Proteins are organic molecules composed of carbon, hydrogen, oxygen, and nitrogen/7%3A_Amino_Acids_Proteins_and_Enzymes). These elements are combined in a specific conformation around a carbon center to form amino acids, which are the building blocks of proteins/7%3A_Amino_Acids_Proteins_and_Enzymes). There are 20 different types of naturally occurring amino acids that are combined in unique sequences to form polypeptides, which then fold into a three-dimensional shape to form a protein/7%3A_Amino_Acids_Proteins_and_Enzymes). The amino acids differ from each other in their side chains, which can be polar, nonpolar, charged, or uncharged, and this chemistry is critical to protein structure because the side chains can bond with one another to hold a length of protein in a certain shape or conformation.