Ch 21 - Lipid Biosynthesis

Question 1

Q. 6: The fatty acid synthase complex has ___ major functional regions. Which ones are those?

Answer 1

The fatty acid synthase complex has seven major functional regions. Which ones are those?

Question 2

Q. 1: The biological synthesis of fatty acids and the breakdown of fatty acids via beta oxidation have major differences with respect to enzymes involved and location. Name the most important ones.

Question 3

Q. 2: Fatty acid biosynthesis requires the synthesis and availability of a ____-carbon intermediate, called _________________.

Question 4

Q. 3: The formation of malonyl-CoA requires two substrates. Which ones are those? Which enzyme catalyzes this chemical reaction? Write down the chemical equation.

Question 5

Q. 4: The bacterial acetyl-CoA carboxylase (ACCase) has ___ separate polypeptide subunits and one of the subunits contains an important coenzyme called ______________. This coenzyme is an important dietary vitamin. Which one is it? Which foods are rich in this vitamin?

Question 6

Q. 5: The first step of fatty acid synthesis is the transfer of a carboxyl group, derived from _________________ to biotin. This endergonic step requires chemical energy supplied by the hydrolysis of ______. Write down the net chemical equation of the ACCase reaction.

Question 7

Q. 7: At the fatty acid synthase, long carbon chains of fatty acids are assembled in a repeating, cyclical ____-step sequence. At every round, a longer getting, saturated _____ group becomes the substrate for the subsequent condensation with an activated ___________ group. After each passage through the cycle, the fatty acyl chain is extended by ____ carbons under release of ____ molecule(s) of carbon dioxide.

Answer 7

At the fatty acid synthase, long carbon chains of fatty acids are assembled in a repeating, cyclical four-step sequence. At every round, a longer getting, saturated acyl group becomes the substrate for the subsequent condensation with an activated malonyl group. After each passage through the cycle, the fatty acyl chain is extended by two carbons under release of one molecule(s) of carbon dioxide.

Question 8

Q. 8: While in beta oxidation of fatty acids, the reduction equivalents are FAD and NAD⁺ (i.e. which serve as electron acceptors), the reducing agent in fatty acid synthesis is ___________.

Answer 8

While in beta oxidation of fatty acids, the reduction equivalents are FAD and NAD⁺ (i.e. which serve as electron acceptors), the reducing agent in fatty acid synthesis is NADPH.

Question 9

Q. 9: In E. coli, the core of the fatty acid synthase complex consists of __________ separate polypeptides and at least three other ones. During the cyclical synthesis of fatty acids from ___________________ and ______________________, the intermediates remain covalently attached to the complex as ______________________ to one of two thiol groups of the synthase complex.

Answer 9

In E. coli, the core of the fatty acid synthase complex consists of seven separate polypeptides and at least three other ones. During the cyclical synthesis of fatty acids from acetyl-CoA and malonyl-CoA, the intermediates remain covalently attached to the complex as thioesters to one of two thiol groups of the synthase complex.

Question 10

Q. 10: The central acyl carrier protein (ACP) of E. coli is a small protein which contains an important, sterically flexible prosthetic group. What is the name of this molecule? Synthesis of this molecule requires an important dietary vitamin. Which one is it?

Answer 10

4’-phosphopantetheine. Synthesis of this molecule requires pantothenic acid (vitamin B₅).

Question 11

Q. 11: What is the first event at the fatty acid synthase (FAS) complex? Which enzyme subunit of the complex catalyzes this chemical reaction?

Answer 11

The acetyl group of acetyl-CoA is transferred to the Cys thiol group of the β-ketoacyl-ACP synthase (KS). This is catalyzed by acetyl-CoA-ACP transacetylase (AT).

Question 12

Q. 12: Write down the second chemical reaction happening at the FAS complex. Which enzyme subunit of the FAS complex catalyzes this chemical reaction? Which dietary vitamin is necessary to allow this to happen?

Answer 12

The second reaction is the transfer of the malonyl group from malonyl-CoA to the thiol group of ACP, and this is catalyzed by malonyl-CoA-ACP transferase (MT). Since this reaction involves malonyl-CoA, vitamin B₅ (pantothenic acid) is necessary for CoA synthesis.

