Abstract
Aromatic amine dehydrogenase (AADH) and methylamine dehydrogenase (MADH) are the only two enzymes known to use the cofactor tryptophan tryptophylquinone (TTQ). Each catalyzes oxidative deamination of a distinct class of primary amines. A detailed comparison of their circular dichroic spectra indicates that both proteins share a similar fold with their TTQ cofactors residing in similar environments and that this may be a useful diagnostic probe for TTQ enzymes. Alcaligenes faecalis cells induced to express AADH also express a large amount of the blue copper protein, azurin. Oxidized azurin is rapidly reduced by a catalytic amount of AADH in the presence of the substrate, tyramine. Three A. faecalis cytochromes-c and three other cytochromes-c were tested for electron transfer activity with AADH. Azurin markedly facilitated electron transfer from AADH to each cytochrome. This suggests that AADH and azurin may form an electron transfer complex with a c-type cytochrome, analogous to the crystallographically determined MADH-amicyanin-cytochrome 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 and AADH plus the demonstration of azurin and multiple cytochromes as functional electron-transfer partners suggest that both TTQ-bearing enzymes share common mechanisms for oxidative deamination and subsequent electron transfer.
Highlights
Aromatic amine dehydrogenase (AADH) and methylamine dehydrogenase (MADH) are the only two enzymes known to use the cofactor tryptophan tryptophylquinone (TTQ)
Azurin markedly facilitated electron transfer from AADH to each cytochrome. This suggests that AADH and azurin may form an electron transfer complex with a c-type cytochrome, analogous to the crystallographically determined MADH-amicyanin-cytochrome c-55li complex
The electron transfer pathway for oxidative deamination involving the TTQ enzyme methylamine dehydrogenase is well understood biochemically, and the proteins that probably link this pathway to cytochrome oxidase have been elucidated with
Summary
Vol 270, No., Issue of March 3, pp. 4293-4298, 1995 Printed in U.S.A. Spectroscopic Evidence for a Common Electron Transfer Pathway for Two Tryptophan Tryptophylquinone Enzymes*. Aromatic amine dehydrogenase (AADH) and methylamine dehydrogenase (MADH) are the only two enzymes known to use the cofactor tryptophan tryptophylquinone (TTQ). The larger subunit (a) displays the 7-fold, circular repetition of small f3-sheets that was first seen in the structure of influenza neuraminidase (Varghese et al, 1983; Bossart-Whitaker et al, 1993) and more recently in galactose oxidase (Ito et al, 1991) and methanol dehydrogenase (Xia et al, 1992) Both MADH and AADH catalyze oxidative deamination. The structure of a ternary complex of MADH, amicyanin, and cytochrome c-55li has been solved and suggests the geometry for an electron transfer complex involving MADH, amicyanin, and cytochrome c-55li (Chen et al, 1993; 1994) This wealth of structural information for MADH prompts the questions (i) does AADH have a similar structure to MADH and. The results of these experiments show that AADH and MADH share many properties and suggest that they may have evolved from a common ancestor
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