amino acids are the monomers that join together to form

Proteins come in a wide variety of shapes, and they are generally between amino acids long. However, the final three-dimensional shape of the protein is still specified by its amino acid sequence: chaperones simply make the folding process more reliable. Chaperones are vital in the crowded conditions of the cytoplasm, since they prevent the temporarily exposed hydrophobic regions in newly synthesized protein chains from associating with each other to form protein aggregates (see p. These proteins bind to partly folded polypeptide chains and help them progress along the most energetically favorable folding pathway. This change in shape is often crucial to the function of the protein, as we see later.Īlthough a protein chain can fold into its correct conformation without outside help, protein folding in a living cell is often assisted by special proteins called molecular chaperones. However, the conformation often changes slightly when the protein interacts with other molecules in the cell. Urea is very soluble in water and unfolds proteins at high concentrations, (more.)Įach protein normally folds up into a single stable conformation. (A) This experiment demonstrates that the conformation of a protein is determined solely by its amino acid sequence. When polar amino acids are buried within the protein, they are usually hydrogen-bonded to other polar amino acids or to the polypeptide backbone ( Figure 3-7). In contrast, polar side chains-such as those belonging to arginine, glutamine, and histidine-tend to arrange themselves near the outside of the molecule, where they can form hydrogen bonds with water and with other polar molecules ( Figure 3-6). This enables them to avoid contact with the water that surrounds them inside a cell. The nonpolar (hydrophobic) side chains in a protein-belonging to such amino acids as phenylalanine, leucine, valine, and tryptophan-tend to cluster in the interior of the molecule (just as hydrophobic oil droplets coalesce in water to form one large droplet). Therefore, an important factor governing the folding of any protein is the distribution of its polar and nonpolar amino acids. As described in Chapter 2, hydrophobic molecules, including the nonpolar side chains of particular amino acids, tend to be forced together in an aqueous environment in order to minimize their disruptive effect on the hydrogen-bonded network of water molecules (see p. As in the previous figure, R is used as a general (more.)Ī fourth weak force also has a central role in determining the shape of a protein. Although a single one of these bonds is quite weak, many of them often form together to create a strong bonding arrangement, as in the example shown. Three types of noncovalent bonds that help proteins fold. The stability of each folded shape is therefore determined by the combined strength of large numbers of such noncovalent bonds ( Figure 3-5).

But many weak bonds can act in parallel to hold two regions of a polypeptide chain tightly together. Individual noncovalent bonds are 30–300 times weaker than the typical covalent bonds that create biological molecules.

The weak bonds are of three types: hydrogen bonds, ionic bonds, and van der Waals attractions, as explained in Chapter 2 (see p. These involve atoms in the polypeptide backbone, as well as atoms in the amino acid side chains. The folding of a protein chain is, however, further constrained by many different sets of weak noncovalent bonds that form between one part of the chain and another. By contrast, rotation can occur about (more.) The peptide bond is planar (gray shading) and does not permit rotation. (A) Each amino acid contributes three bonds (red) to the backbone of the chain. Steric limitations on the bond angles in a polypeptide chain.

Amino acids are the monomers that join together to form