Question 13

Q. 14: What is the third event and actually the first chemical reaction of the FAS complex cycle? Which FAS subunit catalyzes this chemical reaction? What type of chemical reaction is it?

Answer 13

The first reaction of the charged FAS complex is the condensation of the activated acetyl and malonyl groups to form acetoacetyl-ACP. This reaction is catalyzed by β-ketoacyl-ACP synthase. It is a condensation reaction, and it is also a decarboxylation of the malonyl group. Thus, one molecule of CO₂ is released (same carbon that was introduced into malonyl-CoA from bicarbonate). Coupling the condensation to the decarboxylation of the malonyl group makes the overall reaction highly exergonic.

Question 14

Q. 15: What is the fourth event and actually the second chemical reaction of the FAS complex cycle? Which FAS subunit catalyzes this chemical reaction? What type of chemical reaction is it?

Answer 14

The acetoacetyl-ACP formed in the condensation step is reduced at the carbonyl group of C-3 to form β-hydroxybutyryl-ACP. β-ketoacyl-ACP reductase (KR) catalyzes this reaction. It is a redox reaction; β-ketobutyryl-ACP is reduced to β-hydroxybutyryl-ACP, and NADPH is oxidized to NADP⁺.

Question 15

Q. 16: What is the third chemical reaction of the FAS complex cycle? Which FAS subunit catalyzes this chemical reaction? What type of chemical reaction is it?

Answer 15

The third chemical reaction of the FAS complex cycle is the dehydration of β-hydroxybutyryl-ACP to trans-Δ²-butenoyl-ACP. This reaction is catalyzed by β-hydroxyacyl-ACP dehydratase (HD). It is a dehydration reaction.

Question 16

Q. 17: What is the fourth chemical reaction happening during each FAS complex cycle? Which FAS subunit catalyzes this chemical reaction? What type of chemical reaction is it?

Answer 16

The double bond of trans-Δ²-butenoyl-ACP is reduced to form butyryl-ACP. This is catalyzed by enoyl-ACP reductase (ER). It is a redox reaction; trans-Δ²-butenoyl-ACP is reduced to butyryl-ACP, and NADPH is oxidized to NADP⁺.

Question 17

Q: 18: Which of the following is the first event on the fatty acid synthase complex?

A) Transfer of malonyl-CoA onto SH group of ACP

B) Reduction of beta ketoacyl group by the beta ketoacyl-ACP reductase (KR)

C) Transfer of acetyl-CoA onto Cys-SH group of beta ketoacyl synthase (KS)

D) Reduction of trans-Δ2-butenoyl-ACP by enoyl-ACP reducatase

E) Dehydration of beta hydroxybutyryl-ACP by beta hydroxacyl-ACP hydratase

Answer 17

C) Transfer of acetyl-CoA onto Cys-SH group of beta ketoacyl synthase (KS)

Question 18

Q. 18: Explain the final event happening at the FAS complex. Which enzyme subunit of the FAS complex catalyzes this chemical reaction?

Answer 18

Formation of the four-carbon, saturated fatty acyl-ACP marks the completion of one pass through the FAS cycle, but the four-carbon group has to now be transferred from the phosphopantetheine –SH group of ACP to the Cys –SH group fo β-ketoacyl-ACP synthase. This is catalyzed by acetyl-CoA-ACP transacetylase (AT).

Question 19

Q. 19: How many FAS cycles are needed to synthesize the saturated 16-carbon fatty acid palmitic acid? What happens when the fatty acid bound on ACP has reached a length of 16 carbons? Why? Which enzyme(s) are involved in this process?

Answer 19

Seven cycles of condensation and reduction produce the 16-carbon saturated palmitoyl group, still bound to ACP. When the fatty acid chain has reached 16-carbons, chain elongation by the FAS complex usually stops and free palmitate is released from the ACP by a hydrolytic activity in the complex. The reasons for stopping chain elongation by the synthase complex at 16 carbons is not well understood. The newly synthesized 16-carbon fatty acid is hydrolytically released from the FAS complex by a thioesterase enzyme.

