Enzymes as biological catalysts, the lock and key model and active site specificity, the effect of temperature, pH and substrate concentration on the rate of enzyme-controlled reactions, and denaturing of enzymes.

What this dot point is asking

OCR wants you to describe enzymes as biological catalysts, explain why they are specific using the lock and key model, and describe and explain how temperature, pH and substrate concentration change the rate of an enzyme-controlled reaction, including denaturing.

Enzymes as biological catalysts

Enzymes control nearly all the reactions in cells, including respiration (enzymes in the mitochondria), photosynthesis (enzymes in the chloroplasts) and digestion (enzymes that break down food). Because enzymes are proteins, anything that changes a protein's shape changes how the enzyme works.

The lock and key model

Each enzyme has a region called the active site, with a particular shape. The substrate is the molecule the enzyme acts on. Only a substrate with a complementary (matching) shape fits into the active site, like a key fitting one particular lock. When the substrate fits, the reaction happens and the products are released, leaving the active site free to be used again.

This explains why enzymes are specific: because only one substrate shape fits, each enzyme catalyses only one type of reaction.

The effect of temperature

As temperature increases from low values, the enzyme and substrate molecules gain kinetic energy and move faster. They collide more often, and more of the collisions have enough energy to react, so the rate increases. This continues up to the optimum temperature, where the rate is highest (around $37$ to $40$ degrees Celsius for many human enzymes).

Above the optimum, the rate falls sharply. The extra heat breaks the bonds that hold the enzyme in its precise shape, so the active site changes shape and the substrate no longer fits. The enzyme is now denatured, a permanent change. Note that enzymes are not alive, so they are denatured, not "killed".

The effect of pH

Each enzyme has an optimum pH at which it works fastest. For many enzymes this is around pH $7$ (neutral), but some work best in acidic or alkaline conditions: the stomach enzyme pepsin works best in the acidic stomach (around pH $2$), while enzymes in the small intestine work best in alkaline conditions. If the pH moves too far from the optimum, the active site changes shape and the enzyme denatures, so the rate falls.

The effect of substrate concentration

As substrate concentration increases, there are more substrate molecules to collide with the enzymes' active sites, so the rate increases. Once the substrate concentration is high enough that the active sites are working as fast as they can (all of them are occupied much of the time), adding more substrate makes no difference, and the rate levels off. At this point the enzyme concentration is the limiting factor.

The enzyme practical (PAG B2)

A common required practical is investigating the effect of pH on the enzyme amylase, which breaks down starch. You mix amylase with starch at different pH values (using buffers) and time how long it takes for the starch to disappear, testing with iodine solution (which turns from orange-brown to blue-black with starch). The rate is calculated as $\dfrac{1}{\text{time}}$. Exam questions test the method, the control variables (temperature, enzyme and substrate concentration, volume) and how to interpret the results.