Differential Functionalization of Presumed and Alanine Transaminases Has Been Driven by Diversification of Pyridoxal Phosphate Interactions

arose from an interspecies hybridization (allopolyploidiza-tion), followed by Whole Genome Duplication. Diversification analysis of Alt1/Alt2 indicated that while Alt1 is an alanine transaminase, Alt2 lost this activity, constituting an example in which one of the members of the gene pair lacks the apparent ancestral physiological role. This paper analyzes structural organization and pyridoxal phosphate (PLP) binding properties of Alt1 and Alt2 indicating functional diversification could have determined loss of Alt2 alanine transaminase activity and thus its role in alanine metabolism. It was found that Alt1 and Alt2 are dimeric enzymes harboring 67% identity and intact conservation of the catalytic residues, with very similar structures. However, tertiary structure analysis indicated that Alt2 has a more open conformation than that of Alt1 so that under physiological conditions, while PLP interaction with Alt1 allows the formation of two tautomeric PLP isomers (enolimine and ketoenamine) Alt2 preferentially forms the ketoenamine PLP tautomer, indicating a modified polarity of the active sites which affect the interaction of PLP with these proteins, that could result in lack of alanine transaminase activity in Alt2. The fact that Alt2 forms a catalytically active Schiff base with PLP and its position in an independent clade in "sensu strictu" yeasts suggests this protein has a yet undiscovered physiological function.