[The "malic enzyme" from Lactobacillus plantarum and Leuconostoc mesenteroides]

Malolactic enzyme from Oenococcus oeni: heterologous expression in Escherichia coli and biochemical characterization

Malolactic enzymes (MLE) are known to directly convert L-malic acid into L-lactic acid with a catalytical requirement of nicotinamide adenine dinucleotide (NAD⁺) and Mn²⁺; however, the reaction mechanism is still unclear. To study a MLE, the structural gene from Oenococcus oeni strain DSM 20255 was heterologously expressed in Escherichia coli, yielding 22.9 kU l⁻¹ fermentation broth. After affinity chromatography and removal of apparently inactive protein by precipitation, purified recombinant MLE had a specific activity of 280 U mg⁻¹ protein with a recovery of approximately 61%. The enzyme appears to be a homodimer with a molecular mass of 128 kDa consisting of two 64 kDa subunits. Characterization of the recombinant enzyme showed optimum activity at pH 6.0 and 45°C, and K_m, V_max and k_cat values of 4.9 mM, 427 U mg⁻¹ and 456 sec⁻¹ for L-malic acid, 91.4 µM, 295 U mg⁻¹ and 315 sec⁻¹ for NAD⁺ and 4.6 µM, 229 U mg⁻¹ and 244 sec⁻¹ for Mn²⁺, respectively. The recombinant MLE retained 95% of its activity after 3 mo at room temperature and 7 mo at 4°C. When using pyruvic acid as substrate, the enzyme showed the conversion of pyruvic acid with detectable L-lactate dehydrogenase (L-LDH) activity and oxidation of NADH. This interesting observation might explain that MLE catalyzes a redox reaction and hence, the requirements for NAD⁺ and Mn²⁺ during the conversion of L-malic to L-lactic acid.

Purification and Properties of a Malolactic Enzyme from a Strain of Leuconostoc mesenteroides Isolated from Grapes

An enzymatic complex able to transform l-malate to l-lactate was obtained from a Leuconostoc mesenteroides strain isolated from grapes. The molecular weight was about 235,000, the isoelectric point was at pH 4.35, and the optimal pH for activity was 5.75. The malolactic activity followed a sequential pattern concerning the involved substrates. At pH values substantially different from the optimum, a positive cooperativity between malate molecules was observed. Oxamate, fructose-1, 6-diphosphate, and l-lactate acted as noncompetitive inhibitors, whereas succinate, citrate, and tartrate isomers produced a competitive inhibition.

Biochemical basis for glucose-induced inhibition of malolactic fermentation in Leuconostoc oenos

The sugar-induced inhibition of malolactic fermentation in cell suspensions of Leuconostoc oenos, recently reclassified as Oenococcus oeni (L. M. T. Dicks, F. Dellaglio, and M. D. Collins, Int. J. Syst. Bacteriol. 45:395-397, 1995) was investigated by in vivo and in vitro nuclear magnetic resonance (NMR) spectroscopy and manometric techniques. At 2 mM, glucose inhibited malolactic fermentation by 50%, and at 5 mM or higher it caused a maximum inhibitory effect of ca. 70%. Galactose, trehalose, maltose, and mannose caused inhibitory effects similar to that observed with glucose, but ribose and 2-deoxyglucose did not affect the rate of malolactic activity. The addition of fructose or citrate completely relieved the glucose-induced inhibition. Glucose was not catabolized by permeabilized cells, and inhibition of malolactic fermentation was not observed under these conditions. 31P NMR analysis of perchloric acid extracts of cells obtained during glucose-malate cometabolism showed high intracellular concentrations of glucose-6-phosphate, 6-phosphogluconate, and glycerol-3-phosphate. Glucose-6-phosphate, 6-phosphogluconate, and NAD(P)H inhibited the malolactic activity in permeabilized cells or cell extracts, whereas NADP+ had no inhibitory effect. The purified malolactic enzyme was strongly inhibited by NADH, whereas all the other above-mentioned metabolites exerted no inhibitory effect, showing that NADH was responsible for the inhibition of malolactic activity in vivo. The concentration of NADH required to inhibit the activity of the malolactic enzyme by 50% was ca. 25 microM. The data provide a coherent biochemical basis to understand the glucose-induced inhibition of malolactic fermentation in L. oenos.

Cloning and characterization of the genes encoding the malolactic enzyme and the malate permease of Leuconostoc oenos

Using degenerated primers from conserved regions of the protein sequences of malic enzymes, we amplified a 324-bp DNA fragment by PCR from Leuconostoc oenos and used this fragment as a probe for screening a Leuconostoc oenos genomic bank. Of the 2,990 clones in the genomic bank examined, 7 with overlapping fragments were isolated by performing colony hybridization experiments. Sequencing 3,453 bp from overlapping fragments revealed two open reading frames that were 1,623 and 942 nucleotides long and were followed by a putative terminator structure. The first deduced protein (molecular weight, 59,118) is very similar (level of similarity, 66%) to the malolactic enzyme of Lactococcus lactis; as in several malic enzymes, highly conserved protein regions are present. The synthesis of a protein with an apparent molecular mass of 60 kDa was highlighted by the results of labelling experiments performed with Escherichia coli minicells. The gene was expressed in E. coli and Saccharomyces cerevisiae and conferred "malolactic activity" to these species. The second open reading frame encodes a putative 34,190-Da protein which has the characteristics of a carrier protein and may have 10 membrane-spanning segments organized around a central hydrophilic core. Energy-dependent L-[14C]malate transport was observed with E. coli dicarboxylic acid transport-deficient mutants carrying the malate permease-expressing vector. Our results suggest that in Leuconostoc oenos the genes that encode the malolactic enzyme and a malate carrier protein are organized in a cluster.

