Evolution of enzymatic activities in the enolase superfamily: Characterization of the (D)-glucarate/galactarate catabolic pathway in Escherichia coli

The genes encoding the enzymes in the (D)-glucarate/galactarate catabolic pathway have been identified in the Escherichia coli genome. These encode, in three transcriptional units, (D)-glucarate dehydratase (GlucD), galactarate dehydratase, 5-keto-4-deoxy-(D)-glucarate aldolase, tartronate semialdehyde reductase, a glycerate kinase that generates 2-phosphoglycerate as product, and two hexaric acid transporters. We also have identified a gene proximal to that encoding GlucD that encodes a protein that is 72% identical in primary sequence to GlucD (GlucD-related protein or GlucDRP). However, whereas GlucD catalyzes the efficient dehydration of both (D)-glucarate and (L)-idarate as well as their epimerization, GlucDRP is significantly impaired in both reactions. Perhaps GlucDRP is an example of gene duplication and evolution in progress in the E. coli chromosome.