Metabolism of L-rhamnose by Escherichia coli. I. L-rhamnose isomerase.

The methyl pentose, L-rhamnose, is known to be fermented by a number of bacteria, but little information is available concerning the metabolic steps involved in this fermentation. An indication of the pathway of rhamnose metabolism was provided by the finding of Kluyver and Schnellen (1937) that propylene glycol was a major product in the fermentation by Bacillus rhamnosifermentans. Since 50 per cent of the rhamnose carbon could be accounted for as propylene glycol, they suggested that the rhamnose molecule was split into glyceraldehyde and lactaldehyde, the latter being then reduced to propylene glycol. However, it is only recently that other steps in rhamnose fermentation have been investigated. Preliminary work (Wilson and Ajl, 1955) indicated that the initial step in the metabolism of L-rhamnose by Escherichia coli involved the conversion of the methyl pentose to its corresponding ketose (L-rhamnulose or L-rhamnoketose). Tecce et al. (1955) with E. coli and Englesberg (1956) with Pasteurella pestis came independently to the same conclusion. The initial step in the metabolism of another methyl pentose, L-fucose, likewise involves the conversion of the aldose to the corresponding ketose, as shown by the work of Green and Cohen (1955, 1956) with a mutant strain of E. coli. This communication describes detailed experiments on the conversion of L-rhamnose to Lrhamnulose by E. coli grown in the presence of rhamnose.