Question 20

Q. 20: Write down the net equation of fatty acid synthesis for the synthesis of 1 mole of palmitic acid.

Answer 20

Question 21

Q. 21: Fatty acid synthesis is an important cellular ATP consuming ________________ process. Where does the ATP usually come from?

Question 22

Q. 22: In E. coli (and some plants), the active sites of the fatty acid synthase complex reside in __________ separate polypeptides, whereas in vertebrates and humans, the FAS complex is comprised of _________________________ which contains all _______________ necessary enzymatic activities.

Answer 22

In E. coli (and some plants), the active sites of the fatty acid synthase complex reside in seven separate polypeptides, whereas in vertebrates and humans, the FAS complex is comprised of a single large polypeptide which contains all seven necessary enzymatic activities.

Question 23

Q. 22: Where within the cell is the FAS complex located? Which molecule supplies the necessary electrons and protons for the two reduction steps happening per cycle on the complex?

Answer 23

In higher eukaryotes, the FAS complex is in the cytosol. NADPH is the electron carrier for the reduction steps that occur in the FAS complex at β-ketoacyl-ACP reductase and enoyl-ACP reductase.

Question 24

Q. 23: How is it possible that the anabolic chemical reactions of fatty acid synthesis can happen side by side within the cytosol where many catabolic chemical reactions, i.e. glycolysis, take place at the same time?

Answer 24

The cells where these catabolic and anabolic reactions take place, namely hepatocytes, keep the ratio of NADPH to NADP+ high in the cytosol and the ratio of NADH to NAD+ low in the cytosol. This creates a strongly reducing environment for the reductive synthesis of fatty acids that use NADPH in their reduction reactions, and the NAD+-dependent oxidative catabolism of glucose can also take place in the cytosol. Basically, the anabolic and catabolic reactions use different electron carriers (NADPH donates electrons in anabolism and NAD+ accepts electrons in catabolism), and those respective electron carrier concentrations are kept high in the cytosol so the reactions can take place side-by-side in the same compartment at the same time.

Question 25

Q. 24: Which metabolic pathway(s) and/or enzymes are responsible to supply the necessary NADPH + H⁺ reduction equivalents for fatty acid synthesis in hepatocytes and adipocytes?

Answer 25

In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the pentose phosphate pathway and by malic enzyme.

Question 26

Q. 25: Write down the chemical reaction catalyzed by the malic enzyme. Where is it located in human cells?

Answer 26

The malic enzyme is located in the cytosol of human cells.

Question 27

Q. 26: Fatty acid synthesis takes place in the cytosol of cells but the acetyl-CoA, the key raw material for this process is generated within mitochondria. However, acetyl-CoA (as a charged molecule) is impermeable for phospholipid membranes. How do cell solve this problem? Which components and enzymes are involved to assure ample supply of cytosolic acetyl-CoA for fatty acid synthesis?

Answer 27

An indirect shuttle transfers acetyl group equivalents across the inner mitochondrial membrane. Once transferred from the mitochondrial matrix to the cytosol, acetyl-CoA is re-formed. Intramitochondrial acetyl-CoA reacts with oxaloacetate to form citrate via citrate synthase, and citrate is transported out of the mitochondria via the citrate transporter. In the cytosol, citrate is cleaved by citrate lyase, regenerating acetyl-CoA and oxaloacetate in an ATP-dependent reaction. Oxaloacetate cannot be transported back into the mitochondria directly, so it is reduced to malate by malate dehydrogenase which returns to the mitochondria matrix via the malate-α-ketoglutarate transporter. In the matrix, malate is re-oxidized to oxaloacetate to complete the shuttle. The malate in the cytosol has another possible fate, though. It can be used to produce NADPH through the activity of malic enzyme.

Question 28

Q. 27: There is an alternative route established in cells which allows the removal of malate from the cytosol after it has been generated by the cMDH enzyme. Which one is it? Name the enzymes and intermediates involved.

