PMID: 20066665

Authors:

Noda-Garcia L, Camacho-Zarco AR, Verdel-Aranda K, Wright H, Soberon X, Fulop V, Barona-Gomez F

Title:

Identification and analysis of residues contained on beta --> alpha loops of the dual-substrate (betaalpha)(8) phosphoribosyl isomerase a (PriA) specific for its phosphoribosyl anthranilate isomerase activity.

Journal:

Protein Sci. 2010 Jan 11.

Abstract:

A good model to experimentally explore evolutionary hypothesis related to enzyme function is the ancient-like dual-substrate (betaalpha)(8) phosphoribosyl isomerase (PriA), which takes part in both histidine and tryptophan biosynthesis in Streptomyces coelicolor and related organisms. In this study, we determined the Michaelis - Menten enzyme kinetics for both isomerase activities in wild-type PriA from S. coelicolor, as well as in selected single-residue mono-functional mutants, identified after Escherichia coli in vivo complementation experiments. Structural and functional analyses of a hitherto unnoticed residue contained on the functionally important beta --> alpha loop 5, namely, Arg(139), which was postulated on structural grounds to be important for the dual-substrate specificity of PriA, is presented for the first time. Indeed, enzyme kinetics analyses done on the mutant variants PriA_Ser(81)Thr and PriA_Arg(139)Asn showed that these residues, which are contained on beta --> alpha loops and in close proximity to the N-terminal phosphate-binding site, are essential solely for the phosphoribosyl anthranilate isomerase activity of PriA. Moreover, analysis of the X-ray crystallographic structure of PriA_Arg(139)Asn elucidated at 1.95 A herein, strongly implicates the occurrence of conformational changes in this beta --> alpha loop as a major structural feature related to the evolution of the dual-substrate specificity of PriA. It is suggested that PriA has evolved by tuning a fine energetic balance that allows the sufficient degree of structural flexibility needed for accommodating two topologically dissimilar substrates - within a bifunctional and thus highly constrained active site - without compromising its structural stability.