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. Author manuscript; available in PMC: 2019 Nov 13.
Published in final edited form as: Annu Rev Physiol. 2018 Nov 28;81:403–428. doi: 10.1146/annurev-physiol-020518-114640

Figure 3.

Figure 3

Mechanisms for generating membrane and surfactant lipids in type II alveolar epithelial cells. These cells can engage in the de novo synthesis of lipids by utilizing pyruvate from glucose, or citrate from the TCA cycle. Mitochondrial citrate is mobilized to the cytoplasm and converted to acetyl-CoA by the enzyme ATP-citrate lyase. Acetyl-CoA undergoes carboxylation to malonyl-CoA by the enzyme acetyl-CoA carboxylase and subsequent steps require repetition of a 4-step reaction known as condensation, reduction, dehydration, and reduction. Fatty acyl chains can then undergo elongation and/or desaturation before being added to glycerol molecules to synthesize membrane and surfactant phospholipids. Fatty acyl chains for phospholipids can also be acquired from dietary sources or from the extracellular surfactant lipid pool. These diverse mechanisms are believed to explain how alveolar epithelial type II cells can maintain precise control over their surfactant lipid pool under a wide range of stress conditions. Abbreviations: ACLY, ATP-citrate lyase; FAS, fatty acid synthesis; TCA, tricarboxylic acid.