The Mechanism and Factors of Enzyme Action

Enzymes and metabolism

Every living cell carries out chemical reactions. Some break large molecules down, some build new molecules, and others release energy or move substances. The total of these reactions is metabolism.

Many metabolic reactions would occur too slowly to sustain life without enzymes. An enzyme is a biological catalyst: it increases the rate of a reaction without being used up, so the same enzyme molecule can catalyse another reaction afterwards.

Enzymes are proteins made from chains of amino acids. Each chain folds into a particular three-dimensional shape. Part of this shape forms an active site, where a reactant called the substrate can bind. Because active sites have different shapes, enzymes catalyse specific reactions.

The lock and key model

The lock and key model is a simplified way to represent enzyme action. The substrate has a shape complementary to the enzyme’s active site, rather as a key has a shape complementary to its lock.

1. Enzyme and substrate particles move and collide.
2. A correctly shaped substrate binds to the active site, forming a temporary enzyme–substrate complex.
3. The enzyme catalyses the reaction, converting the substrate into product or products.
4. The products no longer fit in the same way and leave the active site.
5. The enzyme is unchanged and can be used again.

The model explains specificity: a differently shaped substrate cannot bind successfully, so the enzyme cannot catalyse its reaction. It is still only a model—real enzyme molecules are flexible—but it provides the simplified explanation required here.

Why temperature changes enzyme activity

For a reaction to occur, enzyme and substrate particles must collide, and the substrate must bind to the active site.

At a low temperature, particles have little kinetic energy. They move slowly, so enzyme and substrate collide less frequently. Fewer enzyme–substrate complexes form each second and the reaction rate is low. The enzyme has not been denatured; warming it can increase its activity again.

As temperature rises, particles gain kinetic energy. They move faster, collide more often and form more enzyme–substrate complexes per second. The reaction rate therefore rises until the optimum temperature, at which the rate is greatest.

Above the optimum, increased particle movement no longer gives the overall benefit. Heat disrupts bonds that maintain the enzyme’s three-dimensional shape. The active site changes shape, so it is no longer complementary to the substrate. Fewer complexes form and the rate falls rapidly. This loss of shape and function is denaturation; at sufficiently high temperatures, the enzyme-controlled reaction may stop.

A graph of rate against temperature therefore usually rises gradually to an optimum and then falls steeply. The falling section is not caused by particles losing kinetic energy—it occurs because increasing numbers of enzyme molecules are denatured.

Why pH changes enzyme activity

The pH describes how acidic or alkaline a solution is. Each enzyme has an optimum pH at which its active site has the most suitable shape and the reaction rate is greatest.

Moving away from the optimum can alter interactions and bonds that maintain the enzyme’s shape. The active site becomes less suitable for the substrate, so fewer enzyme–substrate complexes form and the reaction slows. A sufficiently extreme pH can denature the enzyme.

Different enzymes can have different optimum pH values because they work in different chemical environments. There is no rule that every enzyme works best at pH 7.

Connecting the factors to metabolism

Temperature and pH affect metabolism because metabolic pathways depend on enzyme-controlled reactions. If conditions move away from an enzyme’s optimum, that step in a pathway slows. If an enzyme is denatured, its substrate may no longer be converted into product, disrupting later reactions that depend on that product.

Cells and organisms therefore benefit from keeping internal conditions reasonably stable. This allows their enzymes—and consequently their metabolic reactions—to operate effectively.

Describing and calculating rate

Reaction rate compares a change with the time taken:

$$\text{rate} = \frac{\text{quantity changed}}{\text{time}}$$

The unit depends on what is measured: for example, a change in concentration per second or a volume of product per minute.

Sometimes every test begins with the same quantity of substrate and records only the time taken to reach the same endpoint. A useful relative rate is then:

$$\text{relative rate} = \frac{1}{\text{time}}$$

A shorter completion time gives a larger value and therefore represents a faster reaction. If the time is in seconds, this relative rate has the unit $\text{s}^{-1}$. If a question supplies a different fixed numerator, use the stated expression rather than automatically substituting 1.

Core idea

Enzymes are proteins and biological catalysts. They speed up metabolic reactions without being used up. Their folded shapes contain specific active sites.

Lock and key sequence

Complementary substrate collides → binds at active site → forms → reaction occurs → products leave → is reused.