Why are there so many different kinds of proteins when there are only 20 different amino acids ?

Explanation:

Remember, the 20 kinds of amino acids differ because of their R groups. These R groups also have different chemical bonding. There are R groups that form covalent bonds, some that form ionic bonds, and still others that form hydrogen bonds.

Because amino acids can be arranged in many different combinations, it's possible for your body to make thousands of different kinds of proteins from just the same 21 amino acids. You may see books that say there are only 20 amino acids.

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