Effect of interactions between amino acid residues 43 and 61 on thermal stability of bacterial formate dehydrogenases

NAD+-dependent formate dehydrogenases (EC 1.2.1.2, FDH) of methylotrophic bacteria Pseudomonas sp. 101 (PseFDH) and Mycobacterium vaccae N10 (MycFDH) exhibit high homology. They differ in two amino acid residues only among a total of 400, i.e., Ile35 and Glu61 in MycFDH substitute for Thr35 and Lys61 as in PseFDH. However, the rate constant for MycFDH thermal inactivation in the temperature range of 54-65 degrees C is 4-6-times higher than the corresponding rate constant for the enzyme from Pseudomonas sp. 101. To clarify the role of these residues in FDH stability the dependence of the apparent rate constant for enzyme inactivation on phosphate concentration was studied. Kinetic and thermodynamic parameters for thermal inactivation were obtained for both recombinant wild-type and mutant forms, i.e., MycFDH Glu61Gln, Glu61Pro, Glu61Lys and PseFDH Lys61Arg. It has been shown that the lower stability of MycFDH compared to that of PseFDH is caused mainly by electrostatic repulsion between Asp43 and Glu61 residues. Replacement of Lys61 with an Arg residue in the PseFDH molecule does not result in an increase in stability.

Pilot scale production and isolation of recombinant NAD+- and NADP+-specific formate dehydrogenases

The expression of the recombinant wild-type NAD+- and mutant NADP+-dependent formate dehydrogenases (EC 1.2.1.2., FDH) from the methanol-utilizing bacterium Pseudomonas sp. 101 in Escherichia coli cells has been improved to produce active and soluble enzyme up to the level of 50% of total soluble proteins. The cultivation process for E. coli/pFDH8a and E. coli/pFDH8aNP cells was optimized and scaled up to a volume of 100 L. A downstream purification process has been developed to produce technical grade NAD+- and NADP+-specific formate dehydrogenases in pilot scale, utilizing extraction in aqueous two-phase systems.

Bacterial formate dehydrogenase. Increasing the enzyme thermal stability by hydrophobization of alpha-helices

NAD+-dependent formate dehydrogenase (EC 1.2.1.2, FDH) from methylotrophic bacterium Pseudomonas sp.101 exhibits the highest stability among the similar type enzymes studied. To obtain further increase in the thermal stability of FDH we used one of general approaches based on hydrophobization of protein alpha-helices. Five serine residues in positions 131, 160, 168, 184 and 228 were selected for mutagenesis on the basis of (i) comparative studies of nine FDH amino acid sequences from different sources and (ii) with the analysis of the ternary structure of the enzyme from Pseudomonas sp.101. Residues Ser-131 and Ser-160 were replaced by Ala, Val and Leu. Residues Ser-168, Ser-184 and Ser-228 were changed into Ala. Only Ser/Ala mutations in positions 131, 160, 184 and 228 resulted in an increase of the FDH stability. Mutant S168A was 1.7 times less stable than the wild-type FDH. Double mutants S(131,160)A and S(184,228)A and the four-point mutant S(131,160,184,228)A were also prepared and studied. All FDH mutants with a positive stabilization effect had the same kinetic parameters as wild-type enzyme. Depending on the position of the replaced residue, the single point mutation Ser/Ala increased the FDH stability by 5-24%. Combination of mutations shows near additive effect of each mutation to the total FDH stabilization. Four-point mutant S(131,160,184,228)A FDH had 1.5 times higher thermal stability compared to the wild-type enzyme.

Wild-type and mutant forms of recombinant horseradish peroxidase C expressed in Escherichia coli. Substrate specificity and stability under irradiation

Two horseradish peroxidase C (HRPC) mutants with substitutions in the active center, i.e., Phe41-->His and Phe143-->Glu, were compared to the wild-type recombinant enzyme expressed in Escherichia coli in terms of the enzymatic activity and stability under irradiation. Both mutations caused a significant decrease in activity, but it was still possible to follow the effect of mutations on the key steps of the reaction mechanism. Phe41 can be considered a nonpolar barrier separating histidine residues in the active center and providing a firm noncovalent binding with the highly hydrophobic porphyrin ring. The replacement of Phe41 with the ionizable His residue destabilizes the enzyme. The Phe143-->Glu replacement creates a negative charge at the entrance of the heme-binding pocket, and protects the latter from both donor substrates and free radicals. The radiolytic inactivation of the wild-type and mutant forms of recombinant HRP suggested different binding sites for iodide, 2,2'-bis(3-ethylbenzothiasoline-6-sulfonate (ABTS), guaiacol, and O-phenylene diamine. The study of kinetics and inactivation is in agreement with the direct binding of iodide to the heme porphyrin ring. The results also suggest that the ABTS binding site is less accessible than that for O-phenylene diamine.

Gene cloning, purification, and characterization of thermostable and halophilic leucine dehydrogenase from a halophilic thermophile, Bacillus licheniformis TSN9

A halophilic and thermophilic isolate from the sand of Tottori Dune was found to produce a thermostable and halophilic leucine dehydrogenase (EC 1.4.1.9). It was identified to be a new strain of Bacillus licheniformis. The enzyme gene was cloned into Escherichia coli JM109 with a vector plasmid pUC18. The enzyme was purified to homogeneity from the clone cell extract by ion-exchange column chromatography with a yield of 31%. The enzyme was found to be composed of eight subunits identical in relative molecular mass (43,000). The amino acid sequence of the enzyme, deduced from the nucleotide sequence of the gene, showed an identity of 84.6% with that of the B. stearothermophilus enzyme [Nagata S, Tanizawa K, Esaki N, Sakamoto Y, Oshima T, Tanaka H, Soda K (1988) Biochemistry 27:9056-9062], although both enzymes were similar to each other in various enzymological properties such as thermostability, substrate and coenzyme specificities, and stereospecificity for hydrogen transfer from the C-4 of NADH. However, they were markedly distinct from each other in halophilicity; the B. licheniformis enzyme was much more stable than the other in the presence of high concentrations of salts.

