Under physiological conditions, peptide bond formation and degradation both require enzymes, but only formation requires coupling to GTP hydrolysis. Based on this information, peptide bonds under physiological conditions are:

Question

Under physiological conditions, peptide bond formation and degradation both require enzymes, but only formation requires coupling to GTP hydrolysis.  Based on this information, peptide bonds under physiological conditions are:

Quizplus · Accepted Answer

11ecba2d_0e6b_2462_a3bf_8197cfdc0a8f_MD0008_00 Chemical reactions, including those in biological settings, are controlled both by thermodynamics and kinetics. Thermodynamics describes the energy difference between products and reactants (ΔG), and dictates whether a reaction requires energy input. Kinetics describes the energy required to initiate a reaction (activation energy), and determines how quickly a reaction proceeds. If a reaction has a positive ΔG (as in anabolic processes such as protein synthesis), the product is thermodynamically unstable and the reaction requires energy input. In biological settings, this energy input is usually derived from the hydrolysis of ATP or GTP. The reverse reaction is thermodynamically favorable (negative ΔG) and requires no energy input; however, this does not mean the reverse reaction will proceed quickly. Reactions that are thermodynamically favorable but have high activation energies are slow, and the bonds that are broken in these reactions are said to be kinetically stable. The question states that peptide bond formation requires coupling to GTP hydrolysis, a source of energy. Therefore, formation requires energy input and degradation does not, indicating that peptide bonds are thermodynamically unstable (ie, prone to breaking). However, the question also states that degradation requires enzymes. Enzymes increase the rate of otherwise slow reactions by decreasing the activation energy, so in the absence of enzymes peptide bonds degrade slowly, making them kinetically stable. (Choices A and C) Because their formation requires coupling to an energy source (GTP), peptide bonds are thermodynamically unstable. (Choice D) Although it is true that peptide bonds are thermodynamically unstable, the fact that enzymes are required to break peptide bonds means they are kinetically stable. In the absence of the relevant enzymes (called proteases), peptide bond degradation is so slow as to be undetectable on a physiological time scale. Educational objective: Thermodynamics determines whether a reaction can proceed without energy input, and kinetics determines how quickly a reaction proceeds. A reaction that is thermodynamically favorable but slow involves bonds that are thermodynamically unstable (prone to breaking) but kinetically stable (break slowly). Anabolic processes such as DNA and protein synthesis require energy input (usually from ATP or GTP) but form kinetically stable bonds.

Under Physiological Conditions, Peptide Bond Formation and Degradation Both Require