Ascorbate oxidase structure determination

X-ray crystal structures of four functional derivatives of ascorbate oxidase were determined (149,150). The results of these investigations and implications for the catalytic mechanism of the blue oxidases will be outlined in the next section. 

Various spectroscopic methods have been used to probe the nature of the copper centers in the members of the blue copper oxidase family of proteins (e.g. see ref. 13). Prior to the X-ray determination of the structure of ascorbate oxidase in 1989, similarities in the EPR and UV-vis absorption spectra for the blue multi-copper oxidases including laccase and ceruloplasmin had been observed [14] and a number of general conclusions made for the copper centers in ceruloplasmin as shown in Table 1 [13,15]. It was known that six copper atoms were nondialyzable and not available to chelation directly by dithiocarbamate and these coppers were assumed to be tightly bound and/or buried in the protein. Two of the coppers have absorbance maxima around 610 nm and these were interpreted as blue type I coppers with cysteine and histidine ligands, and responsible for the pronounced color of the protein. However, they are not equivalent and one of them, thought to be involved in enzymatic activity, is reduced and reoxidized at a faster rate than the second (e.g. see ref. 16). There was general concurrence that there are two type HI... 

The type 1-3 terminology to distinguish different Cu protein active sites remains extremely useful. Sub-groupings are appearing however in all three categories particularly in the case of the binuclear (EPR inactive) type 3 centers. Thus, in the recently determined X-ray crystal structure of ascorbate oxidase the type 3 and type 2 centers are present as a single trimer unit [. A discrete binuclear type 3 center is, however, retained in hemocyanin [6]. 

The low-temperature MCD and absorption titration studies (Figure 10) have determined that azide binds to both the type 2 and type 3 centers with similar binding constants. A series of chemical perturbations and stoichiometry studies have shown that these effects are associated with the same azide. This demonstrates that one N3 bridges between the type 2 and type 3 centers in laccase. These and other results from MCD spectroscopy first defined the presence of a trinuclear copper cluster active site in biology (89). At higher azide concentration, a second azide binds to the trinuclear site in laccase. Messerschmidt et al. have determined from X-ray crystallography that a trinuclear copper cluster site is also present in ascorbate oxidase (87, 92) and have obtained a crystal structure for a two-azide-bound derivative (87). It appears that some differences exist between the two-azide-bound laccase and ascorbate oxidase derivatives, and it will be important to spectroscopically correlate between these sites. 

This chapter will concentrate mainly on structural and functional aspects of these enzymes with the major emphasis on ascorbate oxidase and laccase. Significant progress has been achieved in the last 10 years the determination of amino acid sequences of all three enzymes, each from several sources, and the X-ray structure of ascorbate oxidase. The new information forms the basis of a much deeper understanding of the function of the enzymes as will be demonstrated in this chapter. 

It turns out from the recently determined X-ray structure of ascorbate oxidase (73, 74) that the nonblue EPR-active type-2 copper together... 

A tentative catalytic mechanism of ascorbate oxidase has been proposed based on the refined X-ray structure and on spectroscopic and mechanistic studies of ascorbate oxidase and the related laccase. The results of these studies have been discussed in detail (74). The X-ray structure determinations of the fully reduced and peroxide derivatives define two important intermediate states during the catalytic cycle. A proposal for the catalytic mechanism incorporating this new information is given in Messerschmidt et al. (150) and presented in Fig. 14. This scheme should be valid in principle also for laccase due to the close similarities of both blue oxidases. 

This progress is mainly due to the determination of the amino-acid sequences for all members of this group and the X-ray crystal structure of ascorbate oxidase. The three-dimensional structure of ascorbate oxidase showed the nature and spatial arrangement of the copper centers and the three-domain structure. However, modern spectroscopic techniques (e.g., low-temperature MCD and ENDOR) made invaluable contributions as well. 

The blue oxidases-related enzymes include phenoxazinone synthase from S. antibioticus This enzyme is a copper-containing oxidase that catalyzes the coupling of 2-aminophenols to form the 2-aminophenoxazinone chromophore. This reaction constitutes the final step in the biosynthesis of the potent antineoplastic agent actinomycin. The crystal structure of the oxidized form phenoxazinone synthase from S. anibioticus has been determined. It has been solved in his hexameric form. One monomer is very similar to Iaccase or ascorbate oxidase but it contains a long loop, which connects two domains and stabilizes the hexameric structure. Bound... 

Type 1 copper proteins are the class of proteins for which cupredoxins were originally named. Type 1 copper proteins include both proteins with known electron transfer function (e.g., plastocyanin and rusticyanin), and proteins whose biological functions have not been determined conclusively (e.g., stellacyanin and plantacyanin). Although these proteins with unknown function cannot be called cupredoxins by the strict functional definition, they have been classified as cupredoxins because they share the same overall structural fold and metal-binding sites as cupredoxins. In addition, many multidomain proteins, such as laccase, ascorbate oxidase, and ceruloplasmin, contain multiple metal centers, one of which is a type 1 copper. Those cupredoxin centers are also included here. Finally, both the Cua center in cytochrome c oxidase (CcO) and nitrous oxide reductase (N2OR), and the red copper center in nitrocyanin will be discussed in this chapter because their metal centers are structurally related to the type 1 copper center and the protein domain that contains both centers share the same overall structural motif as those of cupredoxins. The Cua center also functions as an electron transfer agent. Like ferredoxins, which contain either dinuclear or tetranuclear iron-sulfur centers, cupredoxins may include either the mononuclear or the dinuclear copper center in their metal-binding sites. 

Soc., 1999, 121, 3435 T. Hoshi, H. Saiki, S. Kuwazawa, Y. Kobayama and A. Anzai, Polyelectrolyte multilayer film-coated electrodes for amperometric determination of hydrogen peroxide in the presence of ascorbic acid, uric acid and acetaminophen, Anal. Sci., 2000, 16, 1009 E.S. Forzani, VM. Solis and E.J. Calvo, Electrochemical behavior of polyphenol oxidase immobilized in self-assembled structures layer by layer with cationic polyallylamine, Anal. Chem., 2000, 72, 5300 Y.M. Lvov, Z. Lu, J.B. Schenkman, X. Zu and J.F. Rusling, Direct electrochemistry of myoglobin and cytochrome P450cam in alternate layer-by-layer film with DNA and other polyions, J. Am. Chem. Soc., 1998, 120, 4073. 

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