L-fucose isomerase (FucIase) and L-arabinose isomerase (AI) family; composed of FucIase, AI and similar proteins. FucIase converts L-fucose, an aldohexose, to its ketose form, which prepares it for aldol cleavage (similar to the isomerization of glucose in glycolysis). L-fucose (or 6-deoxy-L-galactose) is found in various oligo- and polysaccharides in mammals, bacteria and plants. AI catalyzes the isomerization of L-arabinose to L-ribulose, the first reaction in its conversion to D-xylulose-5-phosphate, an intermediate in the pentose phosphate pathway, which allows L-arabinose to be used as a carbon source. AI can also convert D-galactose to D-tagatose at elevated temperatures in the presence of divalent metal ions. D-tagatose, rarely found in nature, is of commercial interest as a low-calorie sugar substitute.
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