The synthesis of fatty acids, a process crucial for energy storage, mainly occurs in the cytoplasm of liver and fat cells and involves three major steps.
Here's a breakdown of the steps involved:
Overview of Fatty Acid Synthesis
Fatty acid synthesis is a multi-step process where acetyl-CoA molecules are used to build long-chain fatty acids. This process is primarily active when the body has ample energy and needs to store excess calories as fat.
Steps Involved
The overall process consists of three primary stages, as noted in the reference material:
1. The Citrate Shuttle
- Purpose: To transport acetyl-CoA from the mitochondria, where it is produced, into the cytoplasm, where fatty acid synthesis takes place.
- Process:
- Acetyl-CoA in the mitochondria condenses with oxaloacetate to form citrate.
- Citrate is transported out of the mitochondria into the cytoplasm.
- In the cytoplasm, citrate is broken down back into acetyl-CoA and oxaloacetate.
- The oxaloacetate is then converted back into malate which can then transport back into the mitochondria.
- Significance: This step is essential because acetyl-CoA cannot directly cross the mitochondrial membrane.
2. Acetyl-CoA Carboxylase (Rate-Limiting Step)
- Purpose: This is the committed step of fatty acid synthesis, converting acetyl-CoA to malonyl-CoA.
- Process: Acetyl-CoA carboxylase (ACC), an enzyme, catalyzes the carboxylation of acetyl-CoA to form malonyl-CoA, using bicarbonate and ATP.
- Regulation: This step is highly regulated and is the rate-limiting step in the overall process. The enzyme is activated by citrate and insulin and inhibited by palmitoyl-CoA and glucagon.
- Significance: Malonyl-CoA is the activated two-carbon unit that is added to the growing fatty acid chain during synthesis.
3. Fatty Acid Synthase (FAS) Complex
- Purpose: This is where the actual synthesis of the fatty acid chain occurs.
- Process: The fatty acid synthase (FAS) is a large multi-enzyme complex which contains all the enzymatic activities to add two-carbon units to the growing fatty acid chain.
- Malonyl-CoA and acetyl-CoA are combined in the first step to build a 4-carbon unit.
- This 4-carbon unit goes through a series of reduction, dehydration, and reduction reactions to form a saturated 4-carbon unit, and the process repeats with more malonyl-CoA added to grow the chain by two carbons at a time.
- This process continues until the fatty acid chain, usually palmitate (16 carbons), is synthesized.
- Significance: The FAS complex ensures that each two-carbon unit is added in a controlled manner with the required processing to form the saturated fatty acid chain.
Table Summarizing Fatty Acid Synthesis Steps
| Step | Location | Key Molecules | Primary Function |
|---|---|---|---|
| Citrate Shuttle | Cytoplasm | Citrate, Acetyl-CoA, Oxaloacetate | Transports acetyl-CoA from mitochondria to the cytoplasm |
| Acetyl-CoA Carboxylase | Cytoplasm | Acetyl-CoA, Malonyl-CoA, Bicarbonate, ATP | Catalyzes the formation of malonyl-CoA, the committed step, and rate-limiting step |
| Fatty Acid Synthase Complex | Cytoplasm | Acetyl-CoA, Malonyl-CoA, Fatty Acid Chain | Elongates the fatty acid chain by adding two-carbon units |
