This is an N-terminal immunoglobulin (Ig)-like domain found in esterases such as EstA. Analysis of the EstA structure confirms that it is a member of the alpha/beta hydrolase family, with a conserved Ser-Asp-His catalytic triad. The Ig-like domain presumably plays a role in the multimerization of EstA into an unusual hexameric structure. Additionally, it may also participate in the catalysis of EstA by guiding the substrate to the active site [1].

1: Crystal structure and biochemical properties of a novel thermostable esterase containing an immunoglobulin-like domain. Levisson M, Sun L, Hendriks S, Swinkels P, Akveld T, Bultema JB, Barendregt A, van den Heuvel RH, Dijkstra BW, van der Oost J, Kengen SW; J Mol Biol. 2009;385:949-962. PMID:19013466