Divergent Evolution of (??)8-Barrel Enzymes

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Abstract
The (?[alpha)8-barrel is the most versatile and most frequently encountered fold among enzymes. It is an interesting question how the contemporary (?[alpha)8-barrels are evolutionarily related and by which mechanisms they evolved from more simple precursors. Comprehensive comparisons of amino acid sequences and threedimensional structures suggest that a large fraction of the known (?[alpha)8-barrels have divergently evolved from a common ancestor. The mutational interconversion of enzymatic activities of several (?[alpha)8-barrels further supports their common evolutionary origin. Moreover, the high structural similarity between the N and Cterminal (?[alpha)4 units of two (?[alpha)8-barrel enzymes from histidine biosynthesis indicates that the contemporary proteins evolved by tandem duplication and fusion of the gene of an ancestral halfbarrel precursor. In support of this hypothesis, recombinantly produced halfbarrels were shown to be folded, dimeric proteins.