Enzyme Notes

Enzymes

Overview

  • Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process.

  • They facilitate reactions involving substrates, leading to the formation of products at the enzyme's active site.

Key Terminology

  • Substrates: The molecules upon which enzymes act.

  • Active Site: The specific region of an enzyme where substrates bind and undergo a chemical reