The other day in class I misspoke when I equated Peptidoglycan with Proteoglycan. Being similarly named, I got tongue-tied and confused them and then further confused myself when I tried to clarify. My problem at the time was that I looked to the etymology of the two words to distinguish them and came up short – this is the kind of faulty ‘on your feet thinking’ that gets teachers and politicians into trouble.
So, to correct things for the record, Peptidoglycan refers to the sugar / amino acid polymer making up the cell wall of bacteria. Peptidoglycan is synthesized in the cytoplasm as monomeric units that are then transported to the cell wall for inclusion.
Because animal cells do not contain peptidoglycan, a number of (naturally occurring) antibiotics target it, or the enzymes that operate in its synthesis. The use of peptidoglycan is a ‘clever’ way that evolution has discovered of escaping immune / enzymatic responses because it includes L-Amino Acids that are unused in protein synthesis by any organism and therefore not susceptible to any typical proteases.
The other molecule, Proteoglycan refers to a complex of protein that is ‘decorated’ with sugars, such as the ones those that are added and modified in the ER / Golgi of Eukaryotic cells. Specifically, proteoglycans are “found in all connective tissues, extracellular matrix (ECM) and on the surfaces of many cell types. Proteoglycans are remarkable for their diversity (different cores, different numbers of GAGs with various lenghts and compositions).”
A good review of the various proteoglycan structures can be found here.