Cyclic pathways

The third type of metabolic pathway is cyclic; a product is assembled, or a substrate is catabolized, attached to a carrier molecule that is reformed at the end of each cycle of reactions.

Figure 2.20 shows a cyclic biosynthetic pathway in cartoon form; the product is built up in a series of reactions, then released, regenerating the carrier molecule. An example of such a pathway is the urea synthesis cycle (section 9.3.1.4).

Figure 2.21 shows a cyclic catabolic pathway in cartoon form; the substrate is bound to the carrier molecule, then undergoes a series of reactions in which parts are removed, until at the end of the reaction sequence the original carrier molecule is left. An example of such a pathway is the citric acid cycle (section 5.4.4).

The intermediates in a cyclic pathway can be considered to be catalysts, in that they participate in the reaction sequence, but at the end they emerge unchanged. Until all the enzymes in a cyclic pathway are saturated (and hence acting at V ), oxidation to C=C

hydration to CH-OH

oxidation to C=O

cleavage

Figure 2.19 A spiral or looped (repeating) metabolic pathway.

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