Surfactant proteins (SP) are essential for proper lung function, with four main types each with a different function. SP-A transports proteins and strengthens the immune system, while SP-B produces tubular myelin. SP-C helps lipids move and promotes multilayered structures, and SP-D maintains phospholipid balance and forms immune system cells. Each type is sensitive to environmental conditions.
Several protein complexes that are crucial for the lungs to function properly are known as surfactant protein (SP). A surfactant protein binds to cell surfaces and helps relieve surface tension on the lungs, allowing for expansion and contraction. There are four main types of these compounds, each with a different function.
These specific proteins change the cell surface properties. The exact function of these complexes depends on the type of protein, as there are four main types in the body. These complexes are sensitive to concentration, pH and other environmental conditions.
Surfactant proteins A and D, known as SP-A and SP-D, are hydrophilic, as they attract water. SP-B and SP-C are hydrophobic and dislike water. The effects of each of these surfactant proteins contain significant similarities. Researchers have found it difficult to assign each one a specific role or function.
SP-A was the first surfactant protein to be discovered. It is the most abundant of the four complexes found in the body. There are two different forms depending on the presence of calcium ions. If calcium is present, the six trimers that make up the complex remain in closed form, otherwise SP-A is present in open form.
SP-A’s function is to help transport other proteins and strengthen the immune system. In particular, it is important for the production of tubular myelin. Tubular myelin is a protein vital to lung cells and allows lung volume to change during respiration. The more surfactants, the lower the surface tension on the lungs, allowing them to inflate and deflate more easily.
SP-B is required to produce tubular myelin. It is important for life, as babies born without this compound die soon after birth. SP-B makes it difficult for large areas to collapse by keeping the layer fluid and able to move.
Of the four types of complexes, SP-C is the smallest. It is also the second most abundant SP found in the body. This is found only in the lungs and is the most hydrophobic of the four. The main functions of this SP-C are to help lipids move and promote compounds to stack into multilayered structures.
Protein-D surfactant is the largest of all surfactants. Unlike the other complexes it is not found at the air-water interface of the lungs, but in type II cells which produce other compounds that reduce tension. A type II cell is an alveolar cell found in the lungs. SP-D is required to maintain proper phospholipid balance and the formation of cells used by the immune system, such as macrophages.
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