Synthesis and Storage of Fats as an Energy Reserve
Synthesis & Storage of Fat as an Energy Reserve
Overview
This lecture covers the synthesis and storage of fats as an energy reserve, focusing on glycolysis, lipogenesis, fatty acid synthesis, inhibition of fatty acid oxidation, and fatty acid modification. It encompasses the biochemical pathways and regulatory mechanisms involved in converting excess nutrients into fat, primarily triacylglycerols (TAGs), for energy storage.
Objectives
After this lecture, you should be able to:
Explain the significance of fatty acid synthesis in humans, emphasizing its role in energy homeostasis and metabolic regulation.
Describe the formation of acetyl CoA from glucose and its transport into the cytosol via the citrate shuttle, linking it to NADPH production through the pentose phosphate pathway and malic enzyme.
Describe malonyl CoA formation by acetyl-CoA carboxylase (ACC) and its effect on fatty acid oxidation through the inhibition of carnitine palmitoyltransferase-1 (CPT-1).
Describe the pathway of fatty acid and triacylglycerol synthesis, including the enzymes involved and the sequential addition of fatty acids to glycerol-3-phosphate.
Describe fatty acid modification including elongation and desaturation and its relation to essential fatty acids, highlighting the importance of dietary intake and the roles desaturases play.
Describe how adipose tissue is regulated to store/release fatty acids at appropriate times, focusing on hormonal control by insulin and glucagon.
Fatty Acids
Fatty acids are the simplest lipid unit, consisting of a hydrophobic hydrocarbon chain with a terminal carboxyl group (hydrophilic). They can differ in length, saturation (number of double bonds), and the position of these double bonds. The general formula for a fatty acid is:
Where 'n' is typically an even number.
Fatty Acid Synthesis Systems
There are two systems for fatty acid synthesis:
De novo synthetic pathway: Occurs extra-mitochondrially (cytoplasmic). This pathway synthesizes palmitic acid from acetyl CoA.
Modification pathway: Occurs in the mitochondria and endoplasmic reticulum (ER). It involves elongation and desaturation of existing fatty acids.
De Novo Synthesis of Fatty Acids
De novo synthesis is the new carbon formation from excess amino acids and carbohydrates in the well-fed state, occurring extra-mitochondrially in the cytoplasm. It is activated when there is an excess of nutrients, particularly glucose, which is converted to acetyl CoA.
Primary Product
The primary product is Palmitic fatty acid (16 carbons, saturated), also called the Stem FA, with the formula , which can be modified by elongation and desaturation in the ER to produce other fatty acids.
Importance of Fatty Acid Synthesis
Fatty acid synthesis is crucial for:
Energy storage: Excess carbohydrates and proteins are converted to fatty acids and stored as triacylglycerols in adipose tissue.
Membrane synthesis: Fatty acids are essential components of phospholipids, which form cellular membranes.
Synthesis of signaling molecules: Fatty acids are precursors for various signaling molecules, such as eicosanoids.
Substrates for Fatty Acid Synthesis
The substrates used in fatty acid synthesis include:
Acetyl CoA: Provides the two-carbon units for fatty acid elongation.
NADPH: Provides the reducing equivalents needed for the reduction steps in fatty acid synthesis.
ATP, Biotin, , : Required for the acetyl CoA carboxylase (ACC) reaction, which forms malonyl CoA.
Sources of NADPH + H+
Hexose Monophosphate pathway (HMP or PPP): Major source, producing NADPH as glucose-6-phosphate is converted to ribulose-5-phosphate.
Malic Enzyme: Cytosolic enzyme that converts malate to pyruvate, generating NADPH.
Cytosolic Isocitrate Dehydrogenase (minimal role): Converts isocitrate to α-ketoglutarate, producing NADPH in the cytosol.
Sources of Acetyl CoA
Carbohydrate → pyruvate dehydrogenase (PDH complex) in the well-fed state: Glucose is converted to pyruvate via glycolysis, then pyruvate is converted to acetyl CoA by PDH in the mitochondria.
Ketogenic amino acids: Amino acids that are degraded to acetyl CoA or acetoacetate.
