The isoelectric point is the point at which the net charge of the protein is zero. Its nomenclature - pI. The overall charge on the molecule is affected by pH of the surrounding environment and can become more positively or negatively charged due to the gain or loss, respectively, of protons (H+). The determination of pI can be applied for studying protein heterogeneity, analysis of protein folding status, prediction of protein-protein interactions.
Isoelectric point is also important for immobilized enzymes. During its immobilization on the carrier, the buffer should have a pH value favoring electrostatic interactions with the carrier surface. Since the silica nanoparticles are negatively charged, the enzyme should be positively charged in the coupling pH conditions.
Nearly all proteins can function as buffers for maintaining pH in human systems.The charged regions of proteins can bind hydrogen and hydroxyl ions, and thus function as buffers. For example, positively charged amino acid Histidine is found in high concentrations in hemoglobin. At a physiological pH of around 7, the Henderson-Hasselbalch equation can be used to give a ratio of deprotonation/protonation of the imidazole side chain (pKa = 6). As it turns out, the histidine side chain is approximately 10% protonated at a neutral pH. That is not a negligible amount and it gives the histidine residue a certain amount of buffering capacity. The basic nitrogen activates imidazole sites as a nucleophile.During the conversion of CO2 into bicarbonate, hydrogen ions liberated in the reaction are buffered by hemoglobin, which is reduced by the dissociation of oxygen. As a result, it maintains the optimal blood pH.
 M. Rita Correro, Sabine Sykora, Philippe F.-X.Corvini, Patrick Shahgaldian, Chapter Four - Enzyme Armoring by an Organosilica Layer: Synthesis and Characterization of Hybrid Organic/Inorganic Nanobiocatalysts, Methods in Enzymology Volume 590, 2017, Pages 77-91