Roles of the second-shell amino acid R266 in other members of the MLE subgroup of the enolase superfamily

Catalytic promiscuity is the coincidental ability to catalyze non-biological reactions in the same active site as the native biological reaction. Several lines of evidence show that catalytic promiscuity plays a role in the evolution of new enzyme functions. Thus, studying catalytic promiscuity can help identify structural features that predispose an enzyme to evolve new functions. This study identifies such a pre-adaptive residue in an N -succinylamino acid racemase/ o -succinylbenzoate synthase (NSAR/OSBS) enzymes from the NSAR/OSBS subfamily. Previously, we identified a point mutation, R266Q, in the catalytically promiscuous Amycolatopsis sp. T-1-60 NSAR/OSBS that has a deleterious effect on NSAR activity with a lesser effect on OSBS activity (Truong et al ., in preparation). We demonstrated that R266 was a pre-adaptive feature that enabled the emergence and evolution of NSAR activity in AmyNSAR/OSBS. We examined the role of the residue R266 in the evolution of NSAR activity by examining the effects of the single substitution R266Q in other members of the NSAR/OSBS subfamily including Enterococcus faecalis NSAR/OSBS, Roseiflexus castenholzii NSAR/OSBS, Lysinibacillus varians NSAR/OSBS, and Listeria innocua NSAR/OSBS, which have been previously characterized to carry out both OSBS and NSAR activities efficiently. RcNSAR/OSBS, LvNSAR/OSBS, EfNSAR/OSBS, and LiNSAR/OSBS are 49, 48, 32, and 28% identical, respectively, to AmyNSAR/OSBS. We found that while the R266Q mutation decreases NSAR activity more than OSBS activity, as expected, in most NSAR/OSBS members, the differential effects of the R266Q substitution on NSAR and OSBS activities are not as striking as observed in AmyNSAR/OSBS. In some homologs, the R266Q mutation has very deleterious effects on both OSBS and NSAR activities. Furthermore, the mutation unexpectedly decreases OSBS activity more than NSAR activity in LiNSAR/OSBS. Thus, the effects of R266Q on NSAR and OSBS activities depend on differences in sequence context between members of the NSAR/OSBS subfamily, demonstrating the complex role of epistasis in the evolution of NSAR activity in the NSAR/OSBS subfamily. ### Competing Interest Statement The authors have declared no competing interest. * OSBS : o -succinylbenzoate synthase NSAR : N -succinylamino acid racemase EfNSAR/OSBS : Enterococcus faecalis NSAR/OSBS RcNSAR/OSBS : Roseiflexus castenholzii NSAR/OSBS LvNSAR/OSBS : Lysinibacillus varians NSAR/OSBS LiNSAR/OSBS : Listeria innocua NSAR/OSBS kex : exchange rate DE : Dipeptide Epimerase NSPG : N -succinylphenylglycine SHCHC : 2-succinyl-6-hydroxy-2,4-cyclohexadiene-1-carboxylate DSF : Differential Scanning Fluorimetry