IRRAS Studies of the Host Defense Effect of Pulmonary Surfactants SP-A and SP-D

The pulmonary surfactant proteins SP-A and SP-D have been proven to play a role in host defense. SP-A binds phosphatidylcholine, galactosylceramide, and the lipid A portion of lipopolysaccharide (LPS), which is found in Gram-negative bacterial cell walls. SP-D binds phosphatidylinositol, glucosylceramide and LPS. Although X-ray crystallography and NMR spectroscopy have provided atomic level information about SP-A and SP-D structures, molecular level information about their binding to biological ligands is lacking. IR reflection-absorption spectroscopy (IRRAS) is used in the current work to study the properties of SP-A and SP-D at air/water interfaces and their interaction with biological ligands. The monolayer properties of recombinant rat neck + carbohydrate recognition domain (NCRD) SP-A and its point mutant D215A SP-A are compared. Both NCRD SP-A and D215A SP-A adsorb to the air/water interface, DPPC monolayers, and Lipid A monolayers, but D215A/PL SP-A displays higher affinity and greater stability. Measurements of surface pressure, Amide 1 intensity, and lipid acyl chain conformational ordering can help to elucidate the mechanism of interaction of SP-A and its ligands. Similar experiments have been carried out with SP-D.