Fatty acid synthesis is the creation of fatty acids from acetyl-CoA and NADPH, facilitated by enzymes called fatty acid synthases. This process primarily occurs when there's an excess of calories, particularly from carbohydrates, although excess protein can also contribute.
The Process Explained
The synthesis happens in several steps:
- Starting Material: The process begins with acetyl-CoA, a two-carbon molecule derived from the breakdown of carbohydrates and other fuels. This is often sourced from dietary carbohydrate.
- Enzyme Action: Enzymes, specifically fatty acid synthases, catalyze the reaction. These enzymes are complex, coordinating multiple steps.
- NADPH Cofactor: Reduced NADP+ (NADPH) acts as a crucial cofactor, providing reducing power needed for the creation of the fatty acid chain. Two NADPH molecules are consumed for each two-carbon unit added to the growing chain.
- Chain Elongation: The fatty acid chain is elongated by the sequential addition of two-carbon units from acetyl-CoA.
- Saturated Form: Fatty acids are initially synthesized in a saturated form (no double bonds).
- Desaturation: Desaturases, a different set of enzymes, catalyze the introduction of cis double bonds later in the process, creating unsaturated fatty acids.
Location of Synthesis
Fatty acid synthesis occurs predominantly in the cytoplasm of liver and fat cells. However, it's important to note that mitochondria also possess a separate, independent pathway for fatty acid synthesis. This mitochondrial pathway is highly conserved across eukaryotes and differs significantly from the cytosolic pathway.
Regulation
The synthesis of fatty acids is tightly regulated. Intracellular levels of cholesterol and fatty acids are controlled via a feedback mechanism. For example, insulin plays a role, though hepatic triglyceride synthesis can also occur independently of insulin.
Different Types of Fatty Acid Synthases
There are two main types of fatty acid synthases:
- Type I: Found in eukaryotes, these synthases are large, multifunctional enzymes.
- Type II: Found in prokaryotes and in mitochondria, these synthases consist of multiple individual enzymes.
In summary: Fatty acid synthesis is a complex metabolic pathway involving multiple enzymes, cofactors, and regulatory mechanisms, resulting in the production of essential fatty acids from readily available precursors. The location and precise mechanisms can vary slightly depending on the organism and cellular compartment.
