Plays a key role in regulating maximal capacity for electron transport and oxidative phosphorylation. May be involved in Fe-S cluster shuttling and/or in redox reactions. Binds 1 [2Fe-2S] cluster per subunit. The [2Fe-2S] cluster is redox-active and pH labile and is significantly less stable at pH 4.5 as compared with pH 7.0. Homodimer. Ubiquitinated by PRKN during mitophagy, leading to its degradation and enhancement of mitophagy. Deubiquitinated by USP30. Belongs to the CISD protein family.