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Enzymes

Enzymes are biological catalysts.

A catalyst is a molecule that increases the rate (speed) of a reaction. Enzymes act as catalysts for chemical reactions in the cell.

It is unlikely glucose molecules will spontaneously form a starch molecule. The cell uses an enzyme (starch synthase) to produce starch quickly.

Like all proteins, enzymes are formed from chains of amino acids. In enzymes, the chains form a globular structure (spherical):

The reacting molecules in enzyme-driven reactions are called substrates.
The reacting molecules in enzyme-driven reactions are called substrates.

There are several important features of enzymes:

  • Highly specific: Each enzyme only acts on a single type of reaction.
  • Unchanged: Enzymes are not altered by their reactions.
  • Reusable: Enzymes can be used multiple times.

Enzymes catalyse reactions by lowering the activation energy of the reactions.

All reactions have an activation energy. This is the energy that is required for the reaction to occur. The larger the activation energy, the slower the reaction will be.

By lowering the activation energy, the reaction will proceed faster.

Enzymes reduce the energy barrier between reactants and products.
Enzymes reduce the energy barrier between reactants and products.

There are several theories suggesting how enzymes decrease the activation energy of reactions.

One popular theory is called the lock-and-key hypothesis:

  1. The substrate (the key) binds to the active site of the enzyme (the lock).
  2. The substrate and active site have complementary shapes.

  3. This forms an enzyme-substrate complex (ES-complex).
  4. The ES-complex lowers the activation energy of the reaction, allowing it to occur.
  5. The products of the reaction do not fit the active site. They are quickly removed.
This enzyme catalyses the breakdown of a molecule into two parts. Other enzymes can join molecules together.
This enzyme catalyses the breakdown of a molecule into two parts. Other enzymes can join molecules together.

The lock-and-key hypothesis is a simple but inaccurate way of understanding enzyme function. The induced fit model is a more accurate version of the lock-and-key hypothesis.

In the induced fit model, the binding of the substrate causes the enzyme to change shape. The shape change activates the enzyme, enabling it to act as a catalyst.

Enzyme activity is dependent on a number of factors:

  • Concentration of enzyme and substrate
  • The higher the concentration of the enzyme or substrate, the faster the reaction.

  • Temperature
  • Enzymes function best within a narrow optimum temperature range. This range varies between enzymes.

    The optimum temperature of human enzymes is 37.5$$^{\circ}$$C.

    When enzymes are subjected to temperatures much higher than their optimum, they denature. The shape of the active site changes irreversibly, preventing substrates from binding.

  • pH (Acidity)
  • Different enzymes vary much more in their optimum pH range than in their optimum temperature. pH values outside the optimal range can also cause enzymes to denature.

    The optimum pH of pancreatic amylase is around 7, while enzymes in the stomach operate at a pH between 1.5 and 3.5.

  • Presence of inhibitors can also limit the function of enzymes.
  • DFP is an insecticide used in farming. It inhibits an important protease enzyme in animals.

Enzyme inhibitors reduce the efficiency of enzymes. There are two main classes of inhibitors:

Competitive inhibitors bind directly to the active site of an enzyme. They compete with the substrate for the active site.

Non-competitive inhibitors bind elsewhere on the enzyme and Change the shape of the active site so the substrate can no longer bind.

Inhibitors may also be reversible or irreversible.

Reversible inhibitors are commonly used by the body to slow down and control enzyme-catalysed reactions. These bind through hydrogen and ionic bonding.

Irreversible inhibitors bind covalently to the enzyme. Toxins often work this way.

It is easier to break ionic and hydrogen bonds compared to covalent bonds. Reversible inhibitors can be removed from enzymes while irreversible inhibitors are bound permanently.

Cyanide is a poisonous gas that acts as an irreversible inhibitor. It is produced by some plants, such as almonds, as a natural defence against predators.

The level of activity of an enzyme depends on the type of inhibitor acting on it.

Competitive inhibitors act by binding to the active site and preventing the substrate from binding.

The level of inhibition of the enzyme is dependent on the relative concentration of the competitive inhibitor to the substrate.

Through increasing the substrate concentration, it is possible to achieve the maximum reaction rate regardless of the presence of an inhibitor.

Non-competitive inhibitors bind elsewhere on the enzyme, but affect the shape of the active site, so the substrate can no longer bind.

When a non-competitive inhibitor is present the maximal reaction rate can never be achieved.