Substrate preparation and the citrate shuttle
Fatty acid synthesis requires three main components: "building blocks" (acetyl-CoA), reducing equivalents (NADPH), and energy (ATP). All of them are generated by the activation of glycolysis and the pentose phosphate pathway.
The main problem is that the carbon chain assembly process takes place in the cytosol, whereas the starting substrate (acetyl-CoA) is produced in the mitochondrial matrix during the oxidative decarboxylation of pyruvate. The mitochondrial membrane is impermeable to acetyl-CoA. To bypass this obstacle, the citrate shuttle mechanism is used:
- Inside the mitochondrion, acetyl-CoA condenses with oxaloacetate in a reaction catalyzed by citrate synthase to form citrate.
- Citrate is transported into the cytoplasm via a translocase.
- In the cytosol, the enzyme ATP-citrate lyase cleaves citrate back into oxaloacetate and acetyl-CoA (consuming ATP and utilizing HS-CoA). The goal is achieved: acetyl-CoA is now in the cytosol.
The remaining oxaloacetate is reduced to malate by cytosolic malate dehydrogenase. Then, malate undergoes oxidative decarboxylation to pyruvate by the action of malic enzyme. This reaction generates NADPH, which is required for lipogenesis, and pyruvate returns to the mitochondrion.
First and regulatory reaction
Lipogenesis starts with the conversion of acetyl-CoA into malonyl-CoA (the donor of two-carbon units for chain elongation).
This is a carboxylation reaction catalyzed by a ligase-class enzyme, acetyl-CoA carboxylase. It requires ATP, carbon dioxide, and the coenzyme biotin. This step is crucial and tightly controlled by hormones.
Multienzyme complex: Fatty acid synthase
The subsequent assembly of the molecule is carried out by a multienzyme complex called fatty acid synthase (palmitate synthase). It is a dimer consisting of two identical polypeptide chains. Each chain contains seven active sites and a special acyl carrier protein (ACP). The function of the ACP is to transfer the growing molecule from one active site to the next.
The structure of the synthase contains two important SH groups (thiol groups). One belongs to the amino acid cysteine, and the other to phosphopantetheine.
The process is a cyclic chain elongation by 2 carbon atoms per pass:
- Acetyl-CoA serves only as a "primer" (provides the first two carbons).
- Malonyl-CoA acts as the donor for all subsequent two-carbon units.
- Hydrogen from NADPH is consumed at the stage of $\beta$-keto group reduction and double bond saturation.
The final product of the complex is always the 16-carbon palmitic acid.
Mechanisms of lipogenesis regulation
The rate of synthesis depends on the state of the regulatory enzyme, acetyl-CoA carboxylase. It is controlled in two ways: by covalent modification (phosphorylation) and allosterically (assembly/disassembly of protomers).
Activation (absorptive state):
- When glucose levels are high, insulin is released.
- Insulin activates a phosphatase that removes phosphate from acetyl-CoA carboxylase. The dephosphorylated form is active.
- The accumulation of citrate in the cytosol (an allosteric activator) stimulates the association of enzyme protomers into an active polymer.
- Additionally, during prolonged excess of carbohydrates, insulin triggers the transcription of lipogenesis enzymes at the gene level. This leads to enhanced conversion of glucose into fat and, consequently, to obesity.
Inhibition (fasting or stress):
- Glucagon (during fasting) or adrenaline (during physical exertion) activates the adenylate cyclase system and protein kinase A.
- Protein kinase A attaches a phosphate group to the enzyme. The phosphorylated form is inactive.
- The end product of synthesis, palmitoyl-CoA (an allosteric inhibitor), causes the polymer to dissociate into individual inactive protomers.
Further fate: triacylglycerol synthesis
Synthesized fatty acids are rapidly stored in the form of triacylglycerols (TAGs) in the liver.
The backbone is provided by the glycolytic intermediate dihydroxyacetone phosphate, which is reduced to glycerol-3-phosphate. Two activated fatty acids (acyl-CoAs) are attached to it, forming phosphatidic acid. Following the removal of the phosphate group, diacylglycerol (DAG) is formed, which accepts a third fatty acid to become the final fat—TAG.
Briefly: answers to topical questions
How is fatty acid synthase (palmitate synthase) structured?
Fatty acid synthase is a multifunctional enzyme that is a dimer of two identical subunits.
- Polypeptide chains — consist of two identical chains.
- Composition of a single chain — includes 7 active sites and an acyl carrier protein (ACP).
- Acyl carrier protein — shuttles the growing chain between active sites.
- Binding sites — each chain contains two sites with thiol (SH) groups belonging to cysteine and phosphopantetheine.
What reactions make up one cycle of fatty acid synthase activity?
One cycle of chain elongation on the multienzyme complex consists of four sequential reactions.
- Condensation — removal of the carboxyl group of malonyl as $CO_2$ and attachment of acetyl.
- Reduction — reduction of the $\beta$-keto group involving NADPH.
- Dehydration — removal of a water molecule from the intermediate.
- Reduction — saturation of the double bond using NADPH, resulting in the formation of a saturated radical (butyryl in the first cycle).
How do elongation and desaturation of fatty acids occur after palmitate is formed?
Following the synthesis of palmitic acid, other fatty acids can be synthesized from it via elongation and desaturation.
- Elongation (chain lengthening) — extension of the carbon skeleton using malonyl-CoA; an example is the synthesis of stearic acid from palmitic acid.
- Desaturation (introduction of double bonds) — stearoyl-CoA desaturase acts on saturated fatty acids, converting them into monoenoic acids; for example, palmitic acid C16:0 is converted to palmitoleic acid C16:1.
The human body cannot synthesize fatty acids with double bonds located beyond the 9th carbon atom.
To memorize the regulation of the key enzyme: "Insulin and Citrate assemble it (activate via dephosphorylation and polymerization), whereas Glucagon and Palmitoyl-CoA disassemble it (inactivate via phosphorylation and dissociation)"
Class and exam questions
What is the function of the citrate shuttle in metabolism?
It is necessary for the transport of acetyl-CoA from the mitochondrial matrix, where it is formed from glycolysis products, to the cytosol, where the enzymes of fatty acid synthesis are located.
What is the role of NADPH and what are its sources?
NADPH is used as a hydrogen donor (reducing agent) in synthesis reactions on the palmitate synthase complex. The main sources of NADPH are the pentose phosphate pathway and the action of cytosolic malic enzyme.
By how many carbon atoms is the chain elongated in one cycle of synthase operation?
The elongation of a fatty acid carbon chain occurs strictly by 2 carbon atoms per cycle. The source of these two-carbon units is malonyl-CoA.
What else to study on this topic
- Structure and mechanism of action of acyl carrier protein (ACP)
- Biochemical reactions within the fatty acid synthase complex
- Genetic induction of lipogenic enzymes by insulin
- The role of malic enzyme in linking carbohydrate and lipid metabolism
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