Probing the stabilizing role of C-terminal residues in trimethylamine dehydrogenase

In trimethylamine dehydrogenase, a homodimeric iron-sulfur flavoprotein, the C-terminal 17 residues of each subunit (residues 713-729) embrace residues on the other subunit. The role of this unusual mode of interaction at the subunit interface was probed by isolating three mutant forms of trimethylamine dehydrogenase in which the C-terminus of the enzyme was deleted by five residues [delta(725-729], 10 residues [delta(720-729)] and 17 residues [delta(713-729)]. The solution properties and conformational states of the three mutant enzymes were investigated using optical, fluorescence and circular dichroism spectroscopies, ANS binding and a novel and conformationally sensitive hydrodynamic method. The data reveal that sequential deletion of the C-terminus of trimethylamine dehydrogenase does not affect significantly dimer stability or the overall structural integrity of the enzyme. However, deletion of the C-terminus severely compromises, but does not abolish, the ability of the enzyme to become covalently coupled with the redox cofactor FMN in the active site, located over 20 A from the C-terminus. Hydrodynamic studies reveal minor conformational changes in the deletion mutants that lead to a more compact enzyme structure. These conformational changes are probably transmitted to the active site via altering the interaction of the C-terminus with the second helix in the beta/alpha barrel of trimethylamine dehydrogenase, leading to poor flavinylation during the folding of the enzyme and assembly with FMN.