Fatty acid oxidation → acetyl CoA during fasting and Diabetes Mellitus I (DM I): During fasting, fatty acids are broken down to acetyl CoA in the mitochondria, but this acetyl CoA is primarily used for ketone body synthesis rather than fatty acid synthesis.
Pyruvate Dehydrogenase
Pyruvate, derived from glycolysis, serves as the acetyl CoA source for fatty acid synthesis.
Pyruvate dehydrogenase converts 3C pyruvate into 2C acetyl CoA. The reaction is:
PDH is regulated; it's inhibited by its products (acetyl CoA and NADH) and stimulated by its substrates (pyruvate, CoA, and NAD+). It is also responsive to the energy charge of the cell (ATP/ADP ratio).
Citrate Shuttle
Acetyl CoA is generated in the mitochondria but is needed in the cytosol for fatty acid synthesis. The citrate shuttle transports acetyl CoA from the mitochondria to the cytosol:
Acetyl CoA combines with oxaloacetate in the mitochondria to form citrate.
Citrate is transported across the mitochondrial membrane to the cytosol.
In the cytosol, citrate is cleaved by ATP-citrate lyase to regenerate acetyl CoA and oxaloacetate.
Oxaloacetate is converted to malate by malate dehydrogenase, and malate is then converted to pyruvate by malic enzyme, generating NADPH.
This process not only transports acetyl CoA into the cytosol but also generates NADPH, which is essential for fatty acid synthesis.
Fatty Acid Synthesis Steps
Acetyl CoA Carboxylase:
Acetyl CoA (2C) → Malonyl CoA (3C) (Biotin, )
This is the rate-limiting step of fatty acid synthesis, catalyzed by acetyl CoA carboxylase (ACC). The reaction is:
ACC is regulated by:
Citrate: Activates ACC by promoting its polymerization.
Palmitoyl CoA: Inhibits ACC by causing its depolymerization.
Insulin: Activates ACC by dephosphorylation.
Glucagon and Epinephrine: Inhibit ACC by phosphorylation.
Fatty Acid Synthase:
Multi-enzyme complex
A homodimer, with each monomer having 7 protein domains.
The acyl carrier protein (ACP) domain is where the nascent fatty acid attaches until it reaches 16C and is hydrolyzed by TE (Thioesterase).
KS = Beta-ketoacyl synthase (cysteine residue)
AT = Acyl transferase
DH = Dehydratase
ER = Enoyl reductase
KR = Ketoacyl reductase
ACP = Acyl carrier protein (pantothenic acid residue, incoming malonyl CoA attachment)
TE = Thioesterase
Steps of Fatty Acid Synthesis by Fatty Acid Synthase
Acetate molecule transferred from Acetyl CoA to the –SH group of cysteine residue of KS.
The ACP accepts a 3C malonate unit from malonyl CoA.
Condensation: The acetyl group is transferred from KS to the malonyl group on ACP, releasing and forming a 4-carbon β-ketoacyl-ACP.
Reduction (using NADPH), dehydration, and reduction (using NADPH): The β-ketoacyl group is reduced to a saturated acyl group through the following steps:
Reduction: The keto group is reduced to a hydroxyl group by β-ketoacyl reductase (KR), using NADPH.
Dehydration: Water is removed by dehydratase (DH), forming a double bond between the α and β carbons.
Reduction: The double bond is reduced by enoyl reductase (ER), using NADPH.
Cycles repeat, and when the fatty acid is 16 carbon atoms long, Thioesterase catalyzes hydrolysis, releasing palmitic acid.
Palmitic acid is the final product of the Fatty Acid Synthase complex (but it may produce short chain fatty acids in the lactating mammary glands).
Further elongation and desaturation are carried out by other enzyme systems.
Summary of Process
Acetyl CoA (2C) → (4C) → (6C) → … → Palmitic FA 16C
Reduction is done using NADPH.
Modification of Fatty Acids
Modifications create a variety of fatty acids with different physical properties.
Human FAS only makes saturated 16C fatty acids, insufficient for all purposes.
Synthesized and ingested fatty acids are modified by altering saturation and length.
These modifications occur in the smooth endoplasmic reticulum of the cell.
Enzymes prefer ω-3 and ω-6 essential fatty acids; if these are low in diet, they act on newly synthesized FAs.
