16: Lipids Anabolism
- Page ID
- 158581
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)- Explain the overall process and purpose of fatty acid anabolism (lipogenesis).
- Describe the role of Acetyl-CoA Carboxylase (ACC) and Fatty Acid Synthase (FAS) in fatty acid synthesis.
- Outline the four key steps involved in fatty acid elongation: condensation, reduction, dehydration, and final reduction.
- Discuss how hormonal regulation (insulin vs. glucagon) influences the direction of fatty acid metabolism.
- Apply your understanding of metabolic states (fed vs. fasting) to predict whether the body will favor lipogenesis or fatty acid oxidation.
- Lipogenesis: The anabolic process of synthesizing fatty acids from acetyl-CoA in the cytoplasm.
- Acetyl-CoA: A central metabolic intermediate that serves as the building block for fatty acid synthesis.
- Malonyl-CoA: A three-carbon intermediate formed from acetyl-CoA, used to elongate fatty acid chains.
- Acetyl-CoA Carboxylase (ACC): The rate-limiting enzyme that catalyzes the formation of malonyl-CoA using biotin and ATP.
- Fatty Acid Synthase (FAS): A multifunctional enzyme complex that performs sequential reactions to elongate fatty acids.
- Acyl Carrier Protein (ACP): A domain of FAS that temporarily holds fatty acid intermediates during synthesis.
- NADPH: A reducing agent derived from the pentose phosphate pathway, used in biosynthetic reactions.
- Condensation: The step in fatty acid synthesis where acetyl-ACP and malonyl-ACP combine, releasing CO₂.
- Hormone-sensitive lipase: An enzyme that breaks down stored triacylglycerols into free fatty acids during fasting.
- Carnitine Palmitoyltransferase I (CPT I): Mitochondrial enzyme that transports fatty acids for β-oxidation, inhibited by malonyl-CoA.
- Why does lipogenesis occur in the cytoplasm, while β-oxidation occurs in the mitochondria?
- What cellular conditions (nutrient availability, hormonal state) would promote fatty acid synthesis?
- What is the advantage of using malonyl-CoA (instead of acetyl-CoA) in the elongation process?
- How might a high-protein, low-carb diet impact the activity of Acetyl-CoA Carboxylase?
Fatty Acid Anabolism (Lipogenesis)
Fatty acid anabolism, also known as lipogenesis, is the metabolic process through which the body synthesizes fatty acids from smaller molecular building blocks. This anabolic process primarily occurs in the cytoplasm of cells, particularly in liver, adipose tissue, and mammary glands. The building blocks for fatty acid synthesis are acetyl-CoA molecules, which are assembled two carbons at a time to produce long-chain fatty acids. This energy-intensive process requires ATP, NADPH, and biotin as cofactors. Acetyl-CoA Carboxylase (ACC) and Fatty Acid Synthase (FAS) are the two primary enzymes involved.
Acetyl-CoA Carboxylase (ACC)
The first committed step of lipogenesis is the carboxylation of acetyl-CoA to form malonyl-CoA, catalyzed by the enzyme Acetyl-CoA Carboxylase (ACC). This enzyme uses ATP and biotin as a coenzyme to add a carboxylic acid group to acetyl-CoA, forming malonyl-CoA. This step is crucial because malonyl-CoA serves as the donor of two-carbon units in fatty acid elongation. In practical terms, when a person eats a carbohydrate-rich meal, glucose is metabolized into acetyl-CoA, and in the presence of insulin, ACC is activated to drive the synthesis of malonyl-CoA. This step is also highly regulated by nutritional and hormonal cues: insulin stimulates ACC, while glucagon and AMPK inhibit it via phosphorylation.
Once acetyl-CoA is converted into malonyl-CoA, both of these intermediates are loaded onto Fatty Acid Synthase (FAS). Within this complex, the intermediates are temporarily attached to a domain called Acyl Carrier Protein (ACP). This carrier domain acts like a molecular "arm," holding and transferring the growing fatty acid chain through the various enzymatic steps. The initial step involves attaching one acetyl group and one malonyl group to the ACP, forming acetyl-ACP and malonyl-ACP, respectively. This setup primes the system for elongation reactions.
Fatty Acid Elongation
The core of fatty acid synthesis involves a repeating cycle of four reactions: condensation, reduction, dehydration, and another reduction, each extending the carbon chain by two units.
