• Diiron-carboxylate proteins by function
• Diiron-carboxylate proteins by binuclear centre type
• Diiron-carboxylate proteins by type and number of prosthetic centres
• Diiron-carboxylate proteins in motif databases
• Diiron-carboxylate proteins in PROMISE
• HET groups in PDB
• Diiron-carboxylate proteins in 3­D databases
• References

• Class I (large helix­bundle proteins) includes ribonucleotide reductase R2­type proteins.
• Class II (simple helix­bundle proteins with overhead connection) includes the iron storage protein ferritin, the haem protein bacterioferritin and rubrerythrin, which is a fusion protein containing an N­terminal diiron­binding domain and C­terminal domain homologous to rubredoxin [2].
• Class III (simple helix­bundle proteins without overhead connection) includes the nonhaem dioxygen­binding proteins haemerythrin and myohaemerythrin.
• Class IV (/ß sandwich proteins) includes purple acid phosphatases (PAPs).

Diiron-carboxylate proteins by function

Function	Protein class
Catalysis Iron enzymes in ENZYME database Iron enzymes in LIGAND database	Hydrolases Purple acid phosphatase Oxidoreductases Alkene hydroxylase Methane monooxygenase Phenol hydroxylase Ribonucleotide reductase Stearoyl­acyl carrier protein 9­desaturase Toluene hydroxylase Bacterioferritin (?) Rubrerythrin (?)
Iron storage	Bacterioferritin Ferritin
Oxygen transport and storage	Haemerythrin Myohaemerythrin

Diiron-carboxylate proteins by binuclear centre type

Binuclear centre	Metal ligands		Protein class
	Me1	Me2
	NHis; 1­OAsp (1­OGlu)	NHis; 1­OGlu	Class I Class II (bacterioferritin)
	µ­1:1­OGlu
	1­OGlu; 2­OGlu	NHis; 2­OGlu	Class II (rubrerythrin)
	2 × µ­1:1­OGlu; µ­O
	NHis; 1­OGlu	1­OGlu; 2­OGlu (1­OGlu)	Class II (ferritin)
	µ­1:1­OGlu
	3 × NHis	2 × NHis	Class III
	µ­1:1­OAsp; µ­1:1­OGlu; µ­OH (µ­O)
	NHis; 2­OAsp or 2­OGlu	NHis; OTyr; OH- (H2O)	Class IV (mammalian PAP)
	µ­1:1­OAsp or µ­1:1­OGlu; µ­O (µ­OH)
	NHis; NHis; OAsn; H2O	NHis; OTyr; OAsp; OH-	Class IV (plant PAP)
	µ­OAsp; µ­O (µ­OH)

Diiron-carboxylate proteins by type and number of prosthetic centres

Iron-sulphur proteins
Simple (single binuclear centre)	Complex
Ferritins Haemerythrin family Purple acid phosphatase Ribonucleotide reductase R2­type proteins	Bacterioferritin: haem + binuclear iron centre Rubrerythrin: binuclear iron centre + Fe(Cys)4 centre

Diiron-carboxylate proteins in motif databases

PRINTS ID	PRINTS AC	PROSITE/BLOCKS ID	PROSITE AC	BLOCKS AC
BACFERRITIN	PR00601	BACTERIOFERRITIN	PS00549	BL00549
-	-	FATTY_ACID_DESATUR_2	PS00574	BL00574
-	-	FERRITIN_1 FERRITIN_2	PS00540 PS00204	BL0540
HEMERYTHRIN	PR00186	HEMERYTHRINS	PS00550	BL00550
-	-	RIBORED_SMALL	PS00368	BL00368

See also a table of diiron-carboxylate proteins in PROMISE

HET groups in PDB (at BSM)

Diiron-carboxylate proteins in 3­D databases

Class	Enzymes	Iron storage proteins	Oxygen transport and storage proteins
Class I	Ribonucleotide reductase R2­type proteins	-	-
Class II	Ferroxidase Bacterioferritin (cytochrome b1) Ferritins Rubrerythrin	Bacterioferritin (cytochrome b1) Ferritins	-
Class III	-	-	Haemerythrin family
Class IV	Purple acid phosphatase	-	-

References

1. Nordlund, P. and Eklund, H. (1995) Di­iron-carboxylate proteins. Curr. Opin. Struct. Biol. 5, 758-766.
2. van Beeumen, J.J., van Driessche, G., Liu, M.Y. and LeGall, J. (1991) The primary structure of rubrerythrin, a protein with inorganic pyrophosphatase activity from Desulfovibrio vulgaris. Comparison with hemerythrin and rubredoxin. J. Biol. Chem. 266, 20645-20653.
3. Sträter, N., Lipscomb, W.N., Klabunde, T. and Krebs, B. (1996) Two­metal ion catalysis in enzymatic acyl­ and phosphoryl­transfer reactions. Angew. Chem. Int. Ed. Engl. 35, 2025-2055.
4. Cowan, J.A. (1993) Inorganic Biochemistry: An Introduction. VCH Publishers, New York.

General references on diiron-carboxylate proteins