How to draw a GalCer molecule

1. Start with sphingosine.

1.1. Draw a C18 chain.

1.2. Add the two OH groups on C1 and C3.

1.3. Add the NH₂ group on C2.

Add the trans double bond between C4 and C5.

2. Add the fatty acid to obtain ceramide.

As a matter of fact, ceramide is a family of closely related molecules differing by their fatty acid. In this case, we have built the ceramide with palmitic acid (saturated fatty acid with 16 carbons, i.e. C16 :0). The fatty acid linked to ceramide is now called a fatty acyl chain.

3. Add galactose to obtain GalCer.

GalCer is formed by addition of the cyclic form of D-galactose, called D-galactopyranose. The bond between galactose and ceramide is a β-glycosidic bond. ‘Glycosidic’ means that the reactive OH group of the sugar is the OH beared by the C1 ; ‘β’ means that at the time the bond is formed, this OH group is over the plane of the sugar ring. You do not have to know the structure of D-galactose ; just remember that it is the C4 epimer of D-glucose (both hexoses just differ by the configuration of C4). Galactose is linked to the C1 of ceramide, so that GalCer is abbreviated Galb1-1Cer.

4. Add the OH group in a to obtain GalCer-HFA.

The fatty acid of glycosphingolipid can bear a OH group located on the carbon in a from the carbonyl group (C=O) of the fatty acyl chain. In this case, GalCer is called α-OH GalCer or GalCer-HFA (for hydroxy fatty acid). GalCer-HFA is Thudichum’s phrenosin. Otherwise, GalCer without α-OH (i.e. Thudichum’s kerasin) is GalCer-NFA (for nonhydroxy fatty acid). The molecule you have drawn is GalCer-HFA with a palmityl α-hydroxylated chain.