The key difference between fatty acid synthase (FAS) in bacteria and mammals lies in its structural organization and enzymatic function: bacterial FAS (FAS II) is a dissociated system with separate enzymes, while mammalian FAS (FAS I) is a multi-functional enzyme complex.
Fatty Acid Synthase Systems: A Comparison
Here's a breakdown of the differences in a more structured format:
| Feature | Bacterial FAS (FAS II) | Mammalian FAS (FAS I) |
|---|---|---|
| Structure | Consists of separate, monofunctional enzymes. | A large, dimeric, multifunctional enzyme complex. |
| Organization | Enzymes are encoded by separate genes. | All enzymatic activities are contained within a single polypeptide chain (in mammals). |
| Acyl Carrier Protein (ACP) | ACP is a separate, diffusible protein. | ACP is a domain within the large FAS I polypeptide. |
| Mechanism | Each step of fatty acid synthesis is catalyzed by a distinct enzyme. | All steps are catalyzed within the FAS I complex, with substrates shuttled between active sites. |
| Evolutionary Origin | Likely arose from the fusion of FAS II genes. | Thought to have arisen through modification of fungal PKS I. |
Detailed Explanation
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Bacterial FAS (FAS II): In bacteria, fatty acid synthesis occurs through a system (FAS II) where each enzymatic step is carried out by a separate, individual enzyme. These enzymes work together in a sequential manner, but they are not physically linked. The acyl carrier protein (ACP), which carries the growing fatty acyl chain, is a separate, diffusible protein.
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Mammalian FAS (FAS I): In contrast, mammalian fatty acid synthesis relies on a large, multifunctional enzyme complex (FAS I). All the enzymatic activities required for fatty acid synthesis are located within a single polypeptide chain (or two identical chains in the active dimer). This organization allows for efficient channeling of substrates between active sites, increasing the overall rate of fatty acid synthesis. The ACP is not a separate protein, but rather a domain within the FAS I polypeptide.
Evolutionary Perspective
The evolutionary origins of these two systems are also distinct. Evidence suggests that FAS I in fungi and certain bacteria evolved through the fusion of FAS II genes. On the other hand, mammalian FAS I is believed to have evolved from a fungal polyketide synthase (PKS I).
In essence, the bacterial system is modular and composed of discrete units, while the mammalian system is a highly integrated and efficient machine.
