Complex sentence: The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding an iron ion, which readily interconverts between the Fe 2 + (reduced) and Fe 3 + (oxidized) states.
Split into simpler sentences: The heme group is a highly conjugated ring system (which means its electrons are very mobile) surrounding a metal ion, which readily interconverts between the oxidation states. The metal ion present is often that of "iron", which interconverts between Fe 2 + (reduced) and Fe 3 + (oxidized) states.