International A Level Biology / Proteins and DNA

Temperature, pH, enzyme and substrate concentration revision notes

Edexcel International A Level Biology (YBI11), chapter 5: Proteins and DNA, specification point 2.8. Free lesson.

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Temperature, pH, enzyme concentration and substrate concentration each affect enzyme reaction rate in a different way. Knowing why each graph has its shape helps you explain the results, not just describe them.

How temperature changes the rate

As temperature rises, molecules gain kinetic energy and move faster. Substrate molecules collide with active sites more often, so more enzyme-substrate complexes form each second. The rate rises to the optimum temperature, about 40 °C for this enzyme. Above the optimum, the rate falls steeply because the enzyme is denatured. Below the optimum, some enzyme molecules are already denaturing. At the optimum, the gain from extra kinetic energy balances the loss from denaturation. Above the optimum, denaturation outweighs the effect of extra kinetic energy, so the rate falls.

What denaturation does

Denaturation changes the shape of the enzyme's active site. At the optimum, the substrate fits the active site. Above the optimum, bonds in the enzyme's tertiary structure break, changing the active site's shape. The substrate is no longer complementary, so an enzyme-substrate complex cannot form. Use the word denatured, not killed. An enzyme is not alive.

How pH affects the active site

Each enzyme has its own optimum pH, where its rate is highest. For this enzyme, the optimum is pH 7. Away from this pH, changes in hydrogen ion concentration alter the charges on R groups in the active site. The substrate binds less well, so fewer enzyme-substrate complexes form. At extreme pH, bonds in the tertiary structure break and the enzyme is denatured. The rate falls on either side of the peak as pH moves away from the optimum. Pepsin works in the stomach and has an optimum pH of about 2.

How enzyme concentration affects the rate

When substrate is in excess, increasing enzyme concentration increases the number of active sites. More complexes can form each second, so the rate rises in a straight line through the origin. This only continues while there is enough substrate to use the extra active sites. If substrate is limited, the rate levels off because there is not enough substrate to fill the active sites. Substrate is then the limiting factor.

Why substrate concentration levels off

Keep enzyme concentration fixed and increase substrate concentration. At first, active sites are free much of the time, so more substrate causes more collisions and more complexes to form. The rate rises steeply, then the curve flattens. At high substrate concentration, all active sites are occupied, or saturated. Extra substrate must wait, so the rate stays the same. At the plateau, enzyme concentration limits the rate because every active site is occupied.

What happens if enzyme concentration doubles?

With twice the enzyme concentration, there are twice as many active sites. More enzyme-substrate complexes can form each second before all the sites are occupied. The substrate concentration curve is steeper at the start and levels off at a higher rate, about twice as high.