2. Give the structure of the peptide Gly-Cys-phen-Ala-Pro.
Is it possible to form a hydrogen bond in this fragment α-helices? Why?
3. Give the scheme of interaction in a protein molecule of spatially close lysine and aspartic acid. Specify the type of interaction.
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Answer:
Most proteins have compact, globular shapes, requiring reversals in the direction of their polypeptide chains. Many of these reversals are accomplished by a common structural element called the reverse turn (also known as the β turn or hairpin bend), illustrated in Figure 3.42. In many reverse turns, the CO group of residue i of a polypeptide is hydrogen bonded to the NH group of residue i + 3. This interaction stabilizes abrupt changes in direction of the polypeptide chain. In other cases, more elaborate structures are responsible for chain reversals. These structures are called loops or sometimes Ω loops (omega loops) to suggest their overall shape. Unlike α helices and β strands, loops do not have regular, periodic structures. Nonetheless, loop structures are often rigid and well defined (Figure 3.43). Turns and loops invariably lie on the surfaces of proteins and thus often participate in interactions between proteins and other molecules. The distribution of α helices, β strands, and turns along a protein chain is often referred to as its secondary structure.
Figure 3.42. Structure of a Reverse Turn.
Figure 3.42
Structure of a Reverse Turn. The CO group of residue i of the polypeptide chain is hydrogen bonded to the NH group of residue i + 3 to stabilize the turn.
Figure 3.43. Loops on a Protein Surface.
Figure 3.43
Loops on a Protein Surface. Image mouse.jpg A part of an antibody molecule has surface loops (shown in red) that mediate interactions with other molecules.
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Bookshelf ID: NBK22580
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The Alpha Helix Is a Coiled Structure Stabilized by Intrachain Hydrogen Bonds
Beta Sheets Are Stabilized by Hydrogen Bonding Between Polypeptide Strands
Polypeptide Chains Can Change Direction by Making Reverse Turns and Loops
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Secondary Structure: Polypeptide Chains Can Fold Into Regular Structures Such as...
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