Protein molecules can take various spatial forms, which are four levels of their structural organization. 1) A chain of many amino acid residues connected by peptide bonds is the primary structure of a protein molecule. Based on the primary structure, other types of structures are created. 2) The secondary structure of the protein occurs as a result of the formation of hydrogen bonds between the hydrogen atoms of the NH groups and the oxygen atoms of the CO groups of different amino acid residues of the polypeptide chain. The polypeptide chain is twisted into a spiral. Hydrogen bonds are weak, but due to a significant amount they ensure the stability of this structure. 3) The tertiary structure is formed due to the formation of hydrogen, ionic and other bonds arising between different groups of atoms of a protein molecule in an aqueous medium. In some proteins, S S bonds (disulfide bonds) between cysteine residues (an amino acid containing sulfur) play an important role in the formation of the tertiary structure. In this case, the polypeptide helix is packed into a kind of ball (globule) in such a way that hydrophobic amino acid radicals are immersed inside the globule, and hydrophilic are located on the surface and interact with water molecules. 4) The composition of the molecules of some proteins includes not one, but several polypeptides (globules) that form a single complex. This forms the Quaternary structure.
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