Enzymes and Rate of Reactions

What is an enzyme and what is its role?

Enzymes are globular proteins that act as biological catalysts. They speed up metabolic reactions by lowering the activation energy required for the reaction to occur, without being used up themselves. They can work both intracellularly (e.g. catalase breaking down hydrogen peroxide inside cells) and extracellularly (e.g. amylase digesting starch in the gut).

What is meant by activation energy and how do enzymes lower it?

Activation energy is the minimum energy required to start a chemical reaction. Enzymes bind to their substrate, forming an enzyme-substrate complex, which strains bonds in the substrate and provides an alternative reaction pathway with a lower activation energy, making the reaction proceed much faster.

What is meant by enzyme specificity?

Each enzyme only catalyses one specific reaction (or type of reaction) because its active site has a shape that is complementary to only one specific substrate. This means only the correct substrate can bind and form an enzyme-substrate complex.

Give the difference between anabolic and catabolic reactions and give an enzyme example of each.

Catabolic reactions break larger molecules into smaller ones (e.g. amylase breaking starch into maltose), this can be used for energy release in an organism like glycogen breakdown.

Anabolic reactions join smaller molecules together to build larger ones (e.g. DNA polymerase joining nucleotides to form DNA), this can be used for growth of an organism.

What is the lock-and-key hypothesis of enzyme action?

The active site of the enzyme has a fixed shape that is exactly complementary to the substrate, like a key fitting into a lock. The substrate fits precisely into the active site, an enzyme-substrate complex forms, the reaction occurs, and the products are released. The active site does not change shape.

What is the induced-fit hypothesis and how does it improve on the lock-and-key model?

In the induced-fit model, the active site is not a rigid, fixed shape. When the substrate approaches, the active site moulds itself around the substrate, changing shape slightly to achieve a closer fit. This conformational change strains bonds within the substrate, further lowering activation energy.

The induced-fit model is considered more accurate because it better explains how enzymes can act on a range of similar substrates and why the active site is not simply a rigid complementary shape.

How does a strain in the bonds of a substrate lower the activation energy work

the active site changes shape slightly to fit it more tightly.

The enzyme forces the substrate into a strained position
As the active site moulds around the substrate, it bends and stretches bonds so Bonds become unstable
Because the bonds are bent or stretched:

• they contain more energy

• they are easier to break

This means the reaction needs less activation energy. Reaction happens → products released
The substrate is converted into products, which no longer fit the active site well and leave.

How does temperature affect enzyme activity ?

As temperature increases, molecules have more kinetic energy, so collisions between enzyme and substrate are more frequent and the rate of reaction increases. However, above the optimum temperature, bonds maintaining the enzyme's tertiary structure begin to break, changing the shape of the active site so substrates can no longer bind — the enzyme denatures.

what is the Q₁₀ value?

The Q₁₀ value is the factor by which the rate of reaction increases for every 10°C rise in temperature (typically around 2 for enzyme-controlled reactions, meaning the rate doubles).

Q10=R₂/R₁

How does pH affect enzyme activity?

Each enzyme has an optimum pH at which it works fastest. Above or below this, excess H⁺ ions (acid) or OH⁻ ions (alkali) interfere with the hydrogen bonds and ionic bonds holding the enzyme's tertiary structure together. This alters the shape of the active site, reducing the rate of enzyme-substrate complex formation. At extremes of pH, complete denaturation occurs and no substrate can bind at all.

How does substrate concentration affect the rate of reaction?

As substrate concentration increases, more enzyme-substrate complexes form per unit time and the rate of reaction increases. However, once all active sites are occupied (the enzyme is saturated), adding more substrate has no further effect and the rate plateaus at Vmax. The rate cannot increase further unless more enzyme is added.

How does enzyme concentration affect the rate of reaction?

Increasing enzyme concentration increases the number of active sites available, so more enzyme-substrate complexes can form simultaneously and the rate of reaction increases — provided substrate is not the limiting factor. If substrate concentration is limiting, adding more enzyme will have no effect.