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Condensation (Step 1): Fatty Acid Synthase catalyzes the condensation of acetyl-ACP and malonyl-ACP, releasing CO₂ and forming a β-keto intermediate called acetoacetyl-ACP. The CO₂ released in this step was originally added by ACC in the formation of malonyl-CoA. This decarboxylation helps drive the condensation reaction forward energetically. This is the step where the fatty acid chain begins to form.
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Reduction (Step 2): The β-ketone group in acetoacetyl-ACP is reduced to a β-hydroxyl group, a reaction that consumes NADPH as a reducing agent. This reduction is analogous to reducing a ketone to an alcohol, and NADPH used here is typically sourced from the pentose phosphate pathway.
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Dehydration (Step 3): The β-hydroxy group undergoes dehydration, resulting in the formation of a double bond between the α and β carbon atoms (α,β-unsaturated intermediate). This step helps stabilize the growing fatty acid chain and prepare it for the next reduction.
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Reduction (Step 4): Finally, the double bond is reduced again by another molecule of NADPH, yielding a fully saturated four-carbon fatty acid intermediate. This product is now ready to go through another round of elongation, with a new malonyl-CoA providing two more carbon units.
These four steps repeat several times (typically seven rounds) until a 16-carbon saturated fatty acid (palmitate) is produced.
Hormonal Regulation
The regulation of lipogenesis is tightly controlled by the nutritional state of the body and hormones such as insulin and glucagon. After a carbohydrate-rich meal, insulin is released, which activates lipogenesis through multiple mechanisms:
- Glucokinase is activated in the liver, promoting glycolysis and the conversion of glucose into pyruvate and then acetyl-CoA.
- Some glucose-6-phosphate is diverted to glycogen synthesis, while glyceraldehyde-3-phosphate serves as a precursor for glycerol synthesis (for triglyceride formation).
- Excess acetyl-CoA is used for fatty acid synthesis.
- Insulin also activates phosphodiesterase, which breaks down cAMP, thereby reversing the actions of glucagon and reducing PKA-mediated phosphorylation.
When fasting or under low glucose conditions:
- Glucagon is released and activates cAMP-PKA signaling, which inhibits pyruvate kinase in the liver, reducing glycolysis and glucose utilization.
- cAMP-dependent phosphorylation also inhibits Acetyl-CoA Carboxylase, thus blocking lipogenesis.
- Simultaneously, glucagon activates hormone-sensitive lipase, which breaks down stored triacylglycerols into free fatty acids and glycerol.
- Glycerol enters gluconeogenesis to help maintain blood glucose, while fatty acids are diverted toward β-oxidation to produce energy, not storage.
If a person is fasting for 24 hours, their liver is flooded with glucagon. As a result, acetyl-CoA carboxylase is phosphorylated and inactivated, preventing the formation of malonyl-CoA. At the same time, low malonyl-CoA levels remove inhibition on carnitine palmitoyltransferase I (CPT I), allowing fatty acids to enter the mitochondria for β-oxidation. Hence, the body stops storing fat and shifts entirely toward burning it for ATP.
- Trace the pathway from excess glucose to palmitate synthesis in the liver. What enzymes are activated along the way?
- Compare and contrast the roles of ACC and FAS in fatty acid synthesis. What happens if ACC is inhibited?
- Sequence the steps of the fatty acid elongation cycle (condensation → reduction → dehydration → reduction). What is the role of NADPH in these steps?
- Explain the metabolic shift that occurs during fasting. Why does glucagon inhibit ACC, and how does this affect fatty acid metabolism?
- A diabetic patient has high blood sugar and insulin resistance. Predict what happens to their lipogenesis pathway and the risk of fatty liver disease.
- A mouse is genetically engineered to overexpress ACC. What metabolic phenotypes would you expect (e.g., body fat, energy usage)?
- Imagine a drug that activates phosphodiesterase in the liver. What would happen to the lipogenesis pathway and why?
- How does malonyl-CoA simultaneously promote fatty acid synthesis and inhibit fatty acid oxidation? Why is this dual regulation biologically significant?
- What was the most conceptually challenging part of this pathway for you?
- How does this pathway connect to what you’ve learned about glycolysis, the TCA cycle, or the pentose phosphate pathway?
- Can you visualize a scenario (e.g., during a long fast, post-meal, or in exercise) where this pathway would be upregulated or downregulated?