(S)-Malic acid ((S)-2-Hydroxysuccinic acid) is a tart-tasting organic dicarboxylic acid that plays a role in many sour or tart foods. Apples contain malic acid, which contributes to the sourness of a green apple. Malic acid can make a wine taste tart, although the amount decreases with increasing fruit ripeness. (wikipedia). In its ionized form malic acid is called malate. Malate is an intermediate of the TCA cycle along with fumarate. It can also be formed from pyruvate as one of the anaplerotic reactions. In humans, malic acid is both derived from food sources and synthesized in the body through the citric acid cycle or Krebs cycle which takes place in the mitochondria. Malate's importance to the production of energy in the body during both aerobic and anaerobic conditions is well established. Under aerobic conditions, the oxidation of malate to oxaloacetate provides reducing equivalents to the mitochondria through the malate-aspartate redox shuttle. During anaerobic conditions, where a buildup of excess of reducing equivalents inhibits glycolysis, malic acid's simultaneous reduction to succinate and oxidation to oxaloacetate is capable of removing the accumulating reducing equivalents. This allows malic acid to reverse hypoxia's inhibition of glycolysis and energy production. In studies on rats it has been found that only tissue malate is depleted following exhaustive physical activity. Other key metabolites from the citric acid cycle needed for energy production were found to be unchanged. Because of this, a deficiency of malic acid has been hypothesized to be a major cause of physical exhaustion. Notably, the administration of malic acid to rats has been shown to elevate mitochondrial malate and increase mitochondrial respiration and energy production.

研究表明，ME对于L. casei中(S)-Malic acid(L-苹果酸)的利用是必要的。此外，如果删除编码组氨酸激酶或TC系统的应答调节器的基因，会导致丧失在(S)-Malic acid上生长的能力，这说明相应的TC系统调控并对ME的表达是必需的。转录分析显示，maeE的表达在(S)-Malic acid存在的情况下被诱导，在葡萄糖存在的情况下被抑制，而TC系统的表达则是由(S)-Malic acid诱导并且不被葡萄糖抑制。