TABLE 3.

Motifs in the five subunits of H. thermophilus FRD

SubunitMotifPosition (aa)Conserved sequence
FrdAFAD-binding motifa 9-15[I/V]GXGX2G
Site of covalent FAD attachmentb 40-49RSH[T/S]X2AX[G/S]G
FrdB[2Fe-2S] clusterc 53-73CX4CX2CX11C
Two [4Fe-4S] clustersd 142-153CX2CX2CX3CP
197-208CX2CX2CX3CP
FrdC[4Fe-4S] clustere 21-58[C/H]X2-3CX3CX27-34CX[K/R]
FrdDNAD-binding motiff 165-191GX1-2GX[I, V, L] GXapprox 20[D/E]
FrdEFour [4Fe-4S]clustersg 14-24CX2CX2CX3C
59-71CX2CX4CX3C
91-101CX2CX2CX3C
118-137CX2CX11-15CX3C
  • a Conserved in all FRD/SDH catalytic subunits (11).

  • b Conserved in the membrane-bound FRD/SDH A subunits (6) (Fig. 3).

  • c Conserved in the membrane-bound FRD/SDH B subunits (1).

  • d Found in the B subunits of some archaeal SDHs. Bacterial and other archaeal FRD/SDH B subunits contain one [3Fe-4S] and one [4Fe-4S] cluster instead of two [4Fe-4S] clusters (1, 44).

  • e Conserved in the catalytic subunits of molybdenum enzymes and NuoG (42).

  • f Conserved in various NAD-binding proteins (9).

  • g Conserved in the iron-sulfur subunits of molybdenum enzymes and hydrogenases (27).