An enzyme reaction may begin to decline when?

1. Analyze the question The question asks which condition makes the rate of an enzyme-catalysed reaction start to fall.

2. Explain the concept Enzymes are proteins. Their activity depends on the shape of the active site, which is held in place by weak bonds. Factors affecting enzyme activity:

• Temperature: the rate rises up to the optimum temperature, then falls when the enzyme denatures at higher temperatures.
• pH: each enzyme has an optimum pH. If the pH moves away from it, the ionic bonds holding the enzyme's shape are disrupted, the active site changes shape, and the rate declines.
• Substrate concentration: raising it increases the rate until all active sites are occupied (saturation). The rate then levels off, but it does not decline.
• Atmospheric pressure: it has no significant effect on enzymes in living cells or in the lab.

3. Evaluate each option

• A. The optimum temperature is attained: wrong. At the optimum temperature, the reaction rate is at its maximum, not declining.
• B. The pH of the medium is altered: correct. A change in pH moves the enzyme away from its optimum conditions, which alters the active site's shape and reduces activity.
• C. Increase in substrate concentration: wrong. This raises the rate until it plateaus, and the rate does not fall.
• D. Atmospheric pressure is altered: wrong. Enzymes are not affected by normal changes in pressure.

4. Answer The correct option is B.

5. Tip Remember: "Temperature too high, or pH gone awry, enzymes denature and activity says goodbye." Also remember that optimum means best, so a rate at the optimum is the peak, not a decline. Saturation causes a plateau, never a drop.