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| Globular proteins have a molecular structure that has the appearance of a glob whose 3D structure is anywhere from a sphere to a cigar. Usually the structure of a globular protein is divided into three or four levels. The primary structure is simply the sequence of amino acids forming the peptide chain. The peptide chain is folded in a repetitive fashion, and these structures with repetitive conformations are called [[Secondary_structure|secondary structures]]. Common examples of secondary structures are [[Helices_in_Proteins|α-helix]] and β-sheets. The tertiary structure is the overall 3D structure of a protein molecule and is produced by folding the secondary structures upon themselves, and in the process the sections of the peptide that were not involved in secondary structures form turns (tight loops) and loops. Some globular proteins have a quaternary structure, and it is formed when two or more globular protein molecules (monomer) join together and form a multimeric unit. One way of characterizing globular proteins is by the number of layers of backbones the tertiary structures contain. A convenient way of classifying globular proteins is to categorize them according to the type and arrangement of secondary structures that are present and the intramolecular forces that are produced by these arrangements. A study of these characteristics and classes using specific examples will be presented in this page.
| | #REDIRECT [[Globular Proteins]] |
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| == Layers of Backbone Present in the Structure ==
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| Layers of backbone in the core of the structure is a feature that many, but not all, globular proteins have. The number of layers and their composition vary for different proteins, but in all case the hydrophobic forces between the layers play a major role in maintaining the tertiary structure.
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| <table width='500' align='right' cellpadding='5'><tr><td rowspan='2'> </td><td bgcolor='#eeeeee'><applet load='1a7v' size='490' frame='true' align='right' scene ='Classes_of_globular_proteins/Two_layers/2' caption='' /></td></tr><tr><td bgcolor='#eeeeee'><center>'''Tertiary Structures of Examples'''<scene name='Classes_of_globular_proteins/Two_layers/2'> (Initial scene)</scene></center></td></tr></table>
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| === Two Layers ===
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| The ribbons representing the backbones show the two layers of α-helices. The <scene name='Classes_of_globular_proteins/Two_layers_phobic/1'>hydrophobic side chains</scene> are shown in ball and stick with one layer colored green and the other cyan. Notice that these side chains are mostly located between the layers and that few are on the exterior of the molecule. The <scene name='Classes_of_globular_proteins/Two_layers_polar/1'>polar residues</scene> are now ball & stick, and they tend to be on the surface of the molecule and not between the layers. <scene name='Classes_of_globular_proteins/Two_layers_polar_rotated/1'>Rotate structure</scene> so that axis of helix aligns with z-axis.
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