Lecture 6: Lipid metabolism - Synthesis Flashcards
(28 cards)
Where does fatty acid synthesis take place?
Cytoplasm
How similar are the processes of synthesis and degradation of fatty acids?
Energetically, each is the exact reverse of the other, but they need distinct enzymes and take place in different places in the cell.
Define ‘committed step’ and give an example from glycolysis.
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.
What is the committed step in fatty acid synthesis?
Acetyl CoA + ATP + bicarbonate —> Malonyl CoA + ADP + Pi + H+ Enzyme: Acetyl CoA carboxylase
Give 5 uses of Acetyl CoA.
- oxidation in the TCA cycle - Making cholesterol - Making amino acids - Making ketone bodies - Fatty acid synthesis (convert to malonyl CoA for fatty acid synthesis)
What is special about the catalysis of fatty acid synthesis?
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.
How does the elongation phase of fatty acid synthesis start?
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
Give the overall stoichiometry for palmitate synthesis.
Acetyl CoA + 7 Malonyl CoA + 14 NADPH + 14 H+ —> Palmitate + 14 NADP+ + 6 CO2 + 8 CoA + 7 H2O
Describe the process by which Acetyl CoA is converted to Malonyl CoA at the start of fatty acid synthesis.
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.
How are all of the intermediates of fatty acid synthesis connected to ACP and how is this similar to fatty acid degradation?
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).
Describe the pathway taking Acetyl-ACP to Palmitate + ACP.
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.
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Give the overall reaction of the synthesis of palmitate from acetyl CoA.
8 Acetyl CoA + 7 ATP + 14 NADPH + 6 H+ —> Palmitate + 7 ADP + 7 Pi + 14 NADP+ + 6 H2O + 8 CoA
How is NADPH produced?
Via the pentose phosphate pathway and the malate pyruvate cycle: malate + NADP+ Pyruvate + NADPH + CO2
How can fatty acids longer than palmitate be synthesised and what are they used for?
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.
How can mono-unsaturated fatty acids be synthesised and what are they used for?
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.
What are polyunsaturated fatty acids used for?
They are used to make prostglandins and thromboxones, which are both important in inflammatory response.
What is special about the synthesis of polyunsaturated fatty acids?
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).
Give an example of a polyunsaturated fatty acid.
Linoleic acids (linoleate), C18:2
Compare the 4 reaction cycle of fatty acid synthesis and degradation.
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)
Describe the cycle which is responsible for transporting Acetyl CoA out of the mitochondrial matrix into the cytoplasm.
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.
How is the cycle responsible for transporting Acetyl CoA out of the mitochondrial matrix regulated?
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).
What is the role of 1,2-Diacylglycerol (DAG)?
As a signalling molecule
What can phosphatidic acid become?
A phospholipid
