Which heme is needed? E. coli synthesizes heme c only in hypoxic conditions, for example. You could use a heme c expression vector such as pEC86 if necessary. If you need heme b, it is synthesized endogenously in E. coli, but make sure to add a rich carbon source such as glucose.

In general, it is best to express enzymes in rich media (such as terrific broth) at room temperature. This reduces expression speed, thus giving the enzyme more time to fold properly and incorporate cofactors.