Christopher R. Goward

Molecular evolution of bacterial cell-surface proteins

The cell-surface proteins of the infective bacteria Streptococcus and Staphylococcus are probably involved in the process of infection. These proteins share many features including secretion signal peptides, cell-wall spanning regions, membrane anchor domains and repeated domains of various functions. These common features may have evolved by gene duplication and swapping of gene fragments.

The importance of arginine 102 for the substrate specificity of Escherichia coli malate dehydrogenase

The malate dehydrogenase from Escherichia coli has been specifically altered at a single amino acid residue by using site-directed mutagenesis. The conserved Arg residue at amino acid position 102 in the putative substrate binding site was replaced with a Gln residue. The result was the loss of the high degree of specificity for oxaloacetate. The difference in relative binding energy for oxaloacetate amounted to about 7 kcal/mol and a difference in specificity between oxaloacetate and pyruvate of 8 orders of magnitude between the wild-type and mutant enzymes. These differences may be explained by the large hydration potential of Arg and the formation of a salt bridge with a carboxylate group of oxaloacetate.

The functional units of a peptostreptococcal protein L

Protein L is a cell‐surface protein from Peptostreptococcus which interacts with immunoglobulin kappa light chains. A gene from Peptostreptococcus strain 3316 coding for protein L and fragments thereof were expressed in Escherichia coli. The peptides were examined for binding to immunoglobulin and serum albumin. The four C units were shown to be responsible for binding to immunoglobulin and the four D units for binding to albumin. This protein L molecule therefore binds to albumin at a site separate from that involved in binding to immunoglobulin. The albumin‐binding units have high amino acid sequence identity with the albumin‐binding units of streptococcal cell‐surface proteins. The gene contains three sites available for internal initiation of translation resulting in three active proteins. The protein L molecule presented in this report was compared with a previously reported protein from Peptostreptococcus strain 312. The two proteins differ in several respects, including size and the number and types of repeat units.

Substitution of the amino acid at position 102 with polar and aromatic residues influences substrate specificity of lactate dehydrogenase

The Gin residue at amino acid position 102 ofBacillus stearothermophilus lactate dehydrogenase was replaced with Ser, Thr, Tyr, or Phe to investigate the effect on substrate recognition. The Q102S and Q102T mutant enzymes were found to have a broader range of substrate specificity (measured bykcat/Km) than the wild-type enzyme. However, it is evident that either Ser or Thr at position 102 are of a size able to accommodate a wide variety of substrates in the active site and substrate specificity appears to rely largely on size discrimination in these mutants. The Q102F and Q102Y mutant enzymes have low catalytic efficiency and do not show this relaxed substrate specificity. However, their activities are restored by the presence of an aromatic substrate. All of the enzymes have a very low catalytic efficiency with branched chain aliphatic substrates.

Colorimetric Glucose Assay Using Thermostable Glucokinase

A method for assaying glucose in serum or plasma samples using a thermostable glucokinase was developed. Glucokinase from Bacillus Stearothermophilus was coupled with glucose-6-phosphate dehydrogenase to produce NADPH, which reduced the tetrazolium dye MTT to its formazan. Detection of the product at 660 nm allowed samples containing up to 30 mmol/L glucose to be assayed with an endpoint method. Use of the optimal wavelength for formazan detection, 570 nm, increased sensitivity for NADPH detection by over threefold compared to UV detection. The stability of glucokinase assay mixtures was extensively studied, with variation in buffers, salt and enzyme stabilizers. Maximal half life for reagent stability at room temperature was approximately 30 days, with storage of assay mixtures in two solutions. Various drugs and metabolites were tested for interference in the method and no significant interferences were found.

Use of Macrosorb kieselguhr composite and CM-sepharose fast flow for the large-scale purification of L-asparaginase from Erwinia chrysanthemi

An incompressible kieselguhr-agarose composite, derivatized with carboxymethyl groups, Macrosorb KAX-CM, is compared with a conventional agarose gel, CM-Sepharose Fast Flow, for the large-scale purification of l-asparaginase from Erwinia chrysanthemi. The Macrosorb KAX-CM has better pressure/flow characteristics than CM-Sepharose Fast Flow, but both matrices are identical on criteria of product purity. CM-Sepharose Fast Flow has a greater capacity than Macrosorb KAX-CM and enzyme is eluted in a smaller volume.

Solubilization of IgG-Binding Proteins from Group A and G Streptococci

The release of IgG‐binding proteins from the cell surface of streptococcal strains AR‐1 and G148 with various proteolytic enzymes, acid, alkali or SDS was investigated. The IgG‐binding proteins were purified by affinity chromatography using IgG‐Sepharose Fast Flow. After SDS‐polyacrylamide gel electrophoresis and immuno‐electroblotting the major proteins identified varied in relative molecular mass from 15,000 to 65,000 depending on the solubilizing agent used. The results showed that solubilization with trypsin gave the highest yield of IgG‐binding proteins, that strain G148 yielded about twice the amount of protein as strain AR‐1, and that elastase released an IgG‐binding protein of high relative molecular mass of 65,000.

Large-scale purification of the chromosomal β-lactamase from Enterobacter cloacae P999

Abstract Homogeneous β-lactamase (β-lactam hydrolase, E.C. 3.5.2.6) from Enterobacter cloacae P99, an enzyme that has an important function in antibiotic resistance, was prepared using a single cation-exchange chromatographic step with CM-Sepharose fast-flow. A 6-g amount of the enzyme was isolated from 5 kg of cell paste, with 84% of the enzyme activity in the cell homogenate being recovered by the single cation-exchange step. The specific activity of the β-lactamase was 587 U/mg protein. The relative molecular mass of the enzyme was determined to be 45 kDa by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate and the isoelectric point was 8.95.

Preparation of DNA polymerase from Bacillus caldotenax

A procedure with four chromatography steps was developed for the purification of DNA polymerase from Bacillus caldotenax by using fast protein liquid chromatography. The procedure was suitable for use with process-scale media. Elution profiles obtained from ion-exchange chromatography and triazine-dye affinity chromatography with fast protein liquid chromatography and process-scale media were similar. The enzyme showed stronger interaction, however, with phenyl-Sepharose FF in the scaled-up process than with the phenyl-Superose used in fast protein liquid chromatography. The surprising binding of the DNA polymerase to sulphonated ion-exchange media at pH 7.5 may be explained by the structure of the enzyme.