FDX atlas

Ferredoxin types and subtypes

Ferredoxins (FDXs) are iron–sulfur electron-transfer proteins classified primarily according to the number and composition of their Fe–S clusters. The principal FDX types include single-cluster [2Fe–2S], [3Fe–4S], and [4Fe–4S] proteins, as well as dicluster 7Fe–8S FDXs containing one [3Fe–4S] and one [4Fe–4S] cluster, 2[4Fe–4S] FDXs containing two [4Fe–4S] clusters, and the distinct Allochromatium vinosum-like 2[4Fe–4S]Alv type. Because cluster-based classification alone does not represent the substantial sequence diversity within each type, we proposed dividing FDX types into subtypes based on the characteristic spacing between cluster-ligating cysteine residues in the Fe–S-binding motif. Ferredoxins with the same cysteine-spacing signature are assigned to the same subtype and designated by the cluster type followed by “ST” and a subtype number, for example, 2Fe–2SST1 or 4Fe–4SST2. This two-level classification distinguishes the physicochemical type of the Fe–S centre from variation in its surrounding ligand-binding motif, thereby providing a sequence-based framework for comparing FDX diversity and evolutionary distribution across organisms despite limited overall sequence conservation.