Harpers

LIPIDS OF PHYSIOLOGIC SIGNIFICANCE / 117SphingosineCeramideOHOCH (CH2 ) 3 12 CH CH CH CH N H C CH(OH) (CH 2 ) 21 CH 3CH2 OH OFatty acid(eg, cerebronic acid)GalactoseHOHOCH 2Figure 14–12. Structure of galactosylceramide(galactocerebroside, R = H), and sulfogalactosylceramide(a sulfatide, R = SO 2− 4 ).HOR3HHOHHSTEROIDS PLAY MANYPHYSIOLOGICALLY IMPORTANT ROLESCholesterol is probably the best known steroid becauseof its association with atherosclerosis. However, biochemicallyit is also of significance because it is the precursorof a large number of equally important steroidsthat include the bile acids, adrenocortical hormones,sex hormones, D vitamins, cardiac glycosides, sitosterolsof the plant kingdom, and some alkaloids.All of the steroids have a similar cyclic nucleus resemblingphenanthrene (rings A, B, and C) to which acyclopentane ring (D) is attached. The carbon positionson the steroid nucleus are numbered as shown in Figure14–14. It is important to realize that in structural formulasof steroids, a simple hexagonal ring denotes acompletely saturated six-carbon ring with all valencessatisfied by hydrogen bonds unless shown otherwise; ie,it is not a benzene ring. All double bonds are shown assuch. Methyl side chains are shown as single bonds unattachedat the farther (methyl) end. These occur typicallyat positions 10 and 13 (constituting C atoms 19and 18). A side chain at position 17 is usual (as in cholesterol).If the compound has one or more hydroxylgroups and no carbonyl or carboxyl groups, it is asterol, and the name terminates in -ol.Because of Asymmetry in the SteroidMolecule, Many StereoisomersAre PossibleEach of the six-carbon rings of the steroid nucleus is capableof existing in the three-dimensional conformationeither of a “chair” or a “boat” (Figure 14–15). In naturallyoccurring steroids, virtually all the rings are in the“chair” form, which is the more stable conformation.With respect to each other, the rings can be either cis ortrans (Figure 14–16). The junction between the A andB rings can be cis or trans in naturally occurringsteroids. That between B and C is trans, as is usually theC/D junction. Bonds attaching substituent groupsabove the plane of the rings (β bonds) are shown withbold solid lines, whereas those bonds attaching groupsbelow (α bonds) are indicated with broken lines. The Aring of a 5α steroid is always trans to the B ring,whereas it is cis in a 5β steroid. The methyl groups attachedto C 10 and C 13 are invariably in the β configuration.Ceramide Glucose Galactose N-Acetylgalactosamine(Acylsphingosine)NeuAcGalactoseorCer Glc Gal GalNAc GalNeuAcFigure 14–13. G M1 ganglioside, a monosialoganglioside,the receptor in human intestine for cholera toxin.Figure 14–14.231812 171113 1619C D1 9151410 8A B754 6The steroid nucleus.