Malic enzymes catalyze the reversible oxidative decarboxylation of L-malate using $NAD(P)^+$ as a cofactor. NADP-dependent malic enzyme (MaeB) from Escherichia coli MG1655 was expressed and purified as a fusion protein. The molecular weight of MaeB was about 83 kDa, as determined by SDS-PAGE. The recombinant MaeB showed a maximum activity at pH 7.8 and $46^{circ}C$. MaeB activity was dependent on the presence of $Mn^{2+}$ but was strongly inhibited by $Zn^{2+}$. In order to understand the physiological roles, recombinant E. coli strains ($icd^{NADP}/{Delta}maeB$ and $icd^{NAD}/{Delta}maeB$) containing NADP-dependent isocitrate dehydrogenase (IDH), or engineered NAD-dependent IDH with the deletion of the maeB gene, were constructed using homologous recombination. During growth on acetate, $icd^{NAD}/{Delta}maeB$ grew poorly, having a growth rate only 60% that of the wild-type strain ($icd^{NADP}$). Furthermore, $icd^{NADP}/{Delta}maeB$ exhibited a 2-fold greater adaptability to acetate than $icd^{NAD}/{Delta}maeB$, which may be explained by more NADPH production for biosynthesis in $icd^{NADP}/{Delta}maeB$ due to its NADP-dependent IDH. These results indicated that MaeB was important for NADPH production for bacterial growth on acetate. We also observed that MaeB activity was significantly enhanced (7.83-fold) in $icd^{NAD}$, which was about 3-fold higher than that in $icd^{NADP}$, when switching from glucose to acetate. The marked increase of MaeB activity was probably induced by the shortage of NADPH in $icd^{NAD}$. Evidently, MaeB contributed to the NADPH generation needed for bacterial growth on two carbon compounds.