Structure of thiocyanate hydrolase: A new nitrile hydratase family protein with a novel five-coordinate cobalt(III) center

Thiocyanate hydrolase (SCNase) of Thiobacillus thioparus THI115 is a cobalt (III)-containing enzyme catalyzing the degradation of thiocyanate to carbonyl sulfide and ammonia. We determined the crystal structures of the apo- and native SCNases at a resolution of 2.0 angstrom. SCNases in both forms had a conserved hetero-dodecameric structure, (alpha beta gamma)(4). Four alpha beta gamma heterotrimers were structurally equivalent. One a alpha beta gamma hetero-trimer was composed of the core domain and the beta N domain, which was located at the center of the molecule and linked the hetero-trimers with novel quaternary interfaces. In both the apo- and native SCNases, the core domain was structurally conserved between those of iron and cobalt-types of nitrile hydratase (NHase). Native SCNase possessed the post-translationally modified cysteine ligands, gamma Cys131-SO2H and gamma Cys133-SOH like NHases. However, the low-spin cobalt(III) was found to be in the distorted square-pyramidal geometry, which had not been reported before in any protein. The size as well as the electrostatic properties of the substrate-binding pocket was totally different from NHases with respect to the charge distribution and the substrate accessibility, which rationally explains the differences in the substrate preference between SCNase and NHase. (c) 2006 Elsevier Ltd. All rights reserved.