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Linker chains of the gigantic hemoglobin of the earthworm Lumbricus terrestris : Primary structures of linkers L2, L3, and L4 and analysis of the connectivity of the disulfide bonds in linker L1
Author(s) -
Kao WenYen,
Qin Jun,
Fushitani Kenzo,
Smith Sandra S.,
Gorr Thomas A.,
Riggs Claire K.,
Knapp James E.,
Chait Brian T.,
Riggs Austen F.
Publication year - 2006
Publication title -
proteins: structure, function, and bioinformatics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.699
H-Index - 191
eISSN - 1097-0134
pISSN - 0887-3585
DOI - 10.1002/prot.20852
Subject(s) - lumbricus terrestris , linker , disulfide bond , chemistry , earthworm , hemoglobin , computer science , biochemistry , biology , ecology , operating system
The extracellular hemoglobin (Hb) of the earthworm, Lumbricus terrestris , has four major kinds of globin chains: a , b , c, and d , present in equimolar proportions, and additional non‐heme, non‐globin scaffolding chains called linkers that are required for the calcium‐dependent assembly of the full‐sized molecule. The amino acid sequences of all four of the globin chains and one of the linkers (L1) have previously been determined. The amino acid sequences via cDNA of each of the three remaining linkers, L2, L3, and L4, have been determined so that the sequences of all constituent polypeptides of the hemoglobin are now known. Each linker has a highly conserved cysteine‐rich segment of ∼ 40 residues that is homologous with the seven ligand‐binding repeats of the human low‐density lipoprotein receptor (LDLR). Analysis of linker L1 shows that the connectivity of the three disulfide bonds is exactly the same as in the LDLR ligand‐binding repeats. The presence of a calcium‐binding site comprising one glutamyl and three aspartyl residues in both the LDLR repeats and in the linkers supports the suggestion that calcium is required for the folding and disulfide connectivity of the linkers as in the LDLR repeats. Linker L2 is markedly heterogeneous and contains unusual glycine‐rich sequences near the NH 2 ‐terminus and a polar zipper‐like sequence with imperfect repeats of Asp‐Asp‐His at the carboxyl terminus. Similar Asp‐Asp‐His repeats have been found in a protein homologous to superoxide dismutase in the hemolymph of certain mussels. These repeats may function as metal‐binding sites. Proteins 2006. © 2006 Wiley‐Liss, Inc.