|
In the peptides of this class side-by-side strands of a β-sheet run in opposite directions. One side of the sheet will have polar side chains projecting from it, and the other side will have hydrophobic chains. The polar side of the sheet can be on the surface of the molecule, but the hydrophobic side must be covered with another nonpolar layer of side chains. Peptides in this class can have as few as two layers of backbone. The examples given below show the hydrophobic layer being covered in different ways. One of these methods result in a barrel or cylindrical shape. The barrel formed by the parallel β-sheet is more symmetrical, but the antiparallel β-barrel is more common. Another way is to cover the hydrophobic sheet with a layer of backbone made up of α-helices and extended loops. The third way is to form two interlocking antiparallel β-sheets. Interlocking means that after laying down an antiparallel strand in one sheet the peptide chain loops over and lays down an antiparallel strand in the opposing sheet. Made 2007 by KMO |
|
Antiparallel β-Sheet
Soybean trypsin inhibitor - As its name implies this protein inhibits the enzyme trypsin, and this inhibitory effect must be deactivated in the process of preparing soybeans for use in animal feed, so that the proteins in soybeans are hydrolyzed by trypsin. β-Barrel: Not as clearly defined as the parallel β-barrel, but you can look through the barrel whenever one of the open ends rotates to face the screen. Notice that the outer layer of α-helices is not present like it is in the parallel β-barrel. Made 2007 by KMO |
|
Antiparallel β-Sheet
Rubredoxin - redoxins are sufhydryl containing proteins that participate in redox reactions. β-Barrel: Barrel is quite poorly defined in this case and some α-helices help in covering the hydrophobic side of the sheet. Made 2007 by KMO |
|
Antiparallel β-Sheet
Subtilisin inhibitor - subtilisin is a bacterial protease, and this protein is an inhibitor of this enzyme. β-Sheet covered with α-helices and loops: As the structure rotates you can see the two distinct layers, an antiparallel β-sheet and the hydrophobic side of this sheet covered with a layer made up of α-helices and loops. Made 2007 by KMO |
|
Antiparallel β-Sheet
Concanavalin - example of a lectin, plasma membrane proteins that bind oligsaccharides and glycoproteins and are involved in cell-cell recognition. Interlocking b-Sheets or Greek Key Topology: There are two antiparallel β-sheets, and the hydrophobic sides of the sheets are facing each other. Observe that after laying down a strand in a sheet, often the peptide chain loops over to the other sheet and makes a strand in that sheet. Made 2007 by KMO |
|
Antiparallel β-Sheet
γ-Crystallin - a protein that is a component of the eye lense Interlocking β-sheet: In this example two of the Greek key bilayers are connected by a looping peptide segment. Made 2007 by KMO |
|
Some of these proteins are small in size and therefore do not have large amounts of backbone that can be organized into layers. Others have layers of backbone, but the layers are not as well organized, and therefore the side chain interactions are not as strong. For the proteins in this class, the disulfide bonds or the bonds formed between metal ions and ligands are as important or more than the hydrophobic interactions of the side chains. Made 2007 by KMO |
|
Disulfide-Rich Proteins
Insulin - among its functions is the regulation of glucose uptake by cells Small peptide contains A and B chains that are connected by disulfide bonds, and the tertiary structure of the A chain is also held in place by a disulfide bond. Made 2007 by KMO |
|
Disulfide-Rich Proteins
Crambin - plant seed peptide Small single chain peptide with no significant backbone layers but has disulfide bonds. Made 2007 by KMO |
|
Disulfide-Rich Proteins
Phospholipase A2 - part of a class of hydrolases that degrade phospholipids. This one specifically hydrolyzes the second acyl group. This example is larger than the other two, but it still does not have well organized backbone layers. Made 2007 by KMO |
|
Metal-Rich Proteins
High-potential iron protein - an iron-sulfur protein that has an unusually high redox potential The Fe's of the iron-sulfur center are complexed with the side chains of cysteines which are part of different loops of the peptide. These bonding forces are important in maintaining the tertiary structure. Made 2007 by KMO |
|
Metal-Rich Proteins
Ferredoxin - protein with two iron-sulfur centers; the major function of iron-sulfur proteins is involvement in redox reaction. Both iron-sulfur centers aid in maintaining the tertiary structure. Made 2007 by KMO |
|
Reference
1. Biochemistry, 3rd ed., R. H. Garrett & C. M. Grisham, Thomson/Brooks/Cole, page 178-184. Made 2007 by KMO |