Dehalogenation, denitration, dehydroxylation, and angular attack on substituted biphenyls and related compounds by a biphenyl dioxygenase

The attack by the bph-encoded biphenyl dioxygenase of Burkholderia sp, stra in LB400 on a number of symmetrical ortho-substituted biphenyls or quasi or tho-substituted biphenyl analogues has been investigated. 2,2 ' -Difluoro-, 2,2 ' -dibromo-, 2,2 ' -dinitro-, and 2,2 ' -dihydroxybiphenyl were accept ed as substrates. Dioxygenation of all of these compounds showed a strong p reference for the semisubstituted pair of vicinal ortho and meta carbons, l eading to the formation of 2 ' -substituted 2,3-dihydroxybiphenyls by subse quent elimination of HX (X = F, Br, NO2, or OH). All of these products were further metabolized by 2,3-dihydroxybiphenyl 1,2-dioxygenases of Burkholde ria sp, strain LB400 or of Rhodococcus globerulus P6, Dibenzofuran and dibe nzodioxin, which may be regarded as analogues of doubly ortho-substituted b iphenyls or diphenylethers, respectively, were attacked at the "quasi ortho " carbon (the angular position 4a) and its neighbor, This shows that an aro matic ring-hydroxylating dioxygenase of class IIB is able to attack angular carbons. The catechols formed, 2,3,2 ' -trihydroxybiphenyl and 2,3,2 ' -tr ihydroxydiphenylether, were further metabolized by 2,3-dihydroxybiphenyl 1, 2-dioxygenase. While angular attack by the biphenyl dioxygenase was the mai n route of dibenzodioxin oxidation, lateral dioxygenation leading to dihydr odiols was the major reaction with dibenzofuran. These results indicate tha t this enzyme is capable of hydroxylating ortho or angular carbons carrying a variety of substituents which exert electron-withdrawing inductive effec ts. They also support the view that the conversions of phenols into catecho ls by ring-hydroxylating dioxygenases, such as the transformation of 2,2'-d ihydroxybiphenyl into 2,3,2'-trihydroxybiphenyl, are the results of di- rat her than of monooxygenations. Lateral dioxygenation of dibenzofuran and sub sequent dehydrogenation and extradiol dioxygenation by a number of biphenyl -degrading strains yielded intensely colored dead-end products. Thus, diben zofuran can be a useful chromogenic indicator for the activity of the first three enzymes of biphenyl catabolic pathways.