Anna Długołęcka

A cold-adapted esterase from psychrotrophic Pseudoalteromas sp. strain 643A

A psychrotrophic bacterium producing a cold-adapted esterase upon growth at low temperatures was isolated from the alimentary tract of Antarctic krill Euphasiasuperba Dana, and classified as Pseudoalteromonas sp. strain 643A. A genomic DNA library of strain 643A was introduced into Escherichiacoli TOP10F’, and screening on tributyrin-containing agar plates led to the isolation of esterase gene. The esterase gene (estA, 621 bp) encoded a protein (EstA) of 207 amino acid residues with molecular mass of 23,036 Da. Analysis of the amino acid sequence of EstA suggests that it is a member of the GDSL-lipolytic enzymes family. The purification and characterization of native EstA esterase were performed. The enzyme displayed 20–50% of maximum activity at 0–20°C. The optimal temperature for EstA was 35°C. EstA was stable between pH 9 and 11.5. The enzyme showed activity for esters of short- to medium-chain (C4 and C10) fatty acids, and exhibited no activity for long-chain fatty acid esters like that of palmitate and stearate. EstA was strongly inhibited by phenylmethylsulfonyl fluoride, 2–mercaptoethanol, dithiothreitol and glutathione. Addition of selected divalent ions e.g. Mg2+, Co2+ and Cu2+ led to the reduction of enzymatic activity and the enzyme was slightly activated (∼30%) by Ca2+ ions.

Structure of the single-stranded DNA-binding protein SSB from Thermus aquaticus.

The crystal structure of the single-stranded DNA-binding protein from Thermus aquaticus has been solved and refined at 1.85 A resolution. Two monomers, each encompassing two oligonucleotide/oligosaccharide-binding (OB) domains and a number of flexible beta-hairpin loops, form an oligomer of approximate D(2) symmetry typical of bacterial SSBs. Comparison with other SSB structures confirms considerable variability in the mode of oligomerization and aggregation of SSB oligomers.

Extracellular secretion of Pseudoalteromonas sp. cold-adapted esterase in Escherichia coli in the presence of Pseudoalteromonas sp. components of ABC transport system

Recently we described identification and characterization of GDSL esterase EstA from psychrotrophic bacterium Pseudoalteromonas sp. 643A. Attempts to obtain heterologous overexpression of this enzyme in Escherichia coli system were not satisfactory. The EstA protein was expressed as inclusion bodies, most of that were inactive after purification step, and the recovery of esterolytic activity was very low after refolding. Based on the sequence analysis we found that the esterase EstA gene is clustered with three genes encoding components of ABC transport system. These genes, designated abc1, abc2, and abc3 encode an ATP-binding protein (ABC1) and two permease proteins (ABC2 and ABC3). In present study, to obtain larger amounts of the active cold-adapted EstA esterase from Pseudoalteromonas sp. 643A, we designed a two-plasmid E. coli expression system where the gene encoding EstA enzyme was cloned into pET30b(+) expression vector and three genes encoding components of ABC transport system were cloned into pACYC-pBAD vector. It was shown that the created expression system was useful for extracellular production of active EstA enzyme which was purified from the culture medium. In the presence of all the three transporter proteins the secretion of EstA was at the highest level. When one or two of these components were missing, EstA secretion was also possible, but not so effective. It indicates that ABC2 and ABC3 proteins of Pseudoalteromonas sp. 643A could be replaced with their homologous proteins of E. coli.

Structure of EstA esterase from psychrotrophic Pseudoalteromonas sp. 643A covalently inhibited by monoethylphosphonate.

The crystal structure of the esterase EstA from the cold-adapted bacterium Pseudoalteromonas sp. 643A was determined in a covalently inhibited form at a resolution of 1.35 A. The enzyme has a typical SGNH hydrolase structure consisting of a single domain containing a five-stranded beta-sheet, with three helices at the convex side and two helices at the concave side of the sheet, and is ornamented with a couple of very short helices at the domain edges. The active site is located in a groove and contains the classic catalytic triad of Ser, His and Asp. In the structure of the crystal soaked in diethyl p-nitrophenyl phosphate (DNP), the catalytic serine is covalently connected to a phosphonate moiety that clearly has only one ethyl group. This is the only example in the Protein Data Bank of a DNP-inhibited enzyme with covalently bound monoethylphosphate.