SPECTROSCOPIC EVIDENCE FOR A COMMON ELECTRON-TRANSFER PATHWAY FOR 2 TRYPTOPHAN TRYPTOPHYLQUINONE ENZYMES

Aromatic amine dehydrogenase (AADH) and methylamine dehydrogenase (MAD H) are the only two enzymes known to use the cofactor tryptophan trypt ophylquinone (TTQ). Each catalyzes oxidative deamination of a distinct class of primary amines. A detailed comparison of their circular dich roic spectra indicates that both proteins share a similar fold with th eir TTQ cofactors residing in similar environments and that this may b e a useful diagnostic probe for TTQ enzymes. Alcaligenes faecalis cell s induced to express AADH also express a large amount of the blue copp er protein, azurin. Oxidized azurin is rapidly reduced by a catalytic amount of AADH in the presence of the substrate, tyramine. Three A. fa ecalis cytochromes-c and three other cytochromes-c were tested for ele ctron transfer activity with AADH. Azurin markedly facilitated electro n transfer from AADH to each cytochrome. This suggests that AADH and a zurin may form an electron transfer complex with a c-type cytochrome, analogous to the crystallographically determined MADH-amicyanin-cytoch rome c-551i complex (Chen, L., Durley, R. C. E., Matthews, F. S., and Davidson, V. L. (1994) Science 264, 86-90). The similarities of MADH a nd AADH plus the demonstration of azurin and multiple cytochromes as f unctional electron-transfer partners suggest that both TTQ-bearing enz ymes share common mechanisms for oxidative deamination and subsequent electron transfer.