21. Lipids and Carbohydrates   Previous PageNext Page

One disadvantage of phosphatidic acid is that it is a negative ion (anion), and needs nearby positive ions (cations) to neutralize its negative charge. This problem can be avoided, while preserving the charged polar character of the head of the molecule, if the phosphate group is esterified again with an alcohol that carries its own positive charge. Such a molecule is ethanolamine, HO-CH2CH2-NH2, which at neutral pH exists in the ionized form: HO-CH2CH2-NH3+. This second esterification produces phosphatidyl ethanolamine, or cephalin (shown here). The cephalin molecule is half hydrophobic and half charged polar, yet electrostatically neutral because it carries its counter ion with it.

Cephalin has one flaw. As a primary amine, it can lose a proton and its positive charge under basic conditions and revert to a negatively charged molecule, such as phosphatidic acid. The ingenious solution that actually is used in membranes is to substitute for ethanolamine a quaternary ammonium ion that cannot deprotonate and lose its positive charge. The main lipid component in biological membranes is phosphatidyl choline, or lecithin (right). In lecithin the phosphate group is esterified with the quaternary ammonium compound choline, HO-CH2CH2-N+(CH3)3, which is identical to ethanolamine except for the three methyl groups on the nitrogen. Lecithin cannot lose its positive charge and therefore must remain a neutral molecule overall. The actual shape of a lecithin molecule is indicated by the space-filling model shown by the right button.

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