AbstractProton (H+) conductivity is important in many natural phenomena including oxidative phosphorylation in mitochondria and archea, uncoupling membrane potentials by the antibiotic Gramicidin, and proton actuated bioluminescence in dinoflagellate. In all of these phenomena, the conduction of H+ occurs along chains of hydrogen bonds between water and hydrophilic residues. These chains of hydrogen bonds are also present in many hydrated biopolymers and macromolecule including collagen, keratin, chitosan, and various proteins such as reflectin. All of these materials are also proton conductors. Recently, our group has discovered that the jelly found in the Ampullae of Lorenzini-shark’s electrosensing organs- is the highest naturally occurring proton conducting substance. The jelly has a complex composition, but we attributed the conductivity to the glycosaminoglycan keratan sulfate (KS). Here, we have measured the proton conductivity of hydrated keratan sulfate using PdHx contacts to be 0.50 ± 0.11 mS cm -1- consistent to that of Ampullae of Lorenzini jelly, 2 ± 1 mS cm -1. Proton conductivity, albeit with lower values, is also shared by other glycosaminoglycans with similar chemical structures including dermatan sulfate, chondroitin sulfate A, heparan sulfate, and hyaluronic acid. This observation confirms the structure property relationship between proton conductivity and the chemical structure of biopolymers.