Table 1.

Putative heme transporters and heme binding proteins.

Protein	Proposed function	Localization	Heme binding affinity	Blood disorders	Reference
HRG-1 (SLC48A1)	Import heme	PM, Endosome/ Lysosome		Inherited iron disorders?	[12]
PCFT/HCP1	Import heme?	PM	Km = 125 µM (heme) Km = 1.3–56 µM (folate)	Hereditary familial folate malabsorption, folate-responsive anemia in newborns	[18,19]
FLVCR2	Import heme?	PM		Fowler syndrome, lethal cerebral vasculopathy	[22,23]
FLVCRla	Export heme	PM		Posterior column ataxia and retinitis pigmentosa?	[26]
FLVCRlb	Export heme	IM		Diamond Blackfan anemia?	[34]
ABCG2/BCRP	Export heme	PM		Increased serum urate level, resulting in gout	[41]
ABCB6	Transport coproporphyrinoge n III, heme?	OM? PM, Endosome/ Lysosome		Familial Pseudohyperkalemia, Ocular coloboma	[45,49]
ABCB10	Stabilize Mfrnl, transport heme?	IM		KO mice displayed deficiency of primitive erythropoiesis	[51,52]
p22HBP	Bind heme/tetrapyrroles	Cytosol	Kd = 26 nM		[56]
HBP23	Bind heme/tetrapyrroles	Cytosol	Kd = 55 nM		[55]
GST	Bind heme/tetrapyrroles	Cytosol	Kd = 100–200 nM		[66]
FABP	Bind heme/tetrapyrroles	Cytosol	Kd = 150 nM		[67]
GAPDH	Bind heme, heme insertion into iNOS	Cytosol	Kd = 24 nM		[58,59]
HSP90	Heme insertion into NOS, sGC	Cytosol			[60,61]
Hemopexin	Bind extracellular heme	Blood	Kd < 1 pM		[63]
Haptoglobin	Bind extracellular hemoglobin	Blood	Kd < 1 pM		[63]
Human serum albumin	Bind extracellular heme	Blood	Kd = 5 nM		[68]
HRG-3	Bind heme	Extracellular in C. elegans			[65]

^*PM: plasma membrane, OM: mitochondrial outer membrane, IM: mitochondrial inner membrane, KO: knockout.