Stability and immunological cross-reactivity of malate dehydrogenases from mesophilic and thermophilic sources

Malate dehydrogenase from the thermophilic bacterium Vulcanithermus medioatlanticus

Thermostable dimeric malate dehydrogenase (MDH) was isolated from the microorganism of hydrothermal vents Vulcanithermus medioatlanticus. The enzyme was electrophoretically homogeneous and possessed the specific activity of 6.9 U/mg. The large molecular weight of the subunits (55 kD) is likely to provide the rigidity of the enzyme structure (the activation energy of the enzymatic reaction is 32.6 kJ/mol). The thermophilic MDH differs little from the mesophilic enzyme in terms of kinetic and regulatory characteristics.

The enzymes from extreme thermophiles: bacterial sources, thermostabilities and industrial relevance

This review on enzymes from extreme thermophiles (optimum growth temperature greater than 65 degrees C) concentrates on their characteristics, especially thermostabilities, and their commercial applicability. The enzymes are considered in general terms first, with comments on denaturation, stabilization and industrial processes. Discussion of the enzymes subsequently proceeds in order of their E.C. classification: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. The ramifications of cloned enzymes from extreme thermophiles are also discussed.

Tetrameric malate dehydrogenase from a thermophilic Bacillus: cloning, sequence and overexpression of the gene encoding the enzyme and isolation and characterization of the recombinant enzyme

The gene encoding the tetrameric malate dehydrogenase (MDH) in a thermophilic Bacillus species (BI) has been cloned in an Escherichia coli plasmid. The nucleotide sequence of the gene, the first to be elucidated for a tetrameric MDH, shows the MDH subunit to contain 312 amino acids and have a molecular mass of 33648 Da, which confirms the experimentally determined value of about 35 kDa. Like the genomic DNA of BI, the MDH gene is relatively AT-rich; this contrasts with the generally GC-rich nature of the DNA of thermophilic Bacillus species. Comparison of amino acid sequences reveals that BI MDH bears greater structural similarity to lactate dehydrogenases (LDHs) than to other (dimeric) MDHs. MDHs and LDHs resemble each other in catalytic mechanism and several other respects. However, whereas MDHs in the majority of organisms are dimers, the tetrameric structure is favoured among LDHs. The stronger structural resemblance that BI MDH has to LDHs than to the dimeric MDHs provides some explanation as to why Bacillus MDH, unlike most other MDHs, is tetrameric. A 1 kb fragment containing the BI MDH gene, produced in a PCR, has been cloned into a high-expression E. coli plasmid vector. BI MDH synthesized from this clone constitutes about 47% of the total protein in cell extracts of the E. coli strain carrying the clone. MDH purified from BI and that purified from the E. coli strain carrying the MDH gene clone appear to be identical proteins by several criteria. A number of characteristics of the MDH have been elucidated, including the molecular masses of the native enzyme and the subunit, N-terminal amino acid sequence, isoelectric point, pH optimum for activity, thermostability, stability to pH, urea and guanidinium chloride and several kinetic parameters. Whereas the MDH is a stable tetramer in the pH range 5-7, it appears to be converted into a stable dimer at pH 3.5. This suggests that the dimer is a stable intermediate in the dissociation of the tetramer to monomers at low pH.

Overexpression of the Thermus aquaticus B malate dehydrogenase-encoding gene in Escherichia coli

Expression of the Thermus aquaticus B malate dehydrogenase (MDH)-encoding gene (mdh), cloned in Escherichia coli, was initially at a relatively low level (0.1% of soluble cell protein) and was effected by read-through from the tac promoter in the plasmid vector used. An enhancement in expression to 0.4% of soluble cell protein was achieved by shortening the intervening sequence between the promoter and the translation start codon of mdh. An NdeI restriction site (5'-CAT-ATG-3') was engineered in the shortened fragment, which also changed the start codon from GTG to ATG. This resulted in an eightfold increase in expression, to 3.2% of soluble cell protein. Expression was further increased by subcloning the mdh gene via the engineered NdeI site, into two plasmid expression vectors, one carrying the E. coli trpP promoter and the other the E. coli mdhP promoter. In both these expression systems, 40-50% of the soluble cell protein was T. aquaticus MDH. This suggests that expression of the cloned T. aquaticus mdh in E. coli is enhanced predominantly by the optimisation of transcription and translation initiation signals. Moreover, the base composition of the coding region and the pattern of codon usage dictated by it appear to have little effect on expression. Heat treatment of the cell extract at 85 degrees C further effected purification of T. aquaticus MDH to over 80% of the soluble cell protein. The MDHs purified to homogeneity from the high-expression clones were identical with the MDH isolated from T. aquaticus B cells with respect to all measured parameters.