MauG is a di-heme enzyme responsible for the posttranslational modification of two tryptophan residues to form the tryptophan tryptophylquinone cofactor (TTQ) of methylamine dehydrogenase (MADH). MauG converts preMADH, containing monohydroxylated-βTrp57, to fully functional MADH by catalyzing the insertion of a second oxygen atom into the indole ring and covalently linking βTrp57 to βTrp108. Here we report the 2.1 Å resolution X-ray crystal structure of MauG complexed with preMADH. The c-type heme irons and the nascent TTQ site are separated by long distances over which electron transfer must occur to achieve catalysis. In addition one of the hemes has an atypical His-Tyr axial ligation. The crystalline protein complex is catalytically competent, as on addition of hydrogen peroxide MauG-dependent TTQ synthesis occurs.

The biosynthesis of methylamine dehydrogenase (MADH) from Paracoccus denitrificans requires four genes in addition to those that encode the two structural protein subunits. None of these gene products have been previously isolated. One of these, mauG, exhibits sequence similarity to diheme cytochrome c peroxidases and is required for the synthesis of the tryptophan tryptophylquinone (TTQ) prosthetic group of MADH. A system was developed for the homologous expression of MauG in P. denitrificans. Its signal sequence was correctly processed, and it was purified from the periplasmic cell fraction. The protein contains two covalent c-type hemes, as predicted from the deduced sequence. EPR spectroscopy reveals that the protein as isolated possesses about equal amounts of one high-spin heme with axial symmetry and one low-spin heme with rhombic symmetry. The low-spin heme contains a major and minor component suggesting a small degree of heme heterogeneity. The high-spin heme and the major low-spin heme component each exhibit resonances that are atypical of c-type hemes and dissimilar to those reported for diheme cytochrome c peroxidases. MauG exhibited only very weak peroxidase activity when assayed with either c-type cytochromes or o-dianisidine as an electron donor. Fully reduced MauG was shown to bind carbon monoxide and could be reoxidized by oxygen. The relevance of these unusual properties of MauG is discussed in the context of its role in TTQ biogenesis.

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The quinoprotein methylamine dehydrogenase (MADH), type I copper protein amicyanin, and cytochrome c-551i form a complex within which interprotein electron transfer occurs. It was known that complex formation significantly lowered the oxidation-reduction midpoint potential (Em) value of amicyanin, which facilitated an otherwise thermodynamically unfavorable electron transfer to cytochrome c-551i. Structural, mutagenesis, and potentiometric studies have elucidated the basis for this complex-dependent change in redox properties. Positively charged amino acid residues on the surface of amicyanin are known to stabilize complex formation with MADH and influence the ionic strength dependence of complex formation via electrostatic interactions. Altering the charges of these residues by site-directed mutagenesis had no effect on the Em value of amicyanin, ruling out charge neutralization as the basis for the complex-dependent changes in redox properties. The Em value of free amicyanin varies with pH and exhibits a pKa value for the reduced form of 7.5. The crystal structure of reduced amicyanin at pH 4.4 reveals that His95, which serves as a ligand for Cu2+, has rotated by 180 degrees about the Cbeta-Cgamma bond relative to its position in oxidized amicyanin and is no longer in the copper coordination sphere. At pH 7.7, the crystal structure of reduced amicyanin contains an approximately equal distribution of two active-site conformers. One is very similar to the structure of reduced amicyanin at pH 4.4, and the other is very similar to the structure of oxidized amicyanin at pH 4.8. Potentiometric analysis of amicyanin in complex with MADH indicates that its Em value is not pH-dependent from pH 6.5 to 8.5, and exhibits an Em value similar to that of free amicyanin at high pH. The structure of reduced amicyanin at pH 4.4, with His95 protonated and "flipped", was modeled into the structure of the complex of oxidized amicyanin with MADH. This showed that in the complex, the redox-linked pH-dependent rotation of His95 is hindered because it would cause an overlap of van der Waals' radii with residues of MADH. These results demonstrate that protein-protein interactions profoundly affect the redox properties of this type I copper protein by restricting a pH-dependent, redox-linked conformational change of one of the copper ligands.

