title: Molecular genetic characterization of the L-lactate dehydrogenase gene (ldhL) of Lactobacillus helveticus and biochemical characterization of the enzyme.
abstract: The Lactobacillus helveticus L-(+)-lactate dehydrogenase (L-LDH) gene (ldhL) was isolated from a lambda library. The nucleotide sequence of the ldhL gene was determined and shown to have the capacity to encode a protein of 323 amino acids (35.3 kDa). The deduced sequence of the 35-kDa protein revealed a relatively high degree of identity with other lactobacillar L-LDHs. The highest identity (80.2%) was observed with the Lactobacillus casei L-LDH. The sizes and 5' end analyses of ldhL transcripts showed that the ldhL gene is a monocistronic transcriptional unit. The expression of ldhL, studied as a function of growth, revealed a high expression level at the logarithmic phase of growth. The ldhL gene is preceded by two putative -10 regions, but no corresponding -35 regions could be identified. By primer extension analysis, the ldhL transcripts were confirmed to be derived from the -10 region closest to the initiation codon. However, upstream of these regions additional putative -10/-35 regions could be found. The L-LDH was overexpressed in Escherichia coli and purified to homogeneity by two chromatographic steps. The purified L-LDH was shown to be a nonaliosteric enzyme, and amino acid residues involved in allosteric regulation were not conserved in L. helveticus L-LDH. However, a slight enhancement of enzyme activity was observed in the presence of fructose 1,6-diphosphate, particularly at neutral pH. A detailed enzymatic characterization of L-LDH was performed. The optimal reaction velocity was at pH 5.0, where the kinetic parameters K(m), and Kcat for pyruvate were 0.25 mM and 643 S-1, respectively.
authors.affiliation: Agricultural Research Centre of Finland, Food Research Institute, Jokioinen, Finland.
authors.forename: K
authors.initials: K
authors.lastname: Savijoki
authors.forename: A
authors.initials: A
authors.lastname: Palva
mesh_terms: D000595:Amino Acid Sequence; D001483:Base Sequence; D003001:Cloning, Molecular; D018387:Codon, Initiator; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D005635:Fructosediphosphates / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D015723:Gene Library; D007770:L-Lactate Dehydrogenase / Q000235:genetics* / Q000378:metabolism*; D007778:Lactobacillus / Q000235:genetics* / Q000378:metabolism*; D007780:Lacticaseibacillus casei / Q000235:genetics; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D011995:Recombination, Genetic; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D014158:Transcription, Genetic
chemical_list: D018387:Codon, Initiator; D005635:Fructosediphosphates; D007770:L-Lactate Dehydrogenase; C029063:fructose-1,6-diphosphate