Abstract
Enzymatic and electron transfer activities have been studied by polarized absorption spectroscopy in single crystals of both binary and ternary complexes of methylamine dehydrogenase (MADH) with its redox partners. Within the crystals, MADH oxidizes methylamine, and the electrons are passed from the reduced tryptophan tryptophylquinone (TTQ) cofactor to the copper of amicyanin and to the heme of cytochrome c551i via amicyanin. The equilibrium distribution of electrons among the cofactors, and the rate of heme reduction after reaction with substrate, are both dependent on pH. The presence of copper in the ternary complex is not absolutely required for electron transfer from TTQ to heme, but its presence greatly enhances the rate of electron flow to the heme.
Highlights
Specific protein recognition is of the utmost importance for all biological systems both for regulation and for transfer of information, metabolites, and other components of living systems
We have undertaken a single crystal polarized absorption study of the reactivity of the methylamine dehydrogenase (MADH)-amicyanin and MADH-amicyanin-cytochrome c551i complexes in their crystalline states
MADH catalyzes the oxidation of methylamine in the periplasm of many methylotrophic and autotrophic bacteria to form ammonia and formaldehyde concomitant with the twoelectron reduction of its redox cofactor tryptophan tryptophylquinone (TTQ) [12]
Summary
(Received for publication, December 26, 1995, and in revised form, February 12, 1996). Enzymatic and electron transfer activities have been studied by polarized absorption spectroscopy in single crystals of both binary and ternary complexes of methylamine dehydrogenase (MADH) with its redox partners. Direct observation of electron transfer complexes in the crystalline state between weakly associating partners has been reported in only three instances These are a complex between cytochrome c and cytochrome c peroxidase [6], a complex between methylamine dehydrogenase (MADH) and amicyanin [7], and a ternary. Rates of the electron transfer reactions between redox centers in the binary and ternary complex in solution have been measured by stopped-flow spectroscopy. The rate for the reduction of copper by MADH in the binary complex is highly dependent on temperature, reaction conditions, and whether the substrate-derived amino group remains bound to reduced TTQ.
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