Bile salts

Structure and synthesis

Bile salts constitute a large family of molecules, composed of a steroid structure with four rings, a five- or eight-carbon side-chain terminating in a carboxylic acid, and several hydroxyl groups, the number and orientation of which is different among the specific bile salts. The four rings are labeled A, B, C, and D, from the farthest to the closest to the side chain with the carboxyl group. The D-ring is smaller by one carbon than the other three. The structure is commonly drawn with A at the left and D at the right. The hydroxyl groups can be in either of two configurations: either up (or out), termed beta (β; often drawn by convention as a solid line), or down, termed alpha (α; displayed as a dashed line). All bile acids have a 3-hydroxyl group, derived from the parent molecule, cholesterol, in which the 3-hydroxyl is beta.