W0257

Structural and Functional Diversity from a Simple Protein Fold. M.L. Hackert, R. Dasgupta, J.J. Almrud, S.R. Ernst, S.C. Wang, W.H. Johnson^†, G. Poelarends^† and C.P. Whitman^†, Dept. of Chemistry and Biochemistry, and^†College of Pharmacy, The Univ. of Texas at Austin, Austin, TX 78712.

The tautomerase superfamily is represented by 4-Oxalocrotonate tautomerase (4-OT), 5-(carboxymethyl)-2-hydroxymuconate isomerase (CHMI), and macrophage migration inhibitory factor (MIF). Members of this enzyme superfamily share a simple folding motif represented by the 4-OT monomer that is remarkable for its small size (62 a.a.) and simple (β-α-β) fold. The CHMI and MIF subunits are nearly twice as large as 4-OT and probably arose by gene duplication. Correspondingly, 4-OT and most of its homologues are hexameric while CHMI and MIF are trimeric. Members of this superfamily also share a key mechanistic feature - an active site amino-terminal proline, which has an unusually low pK_a, as the general base in keto-enol tautomerization.

Several additional members of the 4-OT family have been identified and partially characterized, although the true functions of many of these proteins remain unknown. We have identified at least five sub-families of 4-OT homologues, and crystallized a representative member from each sub-family. Structural diversity includes α2, α3, α6, and α3β3 oligomers, while functional diversity now adds dehalogenase and decarboxylase activities to the tautomerase, isomerase and MIF activities previously reported. The structural results of these studies will be summarized as well as our attempts to use structural information to assign function.