Lung surfactant is a lipid- and protein-containing substance that is secreted from lung and prevents collapse of the alveoli when air is expelled. Lung surfactant contains 50-60% dipalmitoylphosphatidylcholine, a saturated glycerophospholipid. Normally, about 90% of the fatty acids at position two for glycerophospholipids are unsaturated.
How does the palmitate arise at position 2 in lung surfactant? Current evidence rules out the possibility that an unsaturated fatty acid chain at position 2 is modified in situ to a palmitoyl chain. The two remaining possibilities are (1) direct transfer of palmitoyl chains to positions 1 and 2 of glycerphosphorylcholine and (2) hydrolytic cleavage (removal) of the unsaturated chain at position 2 followed by addition of the palmitoyl chain.
One enzyme capable of removing a fatty acid from a glycerophospholipid is phospholipase A2, one of a class of four enzymes that hydrolyze specific bonds in phospholipids (Figure 19.6).
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