Answer 28

Malate can either return to the matrix by the malate-α-ketoglutarate transporter, or malate can convert to pyruvate in the cytosol by a reaction catalyzed by malic enzyme with NADPH and CO₂ products forming as well. Pyruvate transporter allows pyruvate to enter the matrix, where pyruvate can form oxaloacetate with the addition of CO₂ in an ATP-dependent reaction catalyzed by pyruvate carboxylase. Now the alternative pathway is back to the starting point of the acetyl group shuttle.

Question 29

Q. 28: What is the name of the key regulatory enzyme of fatty acid synthesis in cells? Name the regulatory factors which control its catalytic activity. Where is it located?

Answer 29

Acetyl-CoA carboxylase. It is located in the cytosol.

• Palmitoyl-CoA is a feedback inhibitor of acetyl-CoA carboxylase.
• Citrate is an allosteric activator of this enzyme.
• Phosphorylation of acetyl-CoA carboxylase, triggered by glucagon and epinephrine, inactivates the enzyme and reduces its sensitivity to activation by citrate. Phosphorylation also causes the polymerized long filaments of acetyl-CoA carboxylase to dissociate into monomeric subunits, accompanied by loss of activity.
• Acetyl-CoA carboxylase’s active form is dephosphorylated.

Question 30

Q. 29: How do cells prevent the – certainly wasteful – parallel operation of fatty acid synthesis and beta- oxidation?

Answer 30

Malonyl-CoA inhibits carnitine acyltransferase I, thereby blocking β-oxidation. Malonyl-CoA is produced as the first intermediate in fatty acid synthesis; therefore, production of malonyl-CoA shuts down β-oxidation at the level of a transport system in the mitochondrial inner membrane, and it signifies that the cell is focused on synthesizing fatty acids and prevents the opposite (breakdown) from happening.

Question 31

Q. 30: Palmitate, a saturated ___ carbon fatty acid, is the primary and most common end product of fatty acid synthesis. However, cells have the means to lengthened the hydrocarbon chain, e.g. to synthesize the ___ carbon fatty acid stearate, by sequentially adding more acetyl groups. Which enzyme(s) do this? Where do those chemical reactions take place?

Answer 31

Palmitate, a saturated 16 carbon fatty acid, is the primary and most common end product of fatty acid synthesis. However, cells have the means to lengthened the hydrocarbon chain, e.g. to synthesize the 18 carbon fatty acid stearate, by sequentially adding more acetyl groups. Which enzyme(s) do this? Where do those chemical reactions take place?

Palmitate may be elongated to stearate by addition of an acetyl group through the action of fatty acid elongation systems present in the smooth endoplasmic reticulum. There are also other elongation systems found in mitochondria, but the major pathway is in the smooth ER. The enzymes that elongate fatty acids are called elongases. Although location is different, a different enzyme system is involved, and CoA rather than ACP is the acyl carrier in the reaction, the mechanism of elongation in the ER is otherwise identical to palmitate synthesis in the cytosol.

Question 32

Q. 31: Even though humans can elongate palmitic acid to stearic acid and subsequently desaturate the latter to oleic acid (18:1), they cannot convert oleate to ____ [18:2(Δ9,12)] or alpha-linolenate [________________]. Both unsaturated fatty acids are therefore required to be taken up via our diet. in the diet as essential fatty acids. Which food sources are known to be rich in those two unsaturated fatty acids?

Answer 32

Even though humans can elongate palmitic acid to stearic acid and subsequently desaturate the latter to oleic acid (18:1), they cannot convert oleate to linoleate [18:2(Δ9,12)] or alpha-linolenate [18:2(Δ9,12,15)]. Both unsaturated fatty acids are therefore required to be taken up via our diet. in the diet as essential fatty acids. Which food sources are known to be rich in those two unsaturated fatty acids?

Plants can synthesize linoleate and α-linolenate, and these are essential fatty acids for mammals because they are necessary precursors for the synthesis of other products. Therefore, these fatty acids must be obtained from dietary plant material. Some of the food sources of linoleate and linolenate are fish, shellfish, flaxseed, olive oil, sunflower seeds, leafy vegetables, and walnuts.

