Lribulokinase

L-ribulokinase, often referred to by its gene name araB, is a cytosolic enzyme involved in the bacterial metabolism of L-arabinose. It is a key component of the L-arabinose utilization operon, commonly including araA (L-arabinose isomerase) and araD (L-ribulose-5-phosphate 4-epimerase), and is typically studied in Escherichia coli and related bacteria.

The primary reaction catalyzed by L-ribulokinase is the ATP-dependent phosphorylation of L-ribulose to L-ribulose-5-phosphate, producing ADP in the process. This phosphorylation step follows the isomerization of L-arabinose to L-ribulose by L-arabinose isomerase and precedes the conversion of L-ribulose-5-phosphate to enter the pentose phosphate pathway via subsequent enzymatic steps. Thus, L-ribulokinase links the initial recognition and rearrangement of L-arabinose to its ultimate assimilation into central metabolism.

Biochemically, L-ribulokinase is characterized as a carbohydrate kinase and is commonly associated with the PfkB family of kinases, which includes several sugar kinases that act on phosphorylated sugars. Enzymes of this class typically require divalent metal ions such as Mg2+ and display substrate specificity for L-ribulose, with activity integrated into the operon-level regulation that responds to the presence of L-arabinose. In many bacteria, expression of araB is induced when L-arabinose is available, enabling efficient utilization of this sugar as a carbon source.

Across bacteria, L-ribulokinase homologs support the conservation of the L-arabinose utilization pathway, though exact regulatory and kinetic properties can vary by species.