Enzymes

5.1 Enzymes

Enzymes are biological catalysts, which means they speed up chemical reactions in living organisms. They are typically proteins, although some RNA molecules (ribozymes) can also act as enzymes. Enzymes are essential for all metabolic reactions, which are the chemical processes that occur within cells to sustain life.

What are Enzymes?

Enzymes are proteins that have a specific three-dimensional structure. This structure is crucial for their function. The active site is a region on the enzyme where the substrate binds and the chemical reaction takes place.

How Enzymes Work

Enzymes work by lowering the activation energy of a reaction. Activation energy is the minimum amount of energy required to start a chemical reaction. By providing an alternative reaction pathway with a lower activation energy, enzymes allow reactions to occur much faster than they would without them.

The Enzyme-Substrate Interaction

Enzymes have a specific shape that complements their substrate (the molecule the enzyme acts upon). This is often described as a 'lock and key' or 'induced fit' model.

• Lock and Key Model: The enzyme's active site has a very specific shape that only fits one particular substrate.
• Induced Fit Model: The enzyme's active site can change shape slightly when the substrate binds, creating a better fit and increasing the interaction between the enzyme and substrate.

Factors Affecting Enzyme Activity

Several factors can affect how well an enzyme works:

• Temperature: Enzymes have an optimal temperature at which they work best. Below this temperature, the reaction is slow. Above this temperature, the enzyme can denature (lose its shape) and stop working.
• pH: Enzymes also have an optimal pH. Changes in pH can affect the ionization state of amino acids in the enzyme, altering the shape and affecting its activity.
• Substrate Concentration: As substrate concentration increases, the rate of reaction generally increases until all enzyme active sites are occupied. Beyond this point, the rate levels off.
• Enzyme Concentration: Increasing the enzyme concentration generally increases the rate of reaction, assuming there is sufficient substrate available.

Table of Factors Affecting Enzyme Activity

Factor	Effect on Enzyme Activity
Temperature	Increases to an optimum, then decreases as enzyme denatures.
pH	Optimal pH allows for optimal enzyme shape. Deviations can reduce activity.
Substrate Concentration	Increases reaction rate up to a point, then levels off.
Enzyme Concentration	Increases reaction rate (if substrate is not limiting).

Enzyme Specificity

Enzymes are highly specific. This means that each enzyme typically only catalyzes one particular reaction or a small set of closely related reactions. This specificity is due to the unique shape of the enzyme's active site.