Enzyme A catalyzes the conversion of substrate S to product P. When enzyme A is phosphorylated, its kcat value changes from 10 s−1 to 5 s−1 and its KM value changes from 100 μM to 20 μM. If equal concentrations of each enzyme form are used then compared to the unphosphorylated enzyme A, the phosphorylated form:A.has a higher catalytic efficiency and yields a faster reaction rate when [S] < 20 μM.B.has a higher turnover number and yields a faster reaction rate under saturating conditions.C.has a lower affinity for S and yields a slower reaction rate at all [S].D.is allosterically inhibited and yields a lower reaction rate at all [S].
Question
Enzyme A catalyzes the conversion of substrate S to product P. When enzyme A is phosphorylated, its kcat value changes from 10 s−1 to 5 s−1 and its KM value changes from 100 μM to 20 μM. If equal concentrations of each enzyme form are used then compared to the unphosphorylated enzyme A, the phosphorylated form:A.has a higher catalytic efficiency and yields a faster reaction rate when [S] < 20 μM.B.has a higher turnover number and yields a faster reaction rate under saturating conditions.C.has a lower affinity for S and yields a slower reaction rate at all [S].D.is allosterically inhibited and yields a lower reaction rate at all [S].
Solution
The correct answer is A. The phosphorylated form of enzyme A has a higher catalytic efficiency and yields a faster reaction rate when [S] < 20 μM.
Here's why:
Catalytic efficiency is a measure of how efficiently an enzyme converts substrates into products. It is calculated as the ratio of kcat/KM.
For the unphosphorylated enzyme A, the catalytic efficiency is 10 s−1 / 100 μM = 0.1 μM−1s−1.
For the phosphorylated enzyme A, the catalytic efficiency is 5 s−1 / 20 μM = 0.25 μM−1s−1.
So, the phosphorylated form of enzyme A has a higher catalytic efficiency.
When the substrate concentration [S] is less than KM, the reaction rate is approximately proportional to [S]. Since the KM of the phosphorylated enzyme A is lower (20 μM), it will reach its maximum reaction rate at a lower [S] compared to the unphosphorylated form. Therefore, when [S] < 20 μM, the phosphorylated form of enzyme A will yield a faster reaction rate.
Similar Questions
All of the following are true of enzyme-catalyzed reactions EXCEPT:A.the presence or absence of a post-translational modification may alter the activation energy Ea of the reaction.B.the enzyme does not affect the net reaction rate if the ratio of products to reactants equals Keq for the reaction.C.substrates can covalently modify the enzyme to cause a permanent decrease in the enzyme's turnover number kcat.D.the saturation of enzyme active sites by substrate molecules limits the maximum reaction velocity Vmax.Submit
What is the rate of Enzyme reaction and what are the factors that affect it.
Enzymes are able to speed up a reaction by lowering the activation energy:Question 59 Answer Choices A. only if the reaction is exergonic. B. only if the reaction is endergonic. C. if the reaction is endergonic or exergonic D. if ΔG is positive.
Which of the following statements is/are true of enzyme catalysts?Group of answer choicesThey bind to substrates but are never covalently attached to substrate or product.They increase the equilibrium constant for a reaction, thus favoring product formation.They increase the rate at which substrate is converted into product.They lower the activation energy for the conversion of substrate to product.To be effective, they must be present at the same concentration as their substrates.
Which of the following is NOT TRUE of enzymes?Group of answer choicesall enzymes are proteinsenzymes are changed in the reaction they performenzymes catalyze metabolic reactionsenzymes work on substratesenzymes lower activation energy of reactions
Upgrade your grade with Knowee
Get personalized homework help. Review tough concepts in more detail, or go deeper into your topic by exploring other relevant questions.