When protoporphyrin is deficient, such as in sideroblastic anemia, the iron that should be used for heme production remains trapped in the mitochondria because it cannot be incorporated into heme.
If protoporphyrin is deficient, as seen in conditions such as sideroblastic anemia, the iron that would normally be incorporated into heme cannot be used and consequently remains trapped in the mitochondria. In this scenario, iron accumulates within the mitochondria because it cannot be converted into heme due to the lack of protoporphyrin.
Typically, iron is an essential component of hemoglobin, and when it is absorbed, it joins the body's total iron pool. From there, it can be stored in the liver or spleen in the form of ferritin or hemosiderin, or it can be carried through the bloodstream by transferrin. However, if the synthesis of protoporphyrin is inadequate, as in sideroblastic anemia, this step in heme production is inhibited, leaving iron stranded in the mitochondrial environment.
In conclusion, if protoporphyrin is insufficient, iron cannot fulfill its physiological function of contributing to heme synthesis and thus remains trapped within the mitochondria.