Molecular Structures and Interactions of Pulmonary Surfactant Components

The dominating functional property of pulmonary surfactant is to reduce the surface tension at the alveolar air/liquid interface, and thereby prevent the lungs from collapsing at the end of expiration. In addition, the system exhibits host‐defense properties. Insufficient amounts of pulmonary surfactant in premature infants causes respiratory distress syndrome, a serious threat which nowadays can be effectively treated by airway instillation of surfactant preparations. Surfactant is a mixture of many molecular species, mainly phospholipids and specific proteins, surfactant protein A (SP‐A), SP‐B, SP‐C and SP‐D. SP‐A and SP‐D are water‐soluble and belong to the collectins, a family of large multimeric proteins which structurally exhibit collagenous/lectin hybrid properties and functionally are Ca2+‐dependent carbohydrate binding proteins involved in innate host‐defence functions. SP‐A and SP‐D also bind lipids and SP‐A is involved in organization of alveolar surfactant phospholipids. SP‐B belongs to another family of proteins, which includes also lipid‐interacting polypeptides with antibacterial and lytic properties. SP‐B is a 17.4‐kDa homodimer and each subunit contains three intra