Homology model of the quinohaemoprotein alcohol dehydrogenase from Comamonas testosteroni

A molecular model of QH-ADH, the quinohaemoprotein alcohol dehydrogenase from Comamonas testosteroni, has been built by homology modelling. Sequence similarity of N-terminal residues 1-570 with the α-subunit of quinoprotein methanol dehydrogenases (MDHs) from Methylophilus methylotrophus W3A1 and Methylobacterium extorquens provided a basis for the design of the pQQ-binding domain of QH-ADH. Minimal sequence similarity with cytochrome c₅₅₁ from Ectothiorhodospira halophila and cytochrome c₅ from Azotobacter vinelandii has been used to model the C-terminal haem c-binding domain, residues 571-677, absent in MDHs. Distance constraints inferred from ¹⁹F-NMR relaxation studies of trifluoro-methylphenylhydrazine-derivatized PQQ bound to QH-ADH apoenzyme as well as theoretical relations for optimal electron transfer have been employed to position the haem- and PQQ-binding domains relative to each other. The homology model obtained shows overall topological similarity with the crystal structure of cd₁-nitrite reductase from Thiosphera pantotropha. The proposed model accounts for the following: (i) the site that is sensitive to in vivo proteolytic attack; (ii) the substrate specificity in comparison with MDHs; (iii) changes of the spectral properties of the haem c upon reconstitution of apo-enzyme with PQQ; (iv) electronic interaction between haem and PQQ; and (v) enantioselectivity in the conversion of a chiral sec alcohol.