Two azurins with unusual redox and spectroscopic properties isolated from the Pseudomonas chlororaphis strains DSM 50083T and DSM 50135

Cold-Azurin, a New Antibiofilm Protein Produced by the Antarctic Marine Bacterium Pseudomonas sp. TAE6080

Biofilm is accountable for nosocomial infections and chronic illness, making it a serious economic and public health problem. Staphylococcus epidermidis, thanks to its ability to form biofilm and colonize biomaterials, represents the most frequent causative agent involved in biofilm-associated infections of medical devices. Therefore, the research of new molecules able to interfere with S. epidermidis biofilm formation has a remarkable interest. In the present work, the attention was focused on Pseudomonas sp. TAE6080, an Antarctic marine bacterium able to produce and secrete an effective antibiofilm compound. The molecule responsible for this activity was purified by an activity-guided approach and identified by LC-MS/MS. Results indicated the active protein was a periplasmic protein similar to the Pseudomonas aeruginosa PAO1 azurin, named cold-azurin. The cold-azurin was recombinantly produced in E. coli and purified. The recombinant protein was able to impair S. epidermidis attachment to the polystyrene surface and effectively prevent biofilm formation.

Copper nitrite reductase from Sinorhizobium meliloti 2011: Crystal structure and interaction with the physiological versus a nonmetabolically related cupredoxin-like mediator

We report the crystal structure of the copper-containing nitrite reductase (NirK) from the Gram-negative bacterium Sinorhizobium meliloti 2011 (Sm), together with complex structural alignment and docking studies with both non-cognate and the physiologically related pseudoazurins, SmPaz1 and SmPaz2, respectively. S. meliloti is a rhizobacterium used for the formulation of Medicago sativa bionoculants, and SmNirK plays a key role in this symbiosis through the denitrification pathway. The structure of SmNirK, solved at a resolution of 2.5 Å, showed a striking resemblance with the overall structure of the well-known Class I NirKs composed of two Greek key β-barrel domains. The activity of SmNirK is ~12% of the activity reported for classical NirKs, which could be attributed to several factors such as subtle structural differences in the secondary proton channel, solvent accessibility of the substrate channel, and that the denitrifying activity has to be finely regulated within the endosymbiont. In vitro kinetics performed in homogenous and heterogeneous media showed that both SmPaz1 and SmPaz2, which are coded in different regions of the genome, donate electrons to SmNirK with similar performance. Even though the energetics of the interprotein electron transfer (ET) process is not favorable with either electron donors, adduct formation mediated by conserved residues allows minimizing the distance between the copper centers involved in the interprotein ET process.

Electrochemical characterization and application of azurin-modified gold electrodes for detection of superoxide

A novel biosensor for superoxide radical (O(2)(*-)) detection based on Pseudomonas aeruginosa azurin immobilized on gold electrode was designed. The rate constant of azurin reduction by O(2)(*-) was found to be 10(5)M(-1)s(-1) in solution and five times lower, i.e., 0.2 x 10(5)M(-1)s(-1), for azurin coupled to gold by 3,3'-dithiobis(sulfosuccinimidylpropionate) (DTSSP). The electron transfer rate between the protein and the electrode ranged from 2 to 6s(-1). The sensitivity of this biosensor to O(2)(*-) was 6.8 x 10(2)Am(-2)M(-1). The response to the interference substances, such as uric acid, H(2)O(2), and dimethylsulfoxide was negligible below 10 microM. The electrode was applied in three O(2)(*-) generating systems: (i) xanthine oxidase (XOD), (ii) potassium superoxide (KO(2)), and (iii) stimulated neutrophil granulocytes. The latter was compared with luminol-amplified chemiluminescence. The biosensor responded to O(2)(*-) in all three environments, and the signals were antagonized by superoxide dismutase.

Direct electron transfer between copper-containing proteins and electrodes

The electrochemistry of some copper-containing proteins and enzymes, viz. azurin, galactose oxidase, tyrosinase (catechol oxidase), and the "blue" multicopper oxidases (ascorbate oxidase, bilirubin oxidase, ceruloplasmin, laccase) is reviewed and discussed in conjunction with their basic biochemical and structural characteristics. It is shown that long-range electron transfer between these enzymes and electrodes can be established, and the mechanistic schemes of the DET processes are proposed.

Copper-containing nitrite reductase from Pseudomonas chlororaphis DSM 50135

The nitrite reductase (Nir) isolated from Pseudomonas chlororaphis DSM 50135 is a blue enzyme, with type 1 and type 2 copper centers, as in all copper-containing Nirs described so far. For the first time, a direct determination of the reduction potentials of both copper centers in a Cu-Nir was performed: type 2 copper (T2Cu), 172 mV and type 1 copper (T1Cu), 298 mV at pH 7.6. Although the obtained values seem to be inconsistent with the established electron-transfer mechanism, EPR data indicate that the binding of nitrite to the T2Cu center increases its potential, favoring the electron-transfer process. Analysis of the EPR spectrum of the turnover form of the enzyme also suggests that the electron-transfer process between T1Cu and T2Cu is the fastest of the three redox processes involved in the catalysis: (a) reduction of T1Cu; (b) oxidation of T1Cu by T2Cu; and (c) reoxidation of T2Cu by NO(2) (-). Electrochemical experiments show that azurin from the same organism can donate electrons to this enzyme.