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One convenient way of classifying globular proteins is to categorize them according to the type and arrangement of secondary structures that are present(1). Another distinguishing characteristic to be consider is the prominate force which maintains the tertiary structure. There are five classes: Antiparallel α-helix proteins Parallel or mixed β-sheet proteins Antiparallel β-sheet proteins Metal-rich proteins Disulfide-rich proteins The first three have layers of backbone which interact to give strong hydrophobic attractions, but the last two have metallic and disulfide bonds that maintain their tertiary structures. The first two classes are available below, and the other three are available in another MolSlide. Made with MolSlides.ProteinExplorer.Org |
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The peptides in this class have a high contain of α-helix and because of the loops and turns which are present most of the α-helix strands will be antiparallel with respect to each other. The examples which follow are colored N-C rainbow so that the N-terminus and C-terminus of the α-helices can be determined. The amino end of the protein starts with Blue, and moving to the carboxyl end of the peptide the coloration proceeds through the colors of the rainbow and ends with Red. Comparing the colors which are present at the ends of a helical strand one can determine which is the N-terminus and C-terminus. Made with MolSlides.ProteinExplorer.Org |
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Antiparallel α-helix
Myohemerythrin - protein that transports oxygen in some lower animals. Notice that the change in direction produced by the loops creates the antiparallel confirmation. Made with MolSlides.ProteinExplorer.Org |
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Antiparallel α-helix
Tobacco mosaic virus protein - forms the capsid of the virus Again the α-helices, loops and turns are prominent features, and the α-helices are antiparallel. Protein Explorer did not recognize the β-sheets that are present in Figure 6.29 of (1). Made with MolSlides.ProteinExplorer.Org |
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Antiparallel α-helix
Myoglobin - stores molecular oxygen in muscle tissue. Structure is more complex, but again the striking feature is the antiparallel α-helices. Also notice that the backbone can be divided into two layers. Made with MolSlides.ProteinExplorer.Org |
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In addition to having pure parallel β-sheets, some of the proteins in this class contain a b-sheet that has one or two antiparallel strands giving a mixed β-sheet. A characteristic of parallel β-sheets is that both sides of the sheet have hydrophpbic side chains.(1) A consequence of this is that parallel or mixed β-sheets must be located in the interior of the molecule. This type of sheet can not be on the surface exposing the hydrophobic chains to water. Made with MolSlides.ProteinExplorer.Org |
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Parallel or Mixed β-Sheets
Triose phosphate isomerase The β-sheet of the barrel is parallel because after forming a strand of the sheet the peptide chain loops out, forms an α-helix and then loops back to form another strand of the sheet running in the same direction as the previous strand and, thereby, making the sheet parallel. Notice that in this case the backbone can be divided into four layers. Made with MolSlides.ProteinExplorer.Org |
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Parallel/Mixed β-Sheets
Flavodoxin This type of structure is also called doubly wound parallel β-sheet because of the loops of α-helices on both sides of the sheet. In some cases these doubly wound sheets contain a few antiparallel strands forming a mixed β-sheet. Observe that these doubly wound sheets contain three layers of backbone. Made with MolSlides.ProteinExplorer.Org |
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Parallel/Mixed β-Sheet
Phosphoglycerate mutase There is one antiparallel strand in the sheet making it a mixed β-sheet, and the double winding is more extensive. Made with MolSlides.ProteinExplorer.Org |
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Reference
1. Biochemistry, 3rd ed., R. H. Garrett & C. M. Grisham, Thomson/Brooks/Cole, page 178-184. Go to other Three Classes Made with MolSlides.ProteinExplorer.Org |