Modifications cannot cause interconversion between series.
Elongation of Fatty Acids
Elongation occurs in the ER and mitochondria. The process involves adding two-carbon units (derived from malonyl CoA) to the carboxyl end of the fatty acid. Elongases use malonyl-CoA to add 2-carbon units to existing fatty acids to lengthen them.
Desaturation of Fatty Acids
Human cells contain three desaturases: Δ9, Δ6, and Δ5, which desaturate at carbons 9 and 10, 6 and 7, and 5 and 6, respectively. Desaturases introduce double bonds into saturated fatty acids. The most common desaturations occur at the Δ9 position, converting stearic acid (18:0) to oleic acid (18:1).
Triacyl Glycerol (TAG) Synthesis
Glycerol 3-phosphate is the initial acceptor of fatty acids during TAG synthesis.
Two pathways for its production:
In the liver and adipose tissue, glycerol 3-phosphate can be produced from glucose, using the glycolytic pathway to produce dihydroxyacetone phosphate (DHAP).
Second pathway (liver only) uses glycerol kinase to convert free glycerol to glycerol phosphate.
Activation of a free fatty acid:
A fatty acid must be converted to its activated form (bound to CoA) before it can participate in metabolic processes such as TAG synthesis.
Synthesis of Triacylglycerol
Involves the sequential addition of two fatty acids from fatty acyl CoAs, the removal of phosphate, and the addition of the third fatty acid.
TAG Synthesis - Liver vs Adipose
The liver takes up fatty acids from circulation, converts them back into TAG, packages them into VLDLs, and releases them into circulation. The liver synthesizes TAGs from both dietary and de novo synthesized fatty acids.
Circulating VLDLs are a constantly available source of fatty acids for tissues that need them, which use lipoprotein lipase to get the fatty acids. Lipoprotein lipase (LPL) hydrolyzes TAGs in VLDLs, releasing fatty acids that can be taken up by tissues.
Adipose only makes TAG during the fed state and releases fatty acids outside of it, due to hormone-sensitive lipase regulation. Adipose tissue primarily stores TAGs and releases fatty acids during fasting or exercise.
Mobilization of Stored Fat (Lipolysis)
It is phosphorylated by a 3', 5'-cAMP dependent protein kinase. Hormone-sensitive lipase (HSL) is activated by phosphorylation, which is stimulated by hormones such as epinephrine, norepinephrine, glucagon, and cortisol. Insulin inhibits HSL.
Fate of Glycerol
Glycerol released during TAG degradation cannot be metabolized by adipocytes (lack glycerol kinase). It is transported to the liver, where it can be phosphorylated to form TAG or converted to DHAP. In the liver, glycerol can be used for gluconeogenesis or TAG synthesis.
Fate of Fatty Acids
Free fatty acids move through the adipocyte membrane and bind to plasma albumin. They are transported to the tissues, activated to their CoA derivatives, and oxidized for energy in mitochondria. Fatty acids are activated by acyl-CoA synthetase, which attaches CoA to the fatty acid.
Plasma FFAs cannot be used for fuel by red blood cells (RBCs), which have no mitochondria. RBCs rely on glucose for energy.
Fatty Acid Synthesis Overview
Glucose → Pyruvate → Acetyl CoA → Malonyl CoA → Palmitic Acid → Variety of Fatty Acids through Elongation and Modification.
NADPH is used in the process. The entire process is regulated by hormones and substrate availability.
Summary
Fatty acid synthesis occurs mostly during the fed state, in particular tissues, using acetyl CoA from glycolysis. The liver and adipose tissue are the primary sites of fatty acid synthesis.
The way in which acetyl CoA leaves the mitochondria is used by the cell to regulate fatty acid synthesis and provide NADPH. The citrate shuttle plays a key role in this process.
FA synthesis is catalyzed in cyclical steps by Fatty Acid Synthase, producing a 16 C saturated FA. The process requires multiple enzymes and cofactors.
These FAs can be modified in several ways to produce a variety of different fatty acids. Elongation and desaturation allow for the synthesis of a wide range of fatty acids.
The produced fatty acids can then be converted to TAG and stored in adipose tissue or released into circulation by the liver. TAGs are the primary form of energy storage in the body.