The amino acid sequence of Pseudomonas putida azurin

Characterization of a blue-copper protein, auracyanin, of the filamentous anoxygenic phototrophic bacterium Roseiflexus castenholzii

A blue-copper protein auracyanin of the filamentous anoxygenic phototroph Roseiflexus castenholzii was purified and characterized. Genomic sequence analysis showed that R. castenholzii has only one auracyanin, whereas Chloroflexus aurantiacus is known to have two auracyanins, A and B. Absorption spectrum of the Roseiflexus auracyanin was similar to that of auracyanin B of C. aurantiacus. On the other hand, ESR spectrum of the Roseiflexus auracyanin resembles that of auracyanin A of C. aurantiacus. These results suggest that the blue-copper protein auracyanin from R. castenholzii shares features with each of auracyanin A and B. Amino acid sequence alignment of auracyanins from filamentous anoxygenic phototrophs also demonstrated the chimeral feature of the primary structure of the Roseiflexus auracyanin, i.e., auracyanin A-like amino-terminal characteristics and auracyanin B-like one-residue spacing at the Cu-binding loop in the carboxyl-terminus.

Electron transfer from quinohemoprotein alcohol dehydrogenase to blue copper protein azurin in the alcohol oxidase respiratory chain of Pseudomonas putida HK5

A blue copper protein was purified together with a type II quinohemoprotein alcohol dehydrogenase (ADH IIB) from the soluble fraction of Pseudomonas putida HK5 grown on n-butanol. The purified blue copper protein was shown to be azurin, on the basis of several properties such as its absorption maximum (623 nm), its low molecular mass (17 500 Da), its acidic nature (pI of 4.1), its relatively high redox potential (306 mV), the presence of an intramolecular disulfide bond, and N-terminal amino acid sequence homology with respect to azurins from other sources, especially from P. putida NCIB 9869 and Pseudomonas fluorescens. Direct electron transfer from ADH IIB to azurin was shown to occur at a rate of 48-70 s-1. The apparent Km value of ADH IIB for azurin, determined by steady-state kinetics, was decreased several-fold by increasing the ionic strength. Furthermore, the extent of fluorescence quenching of ADH IIB due to the interaction with azurin was increased by increasing the ionic strength, but the binding constant for binding between ADH IIB and azurin was unchanged. The redox potential of azurin was increased 12 mV by incubation with ADH but not vice versa. Furthermore, the redox potential gap between ADH and azurin was increased from 102 to 126 mV by increasing the ionic strength. It is conceivable that a hydrophobic interaction is involved in the electron transfer between both proteins, and it is also suggested that the electron transfer may occur by a freely reversible on and off binding process but may not be related to the global binding process of both proteins. Thus, the results presented here strongly suggest that azurin works as an electron-transfer mediator in a PQQ-dependent alcohol oxidase respiratory chain in P. putida HK5.