Enzymes lower activation energy without being consumed overall. In induced fit, substrate binding changes active-site conformation. Temperature, pH and inhibitors can alter activity. Warming often increases rate until instability or denaturation becomes important; there is no universal optimum temperature for every enzyme.
Calculate rate as product increase divided by elapsed time, keeping the stated units. An initial rate uses an early interval before substantial substrate depletion or product build-up. At fixed enzyme concentration, more substrate can increase rate until active sites approach saturation. A plateau is a region where more substrate produces little further rate increase. When sufficient substrate remains, increasing enzyme concentration can increase total catalytic capacity.
Example 1
An assay makes 24 micromoles of product in 3 minutes during its linear phase. Find the rate.
Rate = product/time =324=8 micromoles per minute. Use the initial linear interval, not a later plateau.
Example 2
Substrate concentration doubles but rate stays at 10 units/min. Explain one possibility.
At this enzyme concentration, active sites may be saturated. Extra substrate then does not increase the maximum catalytic capacity.
Watch out: An enzyme lowers the activation-energy barrier through a suitable reaction pathway.