Page 15 - MALOLACTIC FERMENTATION – IMPORTANCE OF WINE LACTIC ACID BACTERIA IN WINEMAKING

L-malic acid
NAD+ Mn+2
Malolactic enzyme (malate: NAD+ Carboxylyase)
L-lactic acid + CO2
THE CHEMISTRY OF MALOLACTIC FERMENTATION
Figure 1. Malolactic fermentation
The malolactic enzyme from L. oenos (O. oeni) has a molecular mass of 138,000 and consists
of two identical subunits, each with a molecular mass of 65,500 (Kunkee 1991).
ENERGETICS AND BIOLOGICAL ROLE
There has been considerable investigation in recent decades concerning the seemingly ob- scure benefit of malolactic conversion to the bacterial cell. The initiation of MLF in wine usually occurs after LAB have grown beyond a viable cell population of approximately 106 CFU/mL. Al- though providing deacidification and an accompanying increase in pH of up to approximately 0.2 pH units, the malolactic conversion itself appears energetically unfavourable to LAB. It yields little free energy (DG = -8.3 kJ/mole), proceeds without the formation of free interme- diates and does not yield biologically available energy in the form of ATP. Further, although NAD is an essential co-factor, it does not serve an oxidation/reduction function as there is no net change in redox state (Pilone et al. 1976, Renault et al. 1988, Kunkee 1991, Henick-Kling 1993, and Boulton et al. 1998). In overall terms, MLF is not a true fermentation. In addition to supplying little energy for cell growth, it also does not supply a source of carbon for the biosynthetic reactions that are essential for cellular development. Nevertheless, the presence and utilization of malic acid appreciably stimulate the initial growth rate of malolactic bacteria, yet the resulting increase in pH that is associated with MLF does not fully account for this stimulatory effect (Pilone et al. 1976, Renault et al. 1988, and Boulton et al. 1998).
Although the conversion of L-malic acid to L-lactic acid by the malolactic enzyme is energet- ically unfavourable, the MLF has, in fact, been shown to provide energy in the form of ATP to the bacterial cell. This is accomplished by a chemiosmotic mechanism that generates a proton motive force (Dp) across the cell membrane. In this model, the MLF proceeds in three stages. In the first step, entry of L-malic acid into the bacterial cell is facilitated by a specific transport enzyme. In the second step, L-malic acid is decarboxylated within the cell by the malolactic enzyme, yielding L-lactic acid and CO2, which then increases the intracellular pH. In the final stage, L-lactic acid and CO2 are expelled from the cell. For every molecule of lactic acid that leaves the cell, one proton is also translocated outside of the cell. This establishes a proton gradient across the cell membrane between the cytoplasm and the surrounding
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