Oenococcus, the malate permease is a uniporter, whereas in lactococci and lactobacilli, uptake of malate is mediated by an antiporter that exchanges an incoming malic acid for an outgoing lactic acid. No energy is spent for malate transport in either system. The exchange reaction, however, is not electroneu-tral.This is because one extracellular molecule of malic acid, carrying a net electric charge of -2, is exchanged for one of lactic acid that carries a net charge of only -l.This charge difference arises as a result of the decarboxylation reaction, which consumes an intracellular proton. The bottom line is that the cell is able to extrude a proton (or its equivalent) without having to spend energy to do so. That is, the cell conserves energy, in the form of ATP, that it would ordinarily spend to pump protons from the cytoplasm to the extracellular medium.The proton gradient that forms, or the proton motive force, can then either perform other work (e.g.,nutrient transport) for the cell or be used to drive ATP synthesis by the proton-translocating F0 F1 ATPase.
It is important to note that in low-acid grapes, the malolactic fermentation is undesirable, since some acidity is desired in wine. Thus, under some circumstances, the presence of naturally occurring malolactic bacteria is undesirable and the source of potential defects (see below). In contrast, the malolactic fermentation not only is performed for deacidifica-tion, but also to promote flavor stability and balance. Moreover, malolactic bacteria often
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