Brown algae belong to a phylogenetic lineage distantly related to green plants and animals, and are found predominantly, but not exclusively, in the intertidal zone, a harsh and frequently changing environment. Because of their unique evolutionary history and of their habitat, brown algae feature several peculiarities in their metabolism. One of these is the mannitol cycle, which plays a central role in their physiology, as mannitol acts as carbon storage, osmoprotectant, and antioxidant. This polyol is derived directly from the photoassimilate fructose-6-phosphate via the action of a mannitol-1-phosphate dehydrogenase (M1PDH, EC 22.214.171.124) and a mannitol-1-phosphatase (M1Pase, EC 126.96.36.199). This protocol describes the biochemical characterization of the recombinant catalytic domain of one of the three M1PDHs identified in Ectocarpus sp. This recombinant catalytic domain, named hereafter M1PDHcat, catalyzes the reversible conversion of fructose-6-phosphate (F6P) to mannitol-1-phosphate (M1P) using NAD(H) as a cofactor. M1PDHcat activity was assayed in both directions i.e., F6P reduction and M1P oxidation (Figure 1).
Figure 1. Reversible reaction of mannitol-1-phosphate dehydrogenase
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