Metabolism of L rhamnose in Arthrobacter pyridinolis

In A. pyridinolis, a respiration coupled transport system for L rhamnose caused accumulation of free L rhamnose, while a phosphoenolpyruvate: L rhamnose phosphotransferase system caused accumulation of L rhamnose I phosphate. The pathways for subsequent metabolism of L rhamnose and L rhamnose I phosphate have now been investigated. A. pyridinolis contains an inducible L rhamnose isomerase and L rhamnulokinase, as well as a constitutive L rhamnulose I phosphate aldolase. Results with mutants which are unable to metabolize L rhamnose suggest the presence of an L rhamnose I phosphate phosphatase, which forms free L rhamnose by hydrolysis of L rhamnose I phosphate produced by the phosphotransferase system. Mutants which lack this enzyme exhibited severe inhibition of growth in the presence of L rhamnose plus any of a variety of carbon sources. There is some evidence that this inhibition was due to accumulation of L rhamnose I phosphate at toxic concentrations within the bacteria. The metabolism of L rhamnose transported by the phosphotransferase system therefore appears to occur by hydrolysis of L rhamnose I phosphate to free L rhamnose by a phosphatase. Metabolism of the L rhamnose thus produced, and of that accumulated by the respiration coupled transport system, then proceeds by the sequence of reactions: L rhamnose → L rhamnulose → L rhamnulose I phosphate → dihydroxyacetone phosphate plus L lactaldehyde.