Fatty acid synthetases (FAS) are a group of enzymes involved in catalysing the synthesis of fatty acids. Through the elongation cycle, the fatty acid chain is lengthened, and on reaching the requisite length of carbon atoms, released.
Type II FAS are found in bacteria and plants. Individual enzymes catalyse each step, releasing their product into the cytoplasm before the next enzyme stochastically interacts, potentially catalysing the next step.
Type I FAS are generally found in higher eukaryotes (such as vertebrates) and some bacteria and yeasts. Here, all of the required catalytic domains are found on either the same protein, or across 2. This allows for faster synthesis of fatty acids, as the substrate for the next stage is in a high concentration in proximity to the next active site. Faster fatty acid synthesis is ideal in higher eukaryotes, allowing for more energy to be stored in a given period of time, increasing the chances of survival.
However, if there is a rate limiting step in the reaction cycle, type I FAS are unable to upregulate the expression of the specific enzyme. Type II FAS, due to their individual, discrete enzymes, may be able to increase the expression level of a rate-limiting enzyme to allow more flux through. But the release of product into the cytoplasm will likely slow the reaction down due to the low concentrations that would be present, making it more challenging for an enzyme to spontaneously interact successfully with its substrate.