Heat and Denaturing Proteins

[theyellowking]

So when I read EK bio, it says that heating causes all bonds to be broken (More specifically, a table is given: urea disrupts H-Bonds, salt or change in pH disrupts electrostatic bonds, mercaptoethanol disrupts disulfide bonds, organic solvents disrupt hydrophobic forces, and heat disrupts "all forces").

Fast forward to one of the questions in the EK exam, and the question asks:

"Which of the following bonds in a protein is likely to be LEAST stable in the presence of heat?
A. A disulfide bond
B. A Hydrogen bond
C. A polypeptide bond
D. The double bond of a carbonyl"

After getting the question incorrect and jogging my chemistry knowledge, I realized that even by accepting the fact that heat disrupts "all forces," the answer to the question should have been Hydrogen Bonding since it is the weakest out of the four. However, my question is, does heat disrupt all forces? Is that too broad of a term? Looking this up, I found that it disrupts H-Bonds and nonpolar, hydrophobic bonds, but there was no mention of any other forces.

TL;DR Does heat disrupt just H-Bonds + NP Hydrophobic interactions? Or does it include a broader group of forces?

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[Teleologist]

Heat does disrupt all forces.

 

[Vybe]

What did the answer come in the book yellowking?

 

[theyellowking]

It says the following: "The hydrogen bonds and other non-covalent interactions in the secondary and tertiary structures are disrupted. Another way to answer this question is to notice that all the bonds are covalent except hydrogen bond. Although hydrogen bonds are the strongest type of intermolecular bond, they are much weaker than covalent bonds, and will be disrupted before covalent bonds when heat is applied"

 

[Czarcasm]

So when I read EK bio, it says that heating causes all bonds to be broken (More specifically, a table is given: urea disrupts H-Bonds, salt or change in pH disrupts electrostatic bonds, mercaptoethanol disrupts disulfide bonds, organic solvents disrupt hydrophobic forces, and heat disrupts "all forces").

Fast forward to one of the questions in the EK exam, and the question asks:

"Which of the following bonds in a protein is likely to be LEAST stable in the presence of heat?
A. A disulfide bond
B. A Hydrogen bond
C. A polypeptide bond
D. The double bond of a carbonyl"

After getting the question incorrect and jogging my chemistry knowledge, I realized that even by accepting the fact that heat disrupts "all forces," the answer to the question should have been Hydrogen Bonding since it is the weakest out of the four. However, my question is, does heat disrupt all forces? Is that too broad of a term? Looking this up, I found that it disrupts H-Bonds and nonpolar, hydrophobic bonds, but there was no mention of any other forces.

TL;DR Does heat disrupt just H-Bonds + NP Hydrophobic interactions? Or does it include a broader group of forces?

It's important to not over think it. What A, C, and D all have in common is that they are covalent bonds. If you recall, covalent bonds are very difficult to break. Generally, when we boil something, it's the intermolecular forces (hydrogen bonding, dipole-dipole, van der waals forces), that are being disrupted because the heat input is generally sufficient to break these types of bonds. The folding of a protein in solution is predominately due to its hydrophobic nature, however, hydrogen bonding plays an important role in the folding process. In fact, there's some experiments where a protein will not fold correctly in the absence of hydrogen bonding. For instance, disulfide bonds are in large part directed by hydrogen bonding. But all that's really irrelevant to answering this question, especially once you make the distinction between the strength of bonds being broken.

As far as the other factors you mentioned: Various proteins function at optimal pH's and this is because of the individual R groups for each amino acid. Some enzymes for instance, have a functional active site when a certain residue is protonated and because amino acids act like acids/bases - increasing or decreasing pH will alter the protonated species within the aa. Mercaptoethanol is a reducing agent that disrupts the S-S cysteine bridge. Urea and a few other related compounds effect the solubility of the protein in water, which effects the ability of the protein to fold into a certain shape. There's more to it, but most books just say it denatures the protein, which is fine.