AraA

AraA is the designation commonly used for the gene encoding L-arabinose isomerase in many bacteria, including Escherichia coli. The encoded enzyme catalyzes the isomerization of L-arabinose to L-ribulose, the first step in the catabolic pathway that allows bacteria to utilize L-arabinose as a carbon source. In the classical arabinose operon of E. coli, araA is part of the araBAD cluster, which also includes araB (ribulokinase) and araD (L-ribulose-5-phosphate 4-epimerase). The operon is regulated by the AraC transcriptional regulator and is induced in the presence of L-arabinose; glucose can influence expression through catabolite repression, modulating operon activity.

Biochemically, L-arabinose isomerase is typically a metalloenzyme requiring divalent metal ions such as Mn2+ or Co2+ for activity. Enzymes from different organisms show variation in temperature and pH optima, with some thermophilic variants exhibiting enhanced stability. The catalytic mechanism involves rearranging the aldose to form a ketose, producing L-ribulose that can be further processed by AraB and AraD to enter the pentose phosphate pathway.

Besides its native role in sugar metabolism, AraA has attracted interest for industrial and biotechnological applications. Some L-arabinose isomerases can act on other sugars, and engineered or naturally occurring variants have been explored for the isomerization of D-galactose to D-tagatose, a low-calorie sweetener, highlighting potential uses in biomass conversion and food-grade production processes.

See also: araBAD operon, AraC regulator, L-arabinose metabolism.