Surfactant protein B: disulfide bridges, structural properties, and kringle similarities.
The disulfide bridges in porcine hydrophobic surfactant protein B (SP-B) were determined. Results show that three intrachain bridges link half-cystine residues 8 and 77, 11 and 71, and 35 and 46, respectively. This gives SP-B an appearance of three loops, a central big loop surrounded by two smaller ones. In the major form of SP-B, the remaining half-cystine, Cys-48, is probably interchain-linked to its counterpart in another molecule, compatible with the existence of dimeric molecules. A minor fraction, with monomeric SP-B but also lacking free thiols, could be due to polypeptides having Cys-57 (instead of Leu in the major form) and hence an additional intrachain bond (Cys-48-Cys-57). Notably, one of the three intrachain bonds common to all SP-B molecules is analogous to one of the disulfide linkages in the kringle structure of complex serine proteases. SP-B and kringles are also similar in size and in positions of half-cystine residues. SP-B and the kringle of coagulation factor XII exhibit 26% residue identity. This structural similarity of SP-B to a binding domain could reflect functional homology, compatible with the notion that SP-B interacts with surfactant anionic phospholipids, which is also in agreement with an SP-B excess of basic residues. Finally, weak similarities between the perform of SP-B and complex serine proteases are also found. This has implications on further possible relationships between kringles, serine proteases, and antiproteases.
Johansson
J
J
Department of Chemistry I, Karolinska Institutet, Stockholm, Sweden.
Curstedt
T
T
Jörnvall
H
H
D000595:Amino Acid Sequence; D000818:Animals; D003432:Cross-Linking Reagents; D004220:Disulfides; D006801:Humans; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000737:chemistry* / Q000502:physiology; D011487:Protein Conformation; D011510:Proteolipids / Q000737:chemistry* / Q000502:physiology; D011663:Pulmonary Surfactants / Q000737:chemistry* / Q000502:physiology; D012689:Sequence Homology, Nucleic Acid; D013329:Structure-Activity Relationship; D013552:Swine
D003432:Cross-Linking Reagents; D004220:Disulfides; D010446:Peptide Fragments; D011510:Proteolipids; D011663:Pulmonary Surfactants