Binding of calcium to SP-A, a surfactant-associated protein

• Published in: Biochemistry (Easton) Vol. 29; no. 38; pp. 8894 - 8900
• Main Authors: Haagsman, Henk P, Sargeant, Tania, Hauschka, Peter V, Benson, Bradley J, Hawgood, Samuel
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 25.09.1990
• Subjects: Analytical, structural and metabolic biochemistry; Animals; Anti-Infective Agents - chemistry; Anti-Infective Agents - metabolism; Binding and carrier proteins; Binding Sites; Biological and medical sciences; Bronchoalveolar Lavage Fluid - chemistry; calcium; Calcium - metabolism; Chymotrypsin; Dogs; Fluorescence; Fundamental and applied biological sciences. Psychology; Glycoproteins - chemistry; Glycoproteins - metabolism; Humans; Hydrolysis; Protein Conformation; Proteins; Proteolipids - chemistry; Proteolipids - metabolism; Pulmonary Surfactant-Associated Protein A; Pulmonary Surfactant-Associated Proteins; Pulmonary Surfactants - chemistry; Pulmonary Surfactants - metabolism; SP-A protein; Structure-Activity Relationship; Surface Properties; surfactant protein; Human; Fissipedia; Carnivora; Temperature; Purification; Calcium; Lung; Binding site; Aggregation; Proteins; Vertebrata; Mammalia; Binding capacity; Activity concentration relation; Pulmonary surfactant; Molecular association; Enzymatic digestion; Scatchard plot; Dog
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00490a003

SP-A is a lung-specific pulmonary surfactant-associated protein containing a calcium-dependent carbohydrate recognition domain and collagen-like sequence. The protein is a major component of the extracellular form of surfactant known as tubular myelin. SP-A is thought to influence the surface properties of surfactant lipids and regulate the turnover of extracellular surfactant through interaction with a specific cell-surface receptor. These properties of SP-A are dependent on the presence of calcium. We have estimated calcium binding parameters for SP-A from binding data obtained by equilibrium dialysis and gel permeation chromatography. Our results suggest that each SP-A monomer binds two to three calcium ions in conditions chosen as similar to those found in the alveolar lumen. The binding data are best fit to a model incorporating two calcium binding sites with different affinities. Studies with a fragment of SP-A generated by limited proteolysis suggest the higher affinity site for calcium is located in the noncollagenous carboxy-terminal end of SP-A. This region of SP-A contains a carbohydrate recognition domain homologous to other C-type lectins. The binding of calcium to this region of SP-A causes a conformational change as assessed by a small change in the intrinsic fluorescence spectrum and a marked change in the susceptibility to proteolysis. At physiological calcium concentrations, intact SP-A aggregates in a reversible fashion, a property that may be relevant to the formation of tubular myelin.