Sometimes a complete metabolic pathway involves repeating a series of reactions several times over. Thus, the oxidation of fatty acids (section 5.5.2) proceeds by the sequential removal of two-carbon units. The removal of each two-carbon unit involves a repeated sequence of four reactions, and the end-product of each loop of the pathway is a fatty acid that is two carbons shorter than the one that entered the loop. It then undergoes the same sequence of reactions. This is shown in cartoon form in Figure 2.19.
Similarly, the synthesis of fatty acids (section 5.6.1) involves the repeated addition of two-carbon units until the final chain length (commonly 14, 16 or 18 carbon units) has been achieved. The addition of each two-carbon unit involves four separate reaction steps, which are repeated in each loop of the pathway. The synthesis of fatty acids is catalysed by a large multienzyme complex in which the enzymes catalysing each step of the sequence are arranged in a series of concentric rings; the innermost ring catalyses the reaction sequence until the growing fatty acid chain is long enough to reach to the next ring of enzymes outwards from the centre.
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