Bifunctional and moonlighting enzymes: lighting the way to regulatory control

Bifunctional and moonlighting enzymes are unusual multifunctional proteins. Bifunctional enzymes generally contain two large structural domains whose association facilitates metabolic pathway control and/or allows more efficient substrate conversion. Moonlighting enzymes have a recognized catalytic activity and an additional regulatory or structural function. Here, I describe known plant proteins of both groups and consider the specific advantages to their being multifunctional. Bifunctional enzymes are sometimes co-expressed with monofunctional forms. Identified plant moonlighting enzymes often have an alternate function in signal transduction pathways or as structural components. Properties of proteins of both groups reveal unexpected cell regulatory programs and support the idea that eukaryotes use multitasking activities to achieve their densely connected regulatory networks.