Video Transcript
Which of the following best explains the role of hydrogen bonds in protein structure? (A) Hydrogen bonds form between amino acids to hold them in a polypeptide chain. (B) Hydrogen bonds form within amino acids to join the carboxyl group to the amino group. (C) Hydrogen bonds form between amino acids and water molecules to contribute to the quaternary structure of the protein. Or (D) hydrogen bonds form between amino acids to hold the protein in its secondary-structure shape, e.g., 𝛼-helix.
This question asks about the role of hydrogen bonding in protein structure. In order to answer this question correctly, let’s review the bonds found in amino acids and involved in protein structure.
Amino acids are the basic building block of the polypeptide chains that make up proteins. Proteins are made up of a combination of 20 different amino acids. They all have the same basic structure, with a central carbon atom covalently bonded to a carboxyl group one side and an amino group the other. The 20 types of amino acid come about due to different side groups, called R groups, which have different properties.
The polypeptide chains are formed when multiple amino acids join together via peptide bonds. These bonds involve, the carboxyl COOH, and amino, NH2, groups of individual amino acids. The sequence of amino acids along a polypeptide chain is referred to as the primary structure. This means we can now rule out options (A) and (B), as these do not involve hydrogen bonds.
The secondary structure of proteins refers to the 𝛼-helix and 𝛽-sheet shapes that a polypeptide chain can form. These structures are held together by hydrogen bonds. The hydrogen bonds form between the carbonyl group of one amino acid and the amine group of another. Hydrogen bonds are the only type of bond holding these structures together so are essential for the secondary structure. This indicates that option (D) is correct.
To make sure, let’s finish by looking at the tertiary and quaternary structures. The tertiary structure of a protein describes the three-dimensional arrangements of the 𝛼-helices and 𝛽-sheets. It relies on the interaction of the R groups between the various amino acids, and also with the surrounding solution.
Several types of bonds are involved in this structure, including disulfide, ionic, and hydrogen, as well as the hydrophobic and hydrophilic interactions. The hydrophobic interactions — hydro- means water, and phobic means hating — involve the nonpolar R groups, which repel water molecules, clustering together on the inside of the protein. This leaves the polar, hydrophilic R groups — -philic means loving — on the outside to interact with the surrounding water molecules, forming hydrogen bonds.
Even though these hydrogen bonds between the hydrophilic amino acids and water molecules contribute to the shape of the protein, the final shape is also controlled by the other bonds and also the hydrophobic interactions. Many proteins are only made up of one polypeptide chain, and their structure stops at tertiary level. However, some proteins are made up of two or more chains bonded together. And this forms the quaternary structure. In general, this structure is held together by the same bonds and interactions as the tertiary structure.
Now that we have discussed protein formation from primary to quaternary structure, we are able to answer our question correctly.
Our question asks, “Which of the following best explains the role of hydrogen bonds in protein structure?” Although hydrogen bonds do form between the hydrophilic amino acids and their surrounding water molecules, the overall protein structure is also controlled by the other bonds and interactions. So the proportion of the impact of the hydrogen bonds is relatively small. In the secondary structure, the shape of the 𝛼-helices and 𝛽-sheets relies totally on hydrogen bonds. Therefore, the correct answer to our question is answer choice (D).
The statement that best explains the role of hydrogen bonds in protein structure is “Hydrogen bonds form between amino acids to hold the protein in its secondary-structure shape, e.g., 𝛼-helix.”
