What Is a Surfactant Protein?

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  • Written By: Liz Thomas
  • Edited By: Shereen Skola
  • Last Modified Date: 07 October 2019
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Several protein complexes that are crucial for the lungs to work properly are known as a surfactant protein (SP). A surfactant protein binds to cell surfaces and helps relieve surface tension on the lungs, allowing for expansion and contraction to occur. There are four main types of these compounds, each one having a different function.

These specific proteins change the properties of the cell surface. 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 protein A and D, known as SP-A and SP-D are hydrophilic, as they attract water. SP-B and SP-C are hydrophobic and do not like water. The effects of each of these surfactant proteins contain significant similarities. Researchers have found it difficult to assign a specific role or function to each one.

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 a closed form, otherwise SP-A is present in the open form.


The function of SP-A is to help transport other proteins and boost the immune system. In particular, it is important for tubular myelin production. Tubular myelin is a protein that is crucial to lung cells and allows the lung volume to change when breathing. The more surfactants, the less the surface tension on the lungs, allowing them to inflate and deflate more easily.

SP-B is necessary 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 only found in the lungs and is the most hydrophobic of the four. The main functions of this SP-C is to help lipids move and promote compounds to stack into multi layer structures.

Surfactant protein-D is the largest of all the surfactants. Unlike the other complexes it is not found at the air water interface of the lungs, but in Type II cells that produce other tension reducing compounds. A Type II cell is an alveolar cell found in the lungs. SP-D is necessary to maintain the proper balance of phospholipids and formation of cells used by the immune system, such as macrophages.


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