When substrate is present in excess, the rate of an enzyme catalysed reaction is
Correct answer: B. directly proportional to the enzyme concentration
- A. independent of enzyme concentration
- B. directly proportional to the enzyme concentration
- C. inversely proportional to the enzyme concentration
- D. zero
Explanation
With substrate always available, every additional enzyme molecule adds its own turnover to the total, so doubling the enzyme doubles the rate and the graph is a straight line through the origin. The line only levels off if the substrate begins to run short and becomes limiting instead. This is the reverse of the saturation curve obtained when enzyme is fixed and substrate is increased.
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About Factors Affecting Enzyme Action
Enzyme activity changes with temperature, pH, substrate concentration and enzyme concentration because these factors affect molecular collisions and the active site. Questions also examine optimum conditions, denaturation and inhibition, while distinguishing a fall in activity caused by unsuitable conditions from permanent enzyme damage.
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