Cysteine tryptophylquinone

Figure 3 Quinoprotein cofactors. TPQ, 2,4,5-trihydroxyphenylalanine quinone LTQ, lysine tyrosylquinone CTQ, cysteine tryptophylquinone TTQ, tryptophan tryptophylquinone.

These cross-linked amino acid residues appear to have no direct participation in the catalytic mechanism. They do play a structural role in determining the tertiary structure of the 7 suhunit. It is interesting to note that the TTQ dependent methylamine dehydrogenase does not have these thioether cross-linked residues, but does have six intra-subunit disulfide bonds between cysteine residues, which play a structural role in determining the tertiary structure of the TTQ bearing (3 subunit of that enzyme. The a subunit of QHNDH contains two r-type hemes. One heme c is solvent-accessible and the other is fully buried within the a subunit and located approximately 9 A from the tryptophylquinone moiety of CTQ on the 7 subunit. The a and 7 subunits sit on the surface of the / subunit that with the 7 subunit forms the enzyme active site. 

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