So why do proteins have a net charge? So, remember, proteins are composed of 20 different types of amino acids, and these different types of amino acids differ from one another based on their side chain group. Now, some of these side chain groups have a positive charge, such as Lysine and and arginine, while others have a neutral charge, for example, glycine and valine and so forth. And yet other amino acids have a full negative charge, for example, glutamate and aspartate. So we have different types of amino acids. And because proteins are composed of a variation of these different amino acids, the ones that have more side chain groups that are positively charged end up having a net positive charge.