Question 33

Q. 32: Palmitate and stearate serve as precursors of the two most common mono-unsaturated fatty acids of animal tissues. Name those two and write down their correct sum formula annotation. The double bond is introduced into the fatty acid chain of those fatty acids by an enzyme called __________________________________.

Answer 33

Palmitate and stearate serve as precursors of the two most common mono-unsaturated fatty acids of animal tissues. Name those two and write down their correct sum formula annotation. The double bond is introduced into the fatty acid chain of those fatty acids by an enzyme called fatty acyl-CoA desaturase.

Palmitoleate [16:1(Δ⁹)] and oleate [18:1(Δ⁹)].

Question 34

Q. 33: The fatty acyl–CoA desaturase is classified as a mixed-function oxidase. Explain that term.

Answer 34

Mixed-function oxidases oxidize two different substrates simultaneously. In the desaturation of fatty acyl-CoA in vertebrates, saturated fatty acyl-CoA and NADPH are oxidized by molecular oxygen to produce monosaturated fatty acyl-CoA, NADP⁺, and two molecules of water.

Question 35

Q. 34: Fatty acyl–CoA desaturase reduce saturated fatty acyl-CoAs into ______-unsaturated fatty acyl-CoA molecules. This enzyme contains an iron-containing _____________________ and a FAD-containing flavoprotein (cytochrome b5 reductase). The ultimate electron and protons-donating molecule is _____________. What is the path of electrons during desaturation?

Answer 35

Fatty acyl–CoA desaturase reduce saturated fatty acyl-CoAs into mono-unsaturated fatty acyl-CoA molecules. This enzyme contains an iron-containing cytochrome (Cyt b₅) and a FAD-containing flavoprotein (cytochrome b5 reductase). The ultimate electron and protons-donating molecule is NADPH + H⁺. What is the path of electrons during desaturation?

Question 36

Q. 35: Monooxygenases are enzyme which catalyze chemical reactions in which __________ of the two oxygen atoms of O₂ is incorporated into an organic substrate, the other being reduced to ____________. As a consequence, the main substrate of monooxygenases becomes hydroxylated. How do monooxygenases differ from mixed-function oxidases?

Answer 36

Monooxygenases are enzyme which catalyze chemical reactions in which one of the two oxygen atoms of O₂ is incorporated into an organic substrate, the other being reduced to H₂O. As a consequence, the main substrate of monooxygenases becomes hydroxylated. How do monooxygenases differ from mixed-function oxidases?

Monooxygenases catalyze reactions in which only one of the two oxygen atoms of O₂ is incorporated into the organic substrate, and the other is reduced to H₂O. Most monooxygenases catalyze reactions in which the main substrate becomes hydroxylated. Mixed-function oxidases oxidize two different substrates simultaneously, but the main substrate does not always get hydroxylated. Mixed-function oxidases can catalyze oxidations in which molecular oxygen is the electron acceptor, but oxygen does not necessarily appear in the oxidized product (such as the case of double-bond formation). For example, the mixed-function oxidase called fatty acyl-CoA desaturase oxidizes a saturated fatty acyl-CoA to a monounsaturated fatty acyl-CoA, but a hydroxyl group is not added to the fatty acyl-CoA. Instead, a cis double bond is formed.

Question 37

Q. 36: In chemical nomenclature, what is the general name of enzymes that catalyze oxidation reactions in which molecular oxygen is the electron acceptor? Name an important example of this type of enzyme which plays an important role in white blood cells and adipocytes.

Question 38

Q. 37: Arachidonic acid [20:4(Δ5,8,11,14)] is an enormously important poly-unsaturated fatty acid (PUFA). It is synthesized starting from linoleate via first a desaturation step. Name the next steps leading to bio-synthesis of this important PUFA. Where within the cells is it stored?

Answer 38

Arachidonic acid is mostly stored as a fatty acid component in the phospholipids of cell membranes.

Question 39

Q. 38: Arachidonic acid is released from its cellular storage pool only on demand, i.e. after hormonal stimulation. Which hormones are known to stimulate arachidonic acid release into the cytosol? Which enzyme is catalyzing this event?