Streptomycetes are a group of filamentous Gram positive soil

Streptomycetes are a group of filamentous Gram-positive, soil-inhabiting bacteria which have captured enormous screening interest because of their ability to produce and secrete a variety of PD128907 HCl and extracellular proteins [7]. Among the diverse antibiotics produced by Streptomycetes are the nucleoside antibiotics. The sequence of the genome of Streptomyces coelicolor A3(2) has been predicted to contain genes encoding more than 7500 proteins, many of which have yet to be studied and defined [8]. SCO4901 is one of these genes that had been annotated in data bases as a “putative” adenosine deaminase. However, since ADA and ADE proteins have been shown to have approximately the same size, exhibit overall sequence similarity, contain similar folding topology and share the same catalytic machinery, it is often difficult to unambiguously distinguish one from the other. Characterization of the ADE sequences from Aspergillus nidulans, Saccharomyces cerevisiae and Schilzosaccharomyces pombe shows that these proteins are more closely related to the biochemically characterized ADAs rather than Bacillus subtilis and Escherichia coli ADEs. Based on sequence alignment of ascomycete ADEs and the characterized ADAs, a structural rule has been proposed to distinguish whether a protein is an ADA or an ADE [9]. Asp19, Ser103, Ala183, and Gly184 (numbered according to murine ADA) have been proposed to be characteristic of ADA, whereas Glu, Asp, Asp or Ser, and Ser at the same position are characteristic of ADE. Although Streptomycetes are the major producers of nucleoside antibiotics and many of nucleoside antibiotics exhibit inhibitory effects on purine salvaging enzymes, few enzymes of Streptomycetes origin have been biochemically characterized. Therefore, this report describes the expression, characterization of substrate specificity and inhibition of adenosine deaminase from S. coelicolor (ScADA) by transition state analogs.
Materials and methods
Results
Discussion The purposes of this study were to (i) determine whether SCO4901 encodes for ADA or ADE, (ii) purify the recombinant protein and determine its subunit structure, (iii) characterize substrate and inhibitor specificity of the enzyme and (iv) elucidate the evolutionally relationship between ADA and ADE. To this date, no ADA or ADE of Streptomycetes origin has been purified or characterized for substrate specificity, despite of the fact that these organisms are the major producers of nucleoside antibiotics as secondary metabolites. For instance, two of the most potent inhibitors of ADA, coformycin and 2′-deoxycoformycin, are isolated from Streptomyces antibioticus[17]. Since nucleoside antibiotics often inhibit enzymes involved in nucleoside/nucleotide, RNA and DNA metabolism, antibiotic resistance has been shown to be a pre-requisite for antibiotic production. Since no ADA or ADE of Streptomycetes origin has been biochemically characterized, it is unclear whether enzymes in the salvage pathway from this organism may have evolved differently by altering their substrate/inhibitor specificity to avoid the inhibitory effect of antibiotics. ADA, ADE and AMP deaminases are closely related enzymes that utilize a similar catalytic machinery to catalyze mechanistically related reactions. Phylogenic analysis of amino acid sequences of various gene products suggests that these proteins belong to the adenyl-deaminase family [6]. This family contains five subfamilies, which include ADA, ADE, AMP deaminase, adenosine deaminase-like (ADAL), and adenosine deaminase-related growth factors (ADGF) subfamilies. Members of this family share a novel motif consisting of methionine (or iso/leucine), proline, lysine and glycine (MPKG), although these residues are not part of the catalytic machinery (Fig. 4). ADGF proteins contain all four of these residues, while ADAL and ADE proteins contain only the first three. In most ADA, only PK residues are conserved. In addition to this novel motif, eight of the catalytically active residues are highly conserved in all five protein subfamilies.