Glycosaminoglycans are polysaccharides that are polymeric chains consisting of a disaccharide (two sugar molecules) monomer that repeats itself. If the glycosaminoglycans have negatively charged groups in them, then any proteins that bind to them are likely to have many positively charged amino acid side chains present. The positive electrostatic interaction between negative and positive charges is what is important here. Positively charged amino acids include those with protonated amines in their side chains, thus giving them a positive charge. These types of amino acids include arginine (protonated guanidine group), histidine (protonated imidazole ring), and lysine (protonated amino group). Their specific structures can be found in illustrations in the links below.