NAD(P)H regeneration is the key for heterolactic fermentation of hexoses in Oenococcus oeni

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Abstract
Oenococcus oeni (formerly Leuconostoc oenos) can perform malolactic fermentation, converting L-malate to L-lactate and carbon dioxide, in wines. The energy and redox potential required to support the growth of the micro-organism are supplied mainly by the consumption of carbohydrates via the heterolactic pathway. In the first steps of hexose metabolism two molecules of NAD(P)+ are consumed, which must be regenerated in later reactions. The aim of this work was to test if aerobic growth of O. oeni promotes higher cell yields than anaerobic conditions, as has been shown for other lactic acid bacteria. O. oeni M42 was found to grow poorly under aerobic conditions with glucose as the only carbohydrate in the medium. It was demonstrated that O2 inactivates the enzymes of the ethanol-forming pathway, one of the two pathways which reoxidizes NAD(P)+ cofactors in the heterolactic catabolism of glucose. These results suggest that the regeneration of cofactors is the limiting factor for the aerobic consumption of glucose. When external electron acceptors, such as fructose or pyruvate, were added to glucose-containing culture medium the growth of O. oeni was stimulated slightly; fructose was converted to mannitol, oxidizing two molecules of NAD(P)H, and pyruvate was transformed to lactate, enabling the regeneration of NAD+. The addition of cysteine seemed to suppress the inactivation of the ethanol-forming pathway enzymes by O2, enabling glucose consumption in aerobic conditions to reach similar rates to those found in anaerobic conditions.