Recent discovery of MIO-dependent aminomutases on the biosynthetic pathways of biologically active, medicinal compounds in plants and microorganisms raises interest in further understanding how they catalyze their β-amino acid building blocks. MIO-aminomutases are a subclass of isomerase that executes a chemically challenging isomerization of α-amino acids to β-amino acids. A tyrosine aminomutase isolated from Japanese rice, Oryza sativa (OsTAM), converts α-tyrosine to β-tyrosine (75%) and makes an acrylate (25%), as a by-product. OsTAM is the first TAM to have slight phenylalanine aminomutase (PAM) activity (3%). This may not be surprising since the active sites of OsTAM and TcPAM from Taxus plants differ by only two residues positioned similarly near the aryl ring of substrate. According to our previous studies, we anticipated by changing key active site residues of OsTAM to nonpolar side chains found in TcPAM would improve the binding of substituted phenylalanine substrates. Thus, we incubated sixteen ring-substituted phenylalanines with various mutants and compared their turnover to that of wild-type OsTAM. Our preliminary data surprisingly shows that some mutant OsTAM enzymes change the expected product distribution. These findings suggest that the active site residues direct substrate selectivity, and their role along with tandem distal mutations parses the reaction products toward β-amino acids remains elusive.

Understanding the function of active site residues and an ordered loop of an Oryza sativa tyrosine aminomutase towards parsing its mutase and lyase activities

255th National Meeting (2018)

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