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The potential of the frequently encountered (βα)8-barrel fold to acquire new functions was tested by an approach combining random mutagenesis and selection in vivo. For this purpose, the genes encoding 52 different phosphate-binding (βα)8-barrel proteins were subjected to error-prone PCR and cloned into an expression plasmid. The resulting mixed repertoire was used to transform different auxotrophic Escherichia coli strains, each lacking an enzyme with a phosphate-containing substrate. After plating of the different transformants on minimal medium, growth was observed only for two strains, lacking either the gene for the serine phosphatase SerB or the phosphoserine aminotransferase SerC. The same mutants of the E. coli genes nanE (encoding a putative N-acetylmannosamine-6-phosphate 2-epimerase) and pdxJ (encoding the pyridoxine 5′-phosphate synthase) were responsible for rescuing both ΔserB and ΔserC. Unexpectedly, the complementing NanE and PdxJ variants did not catalyze the SerB or SerC reactions in vitro. Instead, RT-qPCR, RNAseq, and transcriptome analysis showed that they rescue the deletions by enlisting the help of endogenous E. coli enzymes HisB and HisC through exclusive up-regulation of histidine operon transcription. While the promiscuous SerB activity of HisB is well-established, our data indicate that HisC is promiscuous for the SerC reaction, as well. The successful rescue of ΔserB and ΔserC through point mutations and recruitment of additional amino acids in NanE and PdxJ provides another example for the adaptability of the (βα)8-barrel fold.

D-glycero-D-manno-Heptose-1,7-bisphosphate phosphatase (GmhB) is a member of the histidinol-phosphate phosphatase (HisB) subfamily of the haloalkanoic acid dehalogenase (HAD) enzyme superfamily. GmhB supports two divergent biochemical pathways in bacteria: the D-glycero-D-manno-heptose-1α-GDP pathway (in S-layer glycoprotein biosynthesis) and the L-glycero-D-manno- heptose-1β-ADP pathway (in lipid A biosynthesis). Herein, we report the comparative analysis of substrate recognition in selected GmhB orthologs. The substrate specificity of the L-glycero-D-manno-heptose-1β-ADP pathway GmhB from Escherichia coli K-12 was evaluated using hexose and heptose bisphosphates, histidinol phosphate, and common organophosphate metabolites. Only D-glycero-D-manno-heptose 1β,7-bisphosphate (kcat/K m=7 × 106 M-1 s-1) and D-glycero-D-manno-heptose 1α,7-bisphosphate (kcat/Km = 7 × 104 M-1 s-1) displayed physiologically significant substrate activity. 31P NMR analysis demonstrated that E. coli GmhB selectively removes the C(7) phosphate. Steady-state kinetic inhibition studies showed that D-glycero-D-manno-heptose 1β-phosphate (Kis = 60 μM, and Kii = 150 μM) and histidinol phosphate (Kis = 1 mM, and Kii = 6 mM), while not hydrolyzed, do in fact bind to E. coli GmhB, which leads to the conclusion that nonproductive binding contributes to substrate discrimination. High catalytic efficiency and a narrow substrate range are characteristic of a well-evolved metabolic enzyme, and as such, E. coli GmhB is set apart from most HAD phosphatases (which are typically inefficient and promiscuous). The specialization of the biochemical function of GmhB was examined by measuring the kinetic constants for hydrolysis of the α- and β-anomers of D-glycero-D-manno-heptose 1β,7-bisphosphate catalyzed by the GmhB orthologs of the L-glycero-D-manno-heptose 1β-ADP pathways operative in Bordetella bronchiseptica and Mesorhizobium loti and by the GmhB of the D-glycero-D-manno-heptose 1α-GDP pathway operative in Bacteroides thetaiotaomicron. The results show that although each of these representatives possesses physiologically significant catalytic activity toward both anomers, each displays substantial anomeric specificity. Like E. coli GmhB, B. bronchiseptica GmhB and M. loti GmhB prefer the β-anomer, whereas B. thetaiotaomicron GmhB is selective for the α-anomer. By determining the anomeric configuration of the physiological substrate (D-glycero-D-manno-heptose 1,7-bisphosphate) for each of the four GmhB orthologs, we discovered that the anomeric specificity of GmhB correlates with that of the pathway kinase. The conclusion drawn from this finding is that the evolution of the ancestor to GmhB in the HisB subfamily provided for specialization toward two distinct biochemical functions.