers and local hormones because of their transience. They are called

eicosanoids because they contain 20 carbon atoms. Prostaglandin synthase, the enzyme catalyzing the first step in the

synthesis of all eicosanoids except the leukotrienes, consists of a cyclooxygenase and a hydroperoxidase. Aspirin

(acetylsalicylate), an anti-inflammatory and antithrombotic drug, irreversibly blocks the synthesis of these eicosanoids.

Key Terms

triacylglycerol (neutral fat, triacylglyceride)

bile salt

chylomicron

acyl adenylate

carnitine

β-oxidation pathway

vitamin B

(cobalamin)

peroxisome

ketone body

acyl carrier protein (ACP)

fatty acid synthase

malonyl CoA

acetyl CoA carboxylase

megasynthase

polyketide

nonribosomal peptide

AMP-dependent protein kinase (AMPK)

protein phosphatase 2A

arachidonate

prostaglandin

eicosanoid

II. Transducing and Storing Energy 22. Fatty Acid Metabolism

Problems

After lipolysis. Write a balanced equation for the conversion of glycerol into pyruvate. Which enzymes are required

in addition to those of the glycolytic pathway?

See answer

From fatty acid to ketone body. Write a balanced equation for the conversion of stearate into acetoacetate.

See answer

Counterpoint. Compare and contrast fatty acid oxidation and synthesis with respect to

(a) site of the process.

(b) acyl carrier.

(d) stereochemistry of the intemediates.

(e) direction of synthesis or degradation.

(f) organization of the enzyme system.

See answer

Sources. For each of the following unsaturated fatty acids, indicate whether the biosynthetic precursor in animals is

palmitoleate, oleate, linoleate, or linolenate.

(a) 18:1 cis-∆

(b) 18:3 cis-∆

,∆

(d) 20:3 cis-∆

,∆

(e) 22:1 cis-∆

(f) 22:6 cis-∆

,∆

See answer

Tracing carbons. Consider a cell extract that actively synthesizes palmitate. Suppose that a fatty acid synthase in this

preparation forms one molecule of palmitate in about 5 minutes. A large amount of malonyl CoA labeled with

C in

each carbon of its malonyl unit is suddenly added to this system, and fatty acid synthesis is stopped a minute later by

altering the pH. The fatty acids in the supernatant are analyzed for radioactivity. Which carbon atom of the palmitate

formed by this system is more radioactive

C-1 or C-14?

See answer

Driven by decarboxylation. What is the role of decarboxylation in fatty acid synthesis? Name another key reaction in

a metabolic pathway that employs this mechanistic motif.

See answer

Kinase surfeit. Suppose that a promoter mutation leads to the overproduction of protein kinase A in adipose cells.

How might fatty acid metabolism be altered by this mutation?

See answer

An unaccepting mutant. The serine residue in acetyl CoA carboxylase that is the target of the AMP-dependent

protein kinase is mutated to alanine. What is a likely consequence of this mutation?

See answer

Blocked assets. The presence of a fuel molecule in the cytosol does not ensure that it can be effectively used. Give

two examples of how impaired transport of metabolites between compartments leads to disease.

See answer

10.

Elegant inversion. Peroxisomes have an alternative pathway for oxidizing polyunsaturated fatty acids. They contain

a hydratase that converts

d-3-hydroxyacyl CoA into trans-∆

-enoyl CoA. How can this enzyme be used to oxidize

CoAs containing a cis double bond at an even-numbered carbon atom (e.g., the cis-∆

double bond of linoleate)?

See answer

11.

Covalent catastrophe. What is a potential disadvantage of having many catalytic sites together on one very long

polypeptide chain?

See answer

12.

Missing acyl CoA dehydrogenases. A number of genetic deficiencies in acyl CoA dehydrogenases have been

described. This deficiency presents early in life after a period of fasting. Symptoms include vomiting, lethargy, and

sometimes coma. Not only are blood levels of glucose low (hypoglycemia), but starvation-induced ketosis is

absent. Provide a biochemical explanation for these last two observations.

See answer

13.

Effects of clofibrate. High blood levels of triacylglycerides are associated with heart attacks and strokes. Clofibrate,

a drug that increases the activity of peroxisomes, is sometimes used to treat patients with such a condition. What is

the biochemical basis for this treatment?

See answer

14.

A different kind of enzyme. Figure 22.27 shows the response of acetyl CoA carboxylase to varying amounts of

citrate. Explain this effect in light of the allosteric effects that citrate has on the enzyme. Predict the effects of

increasing concentrations of palmitoyl CoA.

See answer

Mechanism Problems

15.

Variation on a theme. Thiolase is homologous in structure to the condensing enzyme. On the basis of this

observation, propose a mechanism for the cleavage of 3-ketoacyl CoA by CoA.

See answer

16.

Two plus three to make four. Propose a reaction mechanism for the condensation of an acetyl unit with a malonyl

unit to form an acetoacetyl unit in fatty acid synthesis.

See answer

Chapter Integration Problems

17.

Ill-advised diet. Suppose that, for some bizarre reason, you decided to exist on a diet of whale and seal blubber,

exclusively.

(a) How would lack of carbohydrates affect your ability to utilize fats?

(b) What would your breath smell like?

consume a healthy dose of odd-chain fatty acids. Does your friend have your best interests at heart? Explain.

See answer

18.

Fats to glycogen. An animal is fed stearic acid that is radioactively labeled with [

C]carbon. A liver biopsy reveals

the presence of

C-labeled glycogen. How is this possible in light of the fact that animals cannot convert fats into

carbohydrates?

See answer

Data Interpretation Problem

19.

Mutant enzyme. Carnitine palmitoyl transferase I (CPTI) catalyzes the conversion of long-chain acyl CoA into acyl

carnitine, a prerequisite for transport into mitochondria and subsequent degradation. A mutant enzyme was

constructed with a single amino acid change at position 3 of glutamic acid for alanine. Figures A through C show

data from studies performed to identify the effect of the mutation [data from J. Shi, H. Zhu, D. N. Arvidson, and G.

J. Wodegiorgis. J. Biol. Chem. 274(1999): 9421 9426].

(a) What is the effect of the mutation on enzyme activity when the concentration of carnitine is varied? What are

the K

and V

max

values for the wild-type and mutant enzymes?

(b) What is the effect when the experiment is repeated with varying concentrations of palmitoyl CoA? What are the

and V

max

values for the wild-type and mutant enzymes?

more sensitive to malonyl CoA inhibition?

(d) Suppose that palmitoyl CoA = 100 µM, carnitine = 100 µM, and malonyl CoA = 10 µM. Under these

conditions, what is the most prominent effect of the mutation on the properties of the enzyme?