Multiple Choice
To calculate the turnover number of an enzyme, you need to know:
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6. Enzymes and Enzyme Kinetics
Kcat
Multiple Choice
Studies with mutated forms of an enzyme show that changing some active-site amino acids decrease the enzyme's turnover number (kcat) but do not affect the K m of the reaction. What is the best interpretation of these results?
A
The Km of the enzyme for the substrate does not depend on amino acid side chains found in the active site.
B
The two terms, Km and turnover number, are inversely proportional.
C
The transition state for this reaction is formed prior to the formation of the ES complex.
D
Amino acids involved in stabilizing transition-state complexes can be different than those affecting the ES-complex.
Verified step by step guidance1
Begin by understanding the terms involved: kcat (turnover number) and Km (Michaelis constant). kcat represents the number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is saturated with substrate. Km is the substrate concentration at which the reaction velocity is half of the maximum velocity.
Consider the role of active-site amino acids in enzyme function. These amino acids can be involved in substrate binding, transition state stabilization, or catalysis. Changes in these amino acids can affect different aspects of enzyme activity.
Analyze the effect of mutations on kcat and Km. The decrease in kcat suggests that the mutations affect the catalytic efficiency of the enzyme, possibly by altering the transition state stabilization or the catalytic mechanism.
Note that Km remains unchanged, indicating that the substrate binding affinity is not affected by the mutations. This suggests that the amino acids altered by the mutations are not involved in substrate binding but rather in the catalytic process.
Conclude that the amino acids involved in stabilizing the transition-state complexes can be different from those affecting the enzyme-substrate (ES) complex formation. This explains why kcat is affected while Km remains constant.
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