Long range electron transfer (ET) 1 between proteins is a process which is fundamental to respiration, photosynthesis, and redox reactions of intermediary metabolism. Many physiologic long range biologic ET reactions are bimolecular processes involving donor and acceptor proteins. The overall redox reaction may require several steps, including specific binding of proteins, protein rearrangements which optimize the coupling between redox centers, chemical transformations such as proton transfer, and the actual ET step. Furthermore, because the redox centers reside within a protein matrix, the possibility exists that the reorganizational energy (λ) or electronic coupling between donor and acceptor (H AB ) may vary with reaction conditions as a result of protein conformational fluctuations. Direct application of ET theory to these processes is problematic because the kinetic complexity of the overall reaction often makes it difficult to identify the true ET rate constant. In model studies of protein ET reactions, it has become standard to analyze the variation in ET rate with the systematic variation of ∆G°to verify that the ET event is rate-limiting for the observed reaction and to obtain values for λ and H AB . This approach, however, will not be applicable to most physiologic interprotein ET reactions. This paper describes alternative approaches for the analysis of protein ET reactions. Models are presented for the kinetic analysis of interprotein ET reactions and the interpretation of experimentally derived values of λ for protein ET reactions whose rates may be influenced by non-ET events. APPLICATION OF ET THEORY TO PROTEIN ET REACTIONSET theory predicts that the rate of an ET reaction will vary predictably with temperature (T), ∆G°, and donoracceptor distance (r) according to the relationships given in eqs 1 and 2 (Marcus & Sutin, 1985) where h is Planck's constant, R is the gas constant (the Boltzmann constant may alternatively be used), k 0 is the characteristic frequency of the nuclei which is usually assigned a value of 10 13 s -1 , and r 0 is the close contact distance usually assigned a value of 3.0 Å. H AB is the electronic coupling between redox centers and describes the degree of wave function overlap between donor and acceptor sites. λ is the reorganizational energy. Detailed discussions of the mathematical and physical meaning of H AB and λ may be found in a number of excellent reviews of ET theory

Paracoccus denitrificans methylamine dehydrogenase (MADH) is an enzyme containing a quinone cofactor tryptophan tryptophylquinone (TTQ) derived from two tryptophan residues (betaTrp(57) and betaTrp(108)) within the polypeptide chain. During cofactor formation, the two tryptophan residues become covalently linked, and two carbonyl oxygens are added to the indole ring of betaTrp(57). Expression of active MADH from P. denitrificans requires four other genes in addition to those that encode the polypeptides of the MADH alpha(2)beta(2) heterotetramer. One of these, mauG, has been shown to be involved in TTQ biogenesis. It contains two covalently attached c-type hemes but exhibits unusual properties compared to c-type cytochromes and diheme cytochrome c peroxidases, to which it has some sequence similarity. To test the role that MauG may play in TTQ maturation, the predicted proximal histidine to each heme (His(35) and His(205)) has each been mutated to valine, and wild-type MADH was expressed in the background of these two mauG mutants. The resultant MADH has been characterized by mass spectrometry and electrophoretic and kinetic analyses. The majority species is a TTQ biogenesis intermediate containing a monohydroxylated betaTrp(57), suggesting that this is the natural substrate for MauG. Previous work has shown that MADH mutated at the betaTrp(108) position (the non-oxygenated TTQ partner) is predominantly also this intermediate, and work on these mutants is extended and compared to the MADH expressed in the background of the histidine to valine mauG mutations. In this study, it is unequivocally demonstrated that MauG is required to initiate the formation of the TTQ cross-link, the conversion of a single hydroxyl located on betaTrp(57) to a carbonyl, and the incorporation of the second oxygen into the TTQ ring to complete TTQ biogenesis. The properties of MauG, which are atypical of c-type cytochromes, are discussed in the context of these final stages of TTQ biogenesis.

Rates of electron-transfer (ET) reactions are dependent on driving force, reorganizational energy, distance, and the nature of the medium which the electron must traverse. In kinetically complex biological systems, non-ET reactions may be required to activate the system for ET and may also influence the observed rates. Studies of ET from tryptophan tryptophylquinone to copper to heme in the methylamine dehydrogenase-amicyanin-cytochrome c-551i ET complex, as well as studies of other physiologic redox protein complexes, are used to illustrate the combination of factors which control rates of interprotein ET reactions.

MauG is a novel 42 kDa diheme protein which is required for the biosynthesis of tryptophan tryptophylquinone, the prosthetic group of methylamine dehydrogenase. The visible absorption and resonance Raman spectroscopic properties of each of the two c-type hemes and the overall redox properties of MauG are described. The absorption maxima for the Soret peaks of the oxidized and reduced hemes are 403 and 418 nm for the low-spin heme and 389 and 427 nm for the high-spin heme, respectively. The resonance Raman spectrum of oxidized MauG exhibits a set of marker bands at 1503 and 1588 cm(-1) which exhibit frequencies similar to those of the nu3 and nu2 bands of c-type heme proteins with bis-histidine coordination. Another set of marker bands at 1478 and 1570 cm(-1) is characteristic of a high-spin heme. Two distinct oxidation-reduction midpoint potential (E(m)) values of -159 and -244 mV are obtained from spectrochemical titration of MauG. However, the two nu3 bands located at 1478 and 1503 cm(-1) shift together to 1467 and 1492 cm(-1), respectively, upon reduction, as do the Soret peaks of the low- and high-spin hemes in the absorption spectrum. Thus, the two hemes with distinct spectral properties are reduced and oxidized to approximately the same extent during redox titrations. This indicates that the high- and low-spin hemes have similar intrinsic E(m) values but exhibit negative redox cooperativity. After the first one-electron reduction of MauG, the electron equilibrates between hemes. This makes the second one-electron reduction of MauG more difficult. Thus, the two E(m) values do not describe redox properties of distinct hemes, but the first and second one-electron reductions of a diheme system with two equivalent hemes. The structural and mechanistic implications of these findings are discussed.