Lineage for Fold f.21: Heme-binding four-helical bundle
- Root: SCOP 1.73
Class f: Membrane and cell surface proteins and peptides [56835] (50 folds) 
Fold f.21: Heme-binding four-helical bundle [81344] (3 superfamilies)
core: four transmembrane helices, up-and-down bundle, binds one or two heme groups in between the helices
Superfamilies:
f.21.1: Transmembrane di-heme cytochromes [81342] (2 families) 
Three of the four heme-ligands are conserved between the two families; both heme groups bind similarly but not identically
f.21.2: Fumarate reductase respiratory complex transmembrane subunits [81343] (2 families)
two distinct families: in one family the common fold is contained in a single-chain subunit, in the other it is formed by two chains
f.21.3: Respiratory nitrate reductase 1 gamma chain [103501] (1 family)
possible link between the two other superfamilies: this subunit corresponds to the gamma subunit of a functionally related Formate dehydrogenase N complex but is structurally closer to the Fumarate reductase subunit FrdC
More info for Fold f.21: Heme-binding four-helical bundle
Timeline for Fold f.21: Heme-binding four-helical bundle:
- Fold f.21: Heme-binding four-helical bundle first appeared in SCOP 1.63
- Fold f.21: Heme-binding four-helical bundle appears in SCOP 1.71
- Fold f.21: Heme-binding four-helical bundle appears in SCOP 1.75
- Fold f.21: Heme-binding four-helical bundle appears in the current release, SCOPe 2.08