The crystal structure of a ternary protein complex has been determined at 2.4 angstrom resolution. The complex is composed of three electron transfer proteins from Paracoccus denitrificans , the quinoprotein methylamine dehydrogenase, the blue copper protein amicyanin, and the cytochrome c _551i . The central region of the c _551i is folded similarly to several small bacterial c-type cytochromes; there is a 45-residue extension at the amino terminus and a 25-residue extension at the carboxyl terminus. The methylamine dehydrogenase-amicyanin interface is largely hydrophobic, whereas the amicyanin-cytochrome interface is more polar, with several charged groups present on each surface. Analysis of the simplest electron transfer pathways between the redox partners points out the importance of other factors such as energetics in determining the electron transfer rates.

Diheme Conversion: A dehydrogenase enzyme found in methylotrophic and autotrophic bacteria, which converts methylamine to ammonia and formaldehyde, must be posttranslationally modified to create a covalently bound cofactor. Synthesis of the cofactor is completed by a diheme enzyme, MauG, which oxidatively links two tryptophan residues to form tryptophan tryptophylquinone. Jensen et al. (p. 1392 ; see the Perspective by Bollinger and Matthews ) describe the crystal structure of a catalytically competent complex of MauG bound to the methylamine dehydrogenase precursor. The reactive tryptophans are buried and well-separated from both heme irons. The heme closest to the nascent tryptophylquinone site is a 6-coordinate with an unusual His-Tyr axial link. The axial Tyr likely stabilizes a bis -Fe(IV) state involved in catalysis. Two other tyrosine residues are positioned to participate in long-range interprotein electron and radical transfer.