Proteins are molecules consisting of a linked chain of subunits called amino acids. Twenty different amino acids occur naturally. Different proteins (arising from different genes) will have different amino acid sequences. Each amino acid has a portion called a "side chain" that is unique in chemical composition and properties. The side chain can extend into the aqueous medium surrounding the protein. Some amino acids (glutamate, aspartate) have acidic side chains and so give up a H+ ion at physiological pH and leave the side chain with a "-" charge. Other amino acids (such as arginine and lysine) have basic side chains that tend to take up a H+ ion from the surrounding water and this confers a "+" charge on these units. Some amino acids (glycine, serine) do not interact with H+ ions at all. Thus, the net charge (sum of + and - charges) on any protein at a given pH is strictly a function of its amino acid composition. The pH that causes the net charge to be zero is called the isoelectric point and this parameter is measurable in the laboratory and can vary widely among different proteins.
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