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.