[Production and catalytic properties of point mutants Phe41--->His and Phe143--->Glu of horseradish peroxidase, expressed in Escherichia coli]

Recombinant horseradish peroxidase and its single-point mutants, F4IH and H143E, have been reactivated from E. coli inclusion bodies with a 25% yield. Both mutations affect heme entrapment as well as enzyme stability and activity. A more than 40-fold decrease in the specific activity towards ABTS is associated with different steps of peroxidase catalysis. F41H replacement results in a drop of both rate constants by two and one orders or magnitude for hydrogen peroxide and ABTS, respectively. The mechanism of iodide oxidation by the F41H mutant fits into a ternary interaction. The F143E replacement mainly affects the steps of ABTS oxidation and product dissociation. It is suggested that the role of replaced phenylalanine residues consists in the formation of a highly hydrophobic pocket allowing for strong non-covalent binding of the heme porphyrin ring.

Effect of single-point mutations Phe41-->His and Phe143-->Glu on folding and catalytic properties of recombinant horseradish peroxidase expressed in E. coli

Wild-type recombinant and Phe41-->His and Phe143-->Glu mutant forms of horseradish peroxidase have been expressed in E. coli and reactivated from inclusion bodies with a yield of about 25%. The purified homogeneous preparations have been studied in the reaction of ABTS oxidation. The effect of mutations on heme entrapment and kinetics of ABTS oxidation demonstrates the essential role of the replaced residues in providing the hydrophobic crevice for the non-covalent heme binding.

Formate dehydrogenase from methylotrophic bacterium Pseudomonas sp. 101: gene cloning and expression in Escherichia coli

Using two degenerate 20- and 23-mer oligonucleotide probes, the gene encoding NAD(+)-dependent formate dehydrogenase (FDH) (EC 1.2.1.2) was shown to lie within a 3.5 kb PstI fragment of the chromosomal DNA of methylotrophic bacterium Pseudomonas sp. 101. A phasmid library was prepared in the lambda pSL5 vector with partial EcoRI-digested DNA from Pseudomonas sp. 101. The 12 clones selected contained three types of phasmid: lambda pFDH1, lambda pFDH2 and lambda pFDH3 (with inserts of 15, 17.6 and 18.5 kb respectively). The inserts contained the same 15 kb EcoRI and 3.5 kb PstI fragments and included the complete FDH gene. Further subcloning of the insert resulted in plasmid pFDH2 and a 2.32 kb HindIII-BgIII fragment. Active enzyme was expressed in Escherichia coli TG1 (pFDH2) strain under control of a lac promoter.

Catalytic properties and stability of a Pseudomonas sp.101 formate dehydrogenase mutants containing Cys-255-Ser and Cys-255-Met replacements

Two mutants of bacterial formate dehydrogenase from Pseudomonas sp.101 (EC 1.2.1.2, FDH)-C255S (FDH-S) and C255M (FDH-M), were obtained and its properties were studied. Both mutations provided the high resistance to inactivation by Hg2+. Slow inactivation of mutants by DTNB reveals the presence in FDH molecule of another essential cysteine residue. Specific activities of FDH, FDH-S and FDH-M were 16, 16 and 9.5 U/mg of protein, respectively. Km on formate was 7.5, 7.5 and 20 mM and Km on NAD(+)-0.1, 0.3 and 0.6 mM for FDH, FDH-S and FDH-M, respectively. Mutations of Cys255 on Ser or Met resulted in increasing of enzyme stability at 25 degrees C and decreasing of thermostability (above 45 degrees C). Data obtained show that Cys255 is unique residue for providing both enzyme thermostability and catalytically optimal binding of coenzyme.

Kinetic isotope effect and the presteady-state kinetics of the reaction catalyzed by the bacterial formate dehydrogenase

The primary kinetic isotope effect of the reaction catalyzed by NAD+-dependent formate dehydrogenase (EC 1.2.1.2.) from the methylotrophic bacterium Pseudomonas sp. 101 has been studied. Analysis of the ratios HVm/DVm and H(Vm/KM)/D(Vm/KM) in the pH range 6.1-7.9 showed that the transfer of hydride ion in ternary enzyme-substrate complex is a limiting step of the reaction, and the formate binding to the binary complex (formate dehydrogenase + NAD+) reached equilibrium when the pH of the medium was increased. An approach has been developed to determine the elementary constants of substrate association (kon) and dissociation (koff) at the stages of the binary--ternary enzyme-substrate complexes for the random equilibrium 2-substrate kinetic mechanism. The kon and koff values obtained for the bacterial formate dehydrogenase by using the proposed approach for NAD+ were (4.8 +/- 0.8)*10(5)M-1s-1 and (90 +/- 10) s-1, and for formate (2.0 +/- 1.0)*10(4) M-1s-1 and (60 +/- 20) s-1, respectively.