Electrogenic L-malate transport by Lactobacillus plantarum: a basis for energy derivation from malolactic fermentation

L-Malate transport in Lactobacillus plantarum was inducible, and the pH optimum was 4.5. Malate uptake could be driven by an artificial proton gradient (delta pH) or an electroneutral lactate efflux. Because L-lactate efflux was unable to drive L-malate transport in the absence of a delta pH, it did not appear that the carrier was a malate-lactate exchanger. The kinetics of malate transport were, however, biphasic, suggesting that the external malate concentration was also serving as a driving force for low-affinity malate uptake. Because the electrical potential (delta psi, inside negative) inhibited malate transport, it appeared that the malate transport-lactate efflux couple was electrogenic (net negative) at high concentrations of malate. De-energized cells that were provided with malate only generated a large proton motive force (greater than 100 mV) when the malate concentration was greater than 5 mM, and malate only caused an increase in cell yield (glucose-limited chemostats) when malate accumulated in the culture vessel. The use of the malate gradient to drive malate transport (facilitated diffusion) explains how L. plantarum derives energy from malolactic fermentation, a process which does not involve substrate-level phosphorylation.

Purification and properties of a malolactic enzyme from Leuconostoc oenos ATCC 23278

The malolactic enzyme of Leuconostoc oenos ATCC 23278 was purified 136fold. The molecular weight was estimated at 132,000 when determined by gel filtration. The enzyme contained two identical subunits (Mw = 66,000 using sodium dodecyl sulfate gel electrophoresis). The malolactic enzyme catalyzes the NAD(+)- and Mn(+)-dependent reaction L-malate----L-lactate + CO2. The apparent Km values for malic acid, NAD+, and Mn2+ were 17 mM, 0.044 mM, and 0.017 mM, respectively. The optimal pH and the optimal temperature for activity were 5.0, and 37 degrees C, respectively and the isoelectric point was pH 4.30. L-lactate and ethanol were non-competitive inhibitors, whereas succinate, citrate, and D-tartrate showed competitive type inhibitions.

Chemiosmotic energy from malolactic fermentation

By using the luciferase-luciferin ATP assay and whole cells of Leuconostoc oenos, we have demonstrated that malolactic fermentation does yield ATP. This energy-yielding mechanism did not occur in a cell extract and was inhibited in the presence of dicyclohexylcarbodiimide or an ionophore such as monensin. A lactate:proton efflux mechanism for this proposed pathway is presented.

Malolactic enzyme in Lactobacillus murinus

The malolactic enzyme of Lactobacillus murinus is inducible. The induction is produced by L-malic acid only in the presence of glucose and amino acids and occurs at the transcription level. The enzyme, purified to homogeneity, has a Mr of 220,000 and consists of 2 apparently identical subunits (Mr = 110,000) that were observed after treatment with sodium dodecyl sulphate. NAD+ protected the enzyme against inactivation and its addition, after dissociation, restored the malolactic activity. Maximum enzyme activity was observed at 37 degrees C and pH 5.5. At pH values substantially different from the optimum, a positive cooperativity between substrate molecules was observed. The activation energy of the reaction was 8,000 and 16,200 cal mol-1 for temperatures above and below 30 degrees C, respectively. Malolactic enzyme catalyzes the NAD+ and manganese-dependent reaction; L-malate----L-lactate + CO2. The stoichiometry of the reaction was confirmed. The malolactic transformation occurs by a compulsory-order mechanism. NAD+ bound first to the protein, independently of malate concentration. Mn2+ acts as an allosteric activator. Malate bound to the complex enzyme-NAD-Mn2+. Oxamate, fructose 1,6-diphosphate and malonate acted as non-competitive inhibitors, whereas citrate and L-tartrate produced a competitive inhibition. This enzyme can be distinguished from the malic enzyme of pigeon liver (E.C.1.1.1.40) and from the true malic enzymes (E.C.1.1.1.38 and E.C.1.1.1.39).

Flavor composition of wines: a review

The formation of flavor in fermented beverages is due to various biosynthetic mechanisms. In wine, flavors arise as the result of compounds: 1. Originating from the native fruit (grap) 2. Which are formed or altered during the various processes employed in production 3. Which are developed or transformed by yeast during fermentation 4. Arise during the aging process In this review the results of investigations on the development of flavors in grape and wine will be discussed. Special attention will be devoted to the effects of specific processes in winemaking on the development of flavor.