'Essential' -SH in clostridial GDH is not truly essential.

What is it about?

Clostridial glutamate dehydrogenase has only 2 cysteines in its sequence. With DTNB etc. only one of these Cys reacts, inactivating the enzyme. In this paper Cys 320 was replaced with Ser. The mutated enzyme is still active but no longer reacts with DTNB. The mutated subunit differs from wild-type by only one atom (O for S). The study unambiguously identifies Cys 320 as the 'essential' cysteine. This residue is on the surface of the coenzyme-binding domain

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Why is it important?

1. First of all, in a general sense, this study underlines that 'essential' residues identified by chemical modification may not be truly essential. Blocking them may abolish activity, but, as in this case, it may not be necessary to have precisely that residue at that position for activity. 2. In this specific case, the study made clear that the reason DTNB inactivates this GDH is that it blocks coenzyme binding. 3. The study provided us with a clean way of reversibly inactivating subunits and also a clean way of replacing wild-type subunits with mutated subunits that are virtually indistinguishable from wild-type except that they aren't inactivated by DTNB. This has provided the basis for a series of studies of hybrid hexamers containing both active and inactive subunits to explore allosteric interaction in GDH.