Divergent interactions maintain the quaternary octameric structure of a new family of esterases

Abstract

Protein oligomerization contributes significantly to the stability and function of enzymes, and the interacting interfaces that create the oligomers are expected to be conserved. The acetyl-xylo-oligosaccharide esterase, Axe2, from the thermophilic bacterium Geobacillus stearothermophilus represents a new family of esterases belonging to the SGNH superfamily of hydrolytic enzymes, and has a unique doughnut-like homo-octameric configuration, composed of four homo-dimers. The dimers of Axe2 are held together mainly by clusters of hydrogen bonds involving Tyr184 and Arg192, as was demonstrated by site directed mutagenesis. Dimeric mutants obtained by single amino acid replacements were inactive towards 2-naphthyl acetate, indicating the necessity of the octameric assembly for catalysis. The crystal structure of two homologous proteins (PDB 3RJT and 5JD3) reveal the same tertiary fold and octameric ring structure as of Axe2. Surprisingly, these octameric structures appear to be maintained by different sets of amino acids involving Asn183 in 3RJT and His185 in 5JD3 instead of Tyr184 in Axe2. These findings prompt us to investigate five more homologues proteins, which were found to have similar octameric structures, despite significant changes in their key residues. We revealed a conserved quaternary structure, which is maintained via non-conserved interactions.