Glycerol phosphate is a major precursor in the synthesis of triacylglycerides and related compounds. These are the major lipid components of membranes. Here's a simplified pathway to show the importance of the glycerol backbone. ("R" stands for long-chain fatty acids.) I've deliberately avoided naming the glycerol-phosphate precursor because it requires a bit of thought.
Look at that glycerol phosphate molecule. It's drawn as a Fischer projection so it's unambiguous with respect to the other stereoisomer. However, there's still the possibility of ambiguity with respect to the number of the carbon atoms. I can illustrate this by showing you the four possible configurations and conformations (left).
This can be quite confusing when you have to describe the reactions of lipid metabolism and name the various intermediates. Are they going to be 3-phosphate intermediates or 1-phosphate intermediates? We need a new rule to establish the proper numbering for the carbon atoms so everyone is on the same page.
You start with a Fischer projection where the hydroxyl group on C2 is to the left. The new stereochemical numbering (sn) system specifies that the "top" carbon atom becomes C1 and the bottom one is carbon atom number 3. Thus, L-glycerol 3-phosphate becomes sn-glycerol 3-phosphate to indicate that you are using the stereochemical numbering system. (The nomenclature for glycerol is specific for that molecule. The stereospecific nomenclature for other compounds is based on the RS rules of naming stereospecific compounds and it's much more complicated.)
Is this important for students in an introductory biochemistry course? No. On a scale of 1 to 10, where 1 is the lowest, I'd rate this at -3. Students do not need to know the precise nomenclature of lipid molecules and they do not need to know stereospecific nomenclature for that reason (but see below). This also applies to other molecules, particularly citrate, which also is numbered according to the stereochemical numbering system.
Not everyone agrees. Vasel Mezl wrote an article on the stereochemical numbering system for Biochemical Education back in 1996.1 He said,
The stereospecific use of the prochiral ends of glycerol and of citrate is a key concept that is presented in most biochemical texts, yet a nomenclature to designate those ends is usually not given. In fact, few textbooks present structures in a manner that is consistent with stereochemical priorities; among 30 basic biochemistry textbooks that were examined (including7-12), only three textbooks7-9 consistently orient the structure of the key prochiral intermediate, citrate, going from pro-R to pro-S in keeping with the Hirschmann and Hansen proposal6 and only one textbook7 correctly explain the sn system.
I didn't write that part of the textbook back in 1989 but I'm proud of the fact that we were the only book to correctly explain the nomenclature. However, by the time that Mezl wrote his article in 1996, we had already published the second edition of the big book (Moran, Scrimgeour et al. (1994)) and the first edition of the smaller Principles of Biochemistry book (Horton et al. (1993)). There was no mention of the numbering conventions for glycerol phosphate and citrate in those books. It was information that introductory biochemistry students did not need to know.
Undergraduates will be delighted to learn that the complicated nomenclature rules for citrate have been kept out of the most recent edition of Principles of Biochemistry and most (all?) other introductory biochemistry textbooks. However, I've restored an explanation of sn-glycerol and why that numbering system is important. There's a good reason for this; it's because of all the talk about the different kinds of lipids in archaebacteria (Archaea) and what that tells us about the origin of life.
You may be wondering why anyone should care about the triglycerides in archaebacteria. Well, you certainly won't care if your biochemistry course is all about human physiology and rat liver biochemistry! But if your course is based on an evolutionary/comparative approach to biochemistry then the difference is important because it suggests a fundamental distinction between two classes of bacteria. It means that you can't really tell which kinds of triglycerides were present in the last universal common ancestor (LUCA). It might even suggest that membranes and their lipids arose independently in the two lineages. To some, this means that lipids and membranes were late-comers in the evolution of life (e.g. Koonin, 2012).
1. In the interest of full disclosure, the journal Biochemical Education morphed into Biochemistry and Molecular Biology Education (BAMBED). I am a member of the editorial board of that journal.
Englard, S. and Hansen, K.R. (1969) Stereospecifically labelled citric acid cycle intermediates, Methd. Enzymol. 13:567-598.
Koonin, E.V. (2012) The logic of Chance: The nature and Origin of Biological Evolution. FT Press Science, Pearson Education Inc., Upper Saddle River, New Jersey, USA.
Mezl, V.A. (1996) Straightening out the stereochemical numbering sustem. Biochemical Ediucation 21:29-30. [Straightening out the stereochemical numbering sustem] [doi: 10.1016/0307-4412(95)00159-X]