Amicyanin

denitrificans, amicyanin is an obligatory mediator of electron transfer from MADH to soluble c-type cytochromes (Husain and Davidson, 1986). Each protein is induced in this bacterium during growth on 

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Several copper enzymes will be discussed in detail in subsequent sections of this chapter. Information about major classes of copper enzymes, most of which will not be discussed, is collected in Table 5.1 as adapted from Chapter 14 of reference 49. Table 1 of reference 4 describes additional copper proteins such as the blue copper electron transfer proteins stellacyanin, amicyanin, auracyanin, rusticyanin, and so on. Nitrite reductase contains both normal and blue copper enzymes and facilitates the important biological reaction NO) — NO. Solomon s Chemical Reviews article4 contains extensive information on ligand field theory in relation to ground-state electronic properties of copper complexes and the application of... 

There are four other proteins - stellacyanin, rusticyanin, umecyanin and ami-cyanin (Table 3) which have been fairly extensively studied. A crystal structure determination for amicyanin from Thiobacillus versutus is now under way [61]. A number of other type 1 proteins have been identified. These include pseudo-... 

Two amicyanins have been a recent focus of attention [84, 85], These are from the methylotrophic bacteria Pseudomonas AMI and Thiobacillus versutus,... 

Amicyanin appears to be more homologous with the plastocyanin sequence than with the pseudoazurin sequence. 

The blue protein from A. faecalis strain S-6, which was isolated as a requirement for transferring electrons to a copper-containing nitrite reductase, has since been shown to have sequence homology with proteins arbitrarily designated pseudoazurin by Ambler and Tobari (1985), from Achromobacter cycloclastes and from Pseudomonas AMI. [Pseudomonas AMI also produces amicyanin, which is the recipient of electrons from methylamine dehydrogenase, (see below)]. In A. cycloclastes reduced pseudoazurin donates electrons to a copper nitrite reductase (Liu et ai, 1986), as it does in A. faecalis. Ambler and Tobari (1985)... 

Amicyanin (Husain and Davidson, 1986 Groeneveld etal., 1988) spectroscopically resembles plastocyanin more than pseudoazurin and has about the same number of amino acids, so that its classiflcation has been changed from subgroup II to III (the plastocyanin group see Table II). However, its sequence is distinctly different than the plastocyanins, and the new function may indicate yet another class. 

Although crystals have been reported for two amicyanins (Petratos et al., 1988b Lim et al., 1986), the type 1 blue protein, which is an electron acceptor for methylamine dehydrogenase (Tobari and Harada, 1981 van Houweligen et al., 1989), neither study has yet been completed. The structure of methylamine dehydrogenase from Thiobacillus versutus (not a copper protein) has recently been reported (Vellieux et al., 1989). The amicyanin from P. denitrificans has actually been cocrystallized with methylamine dehydrogenase (F. S. Mathews, personal communication. 

Fig. 21. Rmsd overlays of experimental (blue) and computed (yellow or CPK) backbone carbons (top) and active sites (bottom) for Amicyanin (left), Stellacyanin (middle), and Azurin (right) (PDB codes 1AAC, 1JER, 1DYZ).

A new line of research involving blue copper proteins deals with their unfolding and their structural heterogeneity. The thermal unfolding of amicyanin was studied with calorimetric and spectroscopic methods. It was found to be irreversible and the kinetic data were analyzed in a three state model. 

How is the reduced cofactor reoxidized Presumably the copper ion adjacent to the TPQ functions in this process, passing electrons one at a time to the next carrier in a chain. There is no copper in the TTQ-containing subunits. Electrons apparently must jump about 1.6 ran to the copper ion of amicyanin, then another 2.5 nm to the iron ion of the cytochrome c.472 Reoxidation of the aminoquinol formed in Eq. 15-53, step d, yields a Schiff base whose hydrolysis will release ammonia and regenerate the TTQ. Intermediate states with Cu+ and a TTQ semiquinone radical have been observed.4833... 

Amebic dystentery 187 Amebocytes 23, 25 Ameloblasts 442 Ameloblastin 442 Amelogenins 442 Amicyanin 817, 883 Amide(s)... 

Monovalent cations affect the spectroscopic and electron transfer rates between MADH and amicyanin. The precise mechanism for this is still unknown. 

The copper proteins with a type 1 active site are commonly known as blue copper proteins due to their intense blue color in the Cu11 state. They are usually participants in electron transfer processes, and the best-known representatives of this class include plastocyanin, azurin and amicyanin [1]. The copper center in the type 1 active site is surrounded by two nitrogen donor atoms from two... 

The opposite approach of keeping the bulk of the protein geometry fixed to that observed crystallographically while optimizing the geometry of the active site and its immediate surrounds has also been investigated. In a study of plastocyanin and amicyanin, the geometry about the copper(I) centers was well reproduced -391. ... 

The first class is cupredoxins—single-domain blue copper proteins composed of only one BCB domain. These proteins include plastocyanin, azurin, pseudoazurin, amicyanin, auracyanins, rusticyanin, halocyanin, and sulfocyanin (see Section IV). Plantacyanin of the phytocyanin family (Section V), subunit II of the cytochrome c oxidase, and the recently characterized nitrosocyanin also fall into this class. The last two are single BCB domain polypeptides closely related structurally to cupredoxins, but harboring, respectively, a binuclear copper site known as CuA and a novel type of copper-binding site called red (see Sections IX and X). 

Intriguingly, the blue copper sites, especiaUy those with a carbonyl oxygen at the axial coordination position, display high affinity for Zn + ions. Mutants in which the Met is replaced by Gin or Glu preferentiaUy bind Zn + when expressed in heterologous systems, e.g., Escherichia coli. Examples include azurin, amicyanin, nitrite reductase, and possibly also plastocyanin (Diederix et al., 2000 Hibino et al., 1995 Murphy et al., 1995 Nar et al., 1992a Romero et al., 1993). In the case of azurin it has been shown that both wild-type and the Met—Gin mutant have the same affinity for both Zn +and Cu + (Romero ci a/., 1993). In addition, EXAFS studies showed that some preparations of blue copper proteins purihed from their natural sources also contain small fractions of Zn derivatives (DeBeer George, personal communication). 

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