Crystal Structure of a NifS Homologue CsdB from Escherichia coli (MOLECULAR BIOLOGY AND INFORMATION-Biopolymer Structure)

Abstract

Escherichia coli CsdB is a dimeric NifS-homologue belonging to the fold-type I family of PLPdependent enzymes, and catalyzes the decomposition of L-selenocysteine into selenium and L-alanine with specificity higher than that for a substrate of cysteine. The structure of the enzyme has been determined at 2.8 A resolution by an X-ray crystallographic method. The subunit of CsdB comprises a large domain, a small domain, and an N-terminal segment. A remarkable structural feature of CsdB is that an α-helix in the lobe extending from the small domain to the large domain in one subunit of the dimer interacts with a -hairpin loop protruding from the large domain of the other subunit. Cys364, which is essential for the activity toward cysteine but not toward selenocysteine, is clearly seen on the loop of the extended lobe (Thr362.Arg375) although the corresponding loop (Ser321.Arg332) is disordered in the Thermotoga maritima NifS-like protein, which is closely related to the cysteinespecific NifS and whose crystal structure has recently been determined as the second example.