HOMOLOGY MODEL OF THE QUINOHEMOPROTEIN ALCOHOL-DEHYDROGENASE FROM COMAMONAS-TESTOSTERONI

A molecular model of QH-ADH, the quinohaemoprotein alcohol dehydrogena se from Comamonas testosteroni, has been built by homology modelling. Sequence similarity of N-terminal residues 1-570 with the alpha-subuni t of quinoprotein methanol dehydrogenases (MDHs) from Methylophilus me thylotrophus W3A1 and Methylobacterium extorquens provided a basis for the design of the PQQ-binding domain of QH-ADH. Minimal sequence simi larity with cytochrome c(551) from Ectothiorhodospira halophila and cy tochrome c(5) from Azotobacter vinelandii has been used to model the C -terminal haem c-binding domain, residues 571-677, absent in MDHs. Dis tance constraints inferred from F-19-NMR relaxation studies of trifluo romethylphenylhydrazine-derivatized PQQ bound to QH-ADH apoenzyme as w ell as theoretical relations for optimal electron transfer have been e mployed to position the haem- and PQQ-binding domains relative to each other. The homology model obtained shows overall topological similari ty with the crystal structure of cd(1)-nitrite reductase from Thiosphe rapantotropha. The proposed model accounts for the following: (i) the site that is sensitive to in vivo proteolytic attack; (ii) the substra te specificity in comparison with MDHs; (iii) changes of the spectral properties of the haem c upon reconstitution of ape-enzyme with PQQ; ( iv) electronic interaction between haem and PQQ; and (v) enantioselect ivity in the conversion of a chiral sec alcohol.