Lecture 6: Lipid metabolism - Synthesis

Question 1

Where does fatty acid synthesis take place?

Answer 1

Cytoplasm

Question 2

How similar are the processes of synthesis and degradation of fatty acids?

Answer 2

Energetically, each is the exact reverse of the other, but they need distinct enzymes and take place in different places in the cell.

Question 3

Define ‘committed step’ and give an example from glycolysis.

Answer 3

The first step in the pathway which commits an intermediate to go down that pathway and is therefore a major control point. The committed step in glycolysis is fructose-6-phosphate —> fructose-1,6-phosphate (catalysed by phosphofructokinase 1). This is because this is the first irreversible reaction in glycolysis. Fructose-6-phosphate can be converted back to glucose-6-phosphate, but fructose-1,6-phosphate must continue glycolysis to become pyruvate.

Question 4

What is the committed step in fatty acid synthesis?

Answer 4

Acetyl CoA + ATP + bicarbonate —> Malonyl CoA + ADP + Pi + H+ Enzyme: Acetyl CoA carboxylase

Question 5

Give 5 uses of Acetyl CoA.

Answer 5

• oxidation in the TCA cycle - Making cholesterol - Making amino acids - Making ketone bodies - Fatty acid synthesis (convert to malonyl CoA for fatty acid synthesis)

Question 6

What is special about the catalysis of fatty acid synthesis?

Answer 6

Apart from the first reaction (acetyl CoA —> malonyl CoA), every reaction in fatty acid is catalysed by the same enzyme, ACP. In mammals, ACP is part of a big enzyme complex called fatty acid synthase.

Question 7

How does the elongation phase of fatty acid synthesis start?

Answer 7

It starts with the formation of acetyl-ACP and malonyl-ACP from acetyl-CoA and malonyl-CoA. Acetyl-S-CoA + ACP Acetyl-ACP + CoA Malonyl-S-CoA + ACP Malonyl-ACP + CoA

Question 8

Give the overall stoichiometry for palmitate synthesis.

Answer 8

Acetyl CoA + 7 Malonyl CoA + 14 NADPH + 14 H⁺ —> Palmitate + 14 NADP+ + 6 CO₂ + 8 CoA + 7 H₂O

Question 9

Describe the process by which Acetyl CoA is converted to Malonyl CoA at the start of fatty acid synthesis.

Answer 9

The enzyme Acetyl CoA carboxylase has 2 activities: biotin carboxylase and transcarboxylase. It catalyses the carboxylation reaction of biotin attached to the biotin carrier protein with ATP, releasing ADP and Pi and attaching CO2 (O=C-O-) to the biotin on the biotin carrier protein. Then it catalyses the reaction of acetyl CoA and the activated CO2 on the biotin, producing malonyl CoA and free biotin-biotin carrier protein. Here biotin is acting as a cofactor.

Question 10

How are all of the intermediates of fatty acid synthesis connected to ACP and how is this similar to fatty acid degradation?

Answer 10

The intermediates in fatty acid synthesis are covalently bonded to the reactive thiol group in the phosphopantetheine moiety of ACP, which is connected to the fatty acid synthase enzyme via a serine residue. This is similar to CoA, which is the carrier in fatty acid degradation, which also has a reactive thiol group within the same phosphopantetheine moiety (instead of being attached to a protein, as in ACP, it is attached to adenine).

Question 11

Describe the pathway taking Acetyl-ACP to Palmitate + ACP.

Answer 11

Acetyl-ACP is attacked by Malonyl-ACP in a reaction, catalysed by beta-ketoacyl-ACP synthase, which releases ACP-CO2 (the CO2 is from the bicarbonate in the carboxylation reaction earlier) and produces beta-Ketoacyl-ACP (has a ketone functional group). Ketoacyl-ACP is then converted to D-3-Hydroxyacyl-ACP (has a hydroxyl functional group) by beta-Ketoacyl-ACP reductase, in a reduction reaction in which the electron donor is NADPH + H+ (converted to NADP+ during reaction). D-3-Hydroxyacyl-ACP is then converted to trans-(triangle)2-Enoyl-ACP (has C=C functional group) 3-Hydroxyacyl-ACP dehydrase, in a dehydration reaction (water is removed). Then trans-(triangle)2-Enoyl-ACP is reduced to Butyryl-ACP (alkane functional group) by Enoyl-ACP reductase, in a reduction reaction in which the electron donor is NADPH + H+ (converted to NADP+ during the reaction). This concludes the first cycle. Butyryl-ACP is Acetyl-ACP with two CH2s added to the end of acetyl CoA’s CH3-C=O chain. Then Butyryl-ACP enters another cycle by reacting with another malonyl-ACP. This is repeated until palmitate is reached (C16:0) - takes 7 cycles. Palmitate is the longest fatty acid that can be produced in this way. Shorter fatty acids are made with fewer cycles. All of these reactions are catalysed by fatty acid synthase complex.

Question 12

You’re doing really well! Keep going!

Answer 12

Question 13

Give the overall reaction of the synthesis of palmitate from acetyl CoA.

Answer 13

8 Acetyl CoA + 7 ATP + 14 NADPH + 6 H+ —> Palmitate + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O + 8 CoA

Question 14

How is NADPH produced?

Answer 14

Via the pentose phosphate pathway and the malate pyruvate cycle: malate + NADP+ Pyruvate + NADPH + CO2

Question 15

How can fatty acids longer than palmitate be synthesised and what are they used for?

Answer 15

Elongase enzymes elongate palmitate into longer fatty acids, such as stearate (C18:0). Longer fatty acids are used to make triglycerides or phospholipids on the cytoplasmic face of the ER.

Question 16

How can mono-unsaturated fatty acids be synthesised and what are they used for?

Answer 16

Saturated fatty acids, such as palmitate, are modified by desaturase enzymes to produce mono-unsaturated fatty acids. They are used to make triglycerides or phospholipids on the cytoplasmic face of the ER.

Question 17

What are polyunsaturated fatty acids used for?

Answer 17

They are used to make prostglandins and thromboxones, which are both important in inflammatory response.

Question 18

What is special about the synthesis of polyunsaturated fatty acids?

Answer 18

Animals cannot perform the synthesis of polyunsaturated fatty acids, but plants can, so polyunsaturated fatty acids are essential fatty acids (they must be obtained in the diet from plants).

Question 19

Give an example of a polyunsaturated fatty acid.

Answer 19

Linoleic acids (linoleate), C18:2

Question 20

Compare the 4 reaction cycle of fatty acid synthesis and degradation.

Answer 20

Reaction 1: Synthesis: dehydrogenation (+FAD) Degradation: reduction (+NADPH + H+) Reaction 2: Synthesis: hydration (+H2O) Degradation: dehydration (-H2O) Reaction 3: Synthesis: dehydrogenation and oxidation (+NAD+) Degradation: reduction (+NADPH + H+) Reaction 4: Synthesis: thiolytic cleavage Degradation : condensation (+ malonyl CoA, - CO2)

Question 21

Describe the cycle which is responsible for transporting Acetyl CoA out of the mitochondrial matrix into the cytoplasm.

Answer 21

Acetyl CoA is converted to citrate by citrate synthase. Citrate is then transported out of the mitochondria by a transmembrane pore/protein, which simultaneously transports Pi into the mitochondria. In the cytoplasm, citrate is then converted back to acetyl CoA by citrate lyase. Some of the citrate is converted to oxaloacetate, which is then converted to malate by malate dehydrogenase, with the simultaneously conversion of NADH + H+ to NAD+. Then the malate is transported back into the mitochondria via the same pore/protein as citrate. Once back in the matrix, malate is converted back to oxaloacetate by malate dehydrogenase again, with NAD+ becoming NADH + H+ in the process. Then oxaloacetate is converted to citrate, replenishing the cycle. Some malate is not transported into the mitochondria, but is instead converted into pyruvate by malic enzyme, using up NADP+ and releasing CO2, H+ and importantly, NADPH. Then the pyruvate is transported into the mitochondria, where it is converted to oxaloacetate, using ATP and CO2 and releasing ADP and Pi.

Question 22

How is the cycle responsible for transporting Acetyl CoA out of the mitochondrial matrix regulated?

Answer 22

If ATP levels are low, Acetyl CoA enters the TCA cycle, instead of leaving the mitochondria for fatty acid synthesis. If ATP levels are high, Acetyl CoA is converted to citrate and leaves the mitochondria to perform fatty acid synthesis in the cytoplasm (if ATP levels are high fatty acid synthesis is advisable).

Question 23

What is the role of 1,2-Diacylglycerol (DAG)?

Answer 23

As a signalling molecule

Question 24

What can phosphatidic acid become?

Answer 24

A phospholipid

Question 25

Describe how triacylglycerol is synthesised.

Answer 25

Dihydroxyacetone phosphate from glycolysis or gluconeogenesis is reduced to Glycerol-3-phosphate with by glycerol-3-phosphate dehydrogenase, with the simultaneous oxidation of NADH + H+ to NAD+. Then glycerol-3-phosphate is converted to Lysophosphatidic acid by glycerol-3-phosphate acyltransferase, with the simultaneous conversion of R-C(=O)-SCoA to H-SCoA. Then Lysophosphatidic acid is converted to phosphatidic acid by L-acylglycerol-3-phosphate acyltransferase, with the simultaneous conversion of R’-c(=O)-SCoA to H-SCoA. Then phosphatidic acid, which can be a component of phospholipids, is dephosphorylated by phosphatidic acid phosphatase to give 1,2-Diacylglycerol (DAG), which can be used as a signalling molecule. Then 1,2-Diacylglycerol (DAG) is converted to triacylglycerol by diacylglycerol acyltransferase, with the simultaneous conversion of R’‘-C(=O)-SCoA to H-SCoA.

Question 26

How is acetyl CoA carboxylase controlled?

Answer 26

Phosphorylation, which is in turn stimulated by hormones Inactive when phosphorylated. When blood sugar is high, insulin is released and stimulates a phosphatase to dephosphorylate acetyl CoA carboxylase, activating it. This increases the rate of fatty acid synthesis and removes acetyl CoA, driving glycolysis and using up glucose, thus lowering the blood glucose. When blood sugar is low, glucagon is released and stimulates cAMP-dependent protein kinase (PKA) to phosphorylate acetyl CoA carboxylase, deactivating it. This decreases the rate of fatty acid synthesis, so less acetyl CoA is removed and glycolysis slows, so less glucose is used up. Allosteric activation by citrate, inhibition by palmitoyl CoA. If citrate concentration is high, the TCA cycle is slow and ATP is high, so acetyl CoA can be used for synthesis instead of oxidation to produce more ATP in the TCA cycle. If palmitoyl CoA (end point of fatty acid synthesis) concentration is high, there is no need for more fat, so acetyl CoA can be used to make cholesterol instead.

Question 27

Explain how fatty acid synthesis is an example of coordinated use of metabolic pathways.

Answer 27

TCA cycle - transports oxaloacetate out of the mitochondria Pentose phosphate pathway - provides NADPH (reducing power) Glycolysis and TCA cycle - oxidative phosphorylation provides ATP

Question 28

Well done, have a pretty picture!

Answer 28