Identification of the iron-sulfur center in trimethylamine dehydrogenase

Trimethylamine dehydrogenase [trimethylamine:(acceptor) oxidoreductase (demethylating), EC 1.5.99.7] from a facultative methylotroph bacterium has a molecular weight of 147,000 and contains two types of prosthetic groups, one a covalently bound organic chromophore of uncertain structure and the other containing iron and labile sulfur (S*). The structure of the Fe-S* center has been investigated by reactions of the enzyme with sodium mersalyl, o-xylyl-alpha,alpha'-dithiol, and p-methoxybenzenethiol in a 4:1 vol/vol hexamethylphosphoramide/water reaction medium, which destabilizes tertiary structure. Mersalyl treatment results in reduction of visible absorbance consistent with the presence of a 4-Fe center of the ferredoxin type. Reaction with thiols effects partial bleaching of the organic chromophore, as established by separate studies of a detached chromophore peptide, and results in removal (extrusion) of the core unit of the Fe-s* center in the form of the complexes [Fe4S*4(S2-o-xylyl)2]n2n- and [Fe4S*4(SC6H4OMe)4]2-, which were identified by absorption spectra. These results, in conjunction with control extrusion reactions of oxidized ferredoxins from spinach and Clostridium pasteurianum, establish that trimethylamine dehydrogenase contains one Fe4S*4 core unit most probably present as a ferredoxin-type, cysteinate-ligated cluster [Fe4S*4(S-Cys)4].