Fatty Acid Structure

A fatty acid consists of a hydrophobic hydrocarbon chain with a terminal carboxyl group that has a pKa (see p. 6) of ~4.8 (Fig. 1). At physiologic pH, the terminal carboxyl group (–COOH) ionizes, becoming –COO−. [Note: When the pH is above the pK, the deprotonated form predominates .] This anionic group has an affinity for water, giving the fatty acid its amphipathic nature (having both a hydrophilic and a hydrophobic region). However, for longchain-length fatty acids (LCFA), the hydrophobic portion is predominant. These molecules are highly water insoluble and must be transported in the circulation in association with protein. More than 90% of the fatty acids found in plasma are in the form of fatty acid esters (primarily TAG, cholesteryl esters, and phospholipids) contained in circulating lipoprotein particles . FFA are transported in the circulation in association with albumin, the most abundant protein in serum.

Figure 1:  Structure of a fatty acid.