Kinetic characterization of yeast alcohol dehydrogenases. Amino acid residue 294 and substrate specificity

A three-dimensional model of yeast alcohol dehydrogenase, based on the homologous horse liver enzyme, was used to compare the substrate binding pockets of the three isozymes (I, II, and III) from Saccharomyces cerevisiae and the enzyme from Schizosaccharomyces pombe. Isozyme I and the S. pombe enzyme have methionine at position 294 (numbered as in the liver enzyme, corresponding to 270 in yeast), whereas isozymes II and III have leucine. Otherwise the active sites of the S. cerevisiae enzymes are the same. All four wild-type enzymes were produced from the cloned genes. In addition, oligonucleotide-directed mutagenesis was used to change Met-294 in alcohol dehydrogenase I to leucine. The mechanisms for all five enzymes were predominantly ordered with ethanol (but partially random with butanol) at pH 7.3 and 30 degrees C. The wild-type alcohol dehydrogenases and the leucine mutant had similar kinetic constants, except that isozyme II had 10-20-fold smaller Michaelis and inhibition constants for ethanol. Thus, residue 294 is not responsible for this difference. Apparently, substitutions outside of the substrate binding pocket indirectly affect the interactions of the alcohol dehydrogenases with ethanol. Nevertheless, the substitution of methionine with leucine in the substrate binding site of alcohol dehydrogenase I produced a 7-10-fold increase in reactivity (V/Km) with butanol, pentanol, and hexanol. The higher activity is due to tighter binding of the longer chain alcohols and to more rapid hydrogen transfer.     

Molecular cloning of theEnvC gene ofEscherichia coli

DNA fragments complementing theenvC mutation could be isolated by cloning chromosomal DNA in the vector pUH84. When the frequencies of transformation and the frequencies of restoring theenvC ⁺ phenotype were compared, the high copy number hybrid plasmids complemented with a frequency of 10^–5. After subcloning theenvC-complementing DNA fragment into the low copy number plasmid pLG339, efficient complementation was achieved by spontaneous integration of the IS2 element ofEscherichia coli. By nucleotide sequence analysis, a potential promoter, a ribosome-binding site, and an unidentified reading frame were detected in the respective DNA fragment.     

Degradation of Linuron and Some Other Herbicides and Fungicides by a Linuron-Inducible Enzyme Obtained from Bacillus sphaericus

Linuron [3-(3,4-dichlorophenyl)-1-methoxy-1-methylurea] induces the formation of an enzyme (acylamidase) responsible for the degradation of a large variety of different herbicides and fungicides of the acylanilide and phenylurea type. The former type is degraded at a rate at least 10 times higher than the latter.     

Biochemical characterization and crystallization of porin from Rhodopseudomonas blastica

Oligomeric porin of the phototrophic bacterium Rhodopseudomonas blastica DSM 2131 was obtained from cell envelopes by differential temperature extraction in the presence of detergent and salt. The isolated porin exhibited strong porin activity after reconstitution into lipid bilayer membranes. The effective channel diameter for the trimer was estimated as 1.5 nm from single channel conductance measurements in the presence of 1 M KCl. Moderate cation-selectivity was observed. Oligomeric porin migrated as a single band (apparent molecular weight 81 kDa) on sodium dodecyl sulfate polyacrylamide gelelectrophoresis when solubilized below 70 °C. The oligomers were converted into monomers on heating to 70 °C or above forming two bands with apparent molecular weight of 36 kDa and 35 kDa. The oligomer was not sensitive to EDTA. Its molecular weight was determined to be 119.3 kDa by analytical ultracentrifugation. The isoelectric point was 5.7. Circular dichroism data indicated a high content of -sheet structure. Gasphase sequencing of the N-terminal residues revealed the sequence: NH₂-Glu-Ile-Ser-Leu-Asn-Gly-Tyr-Gly-Arg-Phe. Crystals with a maximal side length of 300 m and diffracting to 0.32 nm resolution were obtained with the porin oligomer in the presence of C8E4 and 1,2,3-heptanetriol by using the vapor phase equilibration technique.Abbreviations C8E4 n-octyl tetraoxyethylene - M_r apparent molecular weight - Octyl-POE n-octyl polyoxyethylene - LDAO N,N-dimethyl dodecyl aminoxide - LPS lipopolysaccharide - PAGE polyacrylamide gel-electrophoresis - PEG polyethylene glycol     

Improvement of the catalytic properties of penicillin G acylase from Escherichia coli ATCC 11105 by selection of a new substrate specificity

Cloned penicillin G acylase (PGA) from Escherichia coli ATCC 11105 was mutagenized in vivo using N-methyl-N-nitrosoguanidine. Mutants of PGA were selected by their ability to allow growth of the host strain E. coli M8820 with the new substrates phenylacetyl--alanyl-l-proline (PhAc-Ala-Pro) phthalyl-l-leucine (Pht-Leu) or phthalylglycyl-l-proline (Pht-Gly-Pro) as sole source of proline and leucine respectively. PGA mutants were purified and immobilized onto spherical methacrylate (G-gel). The immobilized form of mutant PGA selected with (PhAc-gbAla-Pro) hydrolyzed 95% of 9 mmol penicillin G 30% faster than wild-type PGA using the same specific activities. The specific activity of the soluble enzyme was 2.7-fold, and inhibition by phenylacetic acid was halved. Immobilized PGA mutant selected with Pht-Gly-Pro hydrolyzed penicillin G 20% faster than wild-type PGA. The K _m of the soluble enzyme was increased 1.7-fold. Furthermore, the latter two mutants were also 3.6-fold more stable at 45° C than wild-type PGA. The specific activity of the mutant selected with Pht-Leu was 6.3-fold lower, and inhibition by phenylacetic acid was increased 13-fold.