Erythropoietic protoporphyria (EPP) is caused by mutations in the gene coding for ferrochelatase, the enzyme that catalyzes the last step in the heme biosynthesis pathway. In this study, we suggest a molecular explanation for the decrease in ferrochelatase activity for a number of EPP missense mutations. We constructed homodimeric and heterodimeric mutant ferrochelatases; we measured their activities and examined their physical properties to see if the loss of activity is additive or greater than expected. The results showed that heterodimers with the mutation in the dimer interface were not isolated suggesting that they were unstable. Mutations in the active site resulted in loss of activity that differed between homodimers and heterodimers. Mutations in the C-terminal extension resulted in loss of the [2Fe-2S] cluster and of enzyme activity for both homodimeric and heterodimeric mutants. Western blot and dynamic light scattering (DLS) results suggest that human ferrochelatase may form a dimer with one or no cluster but it needs both clusters to maintain its stability and activity. Our data also suggest a possible different entry path of the iron substrate of ferrochelatase to the active site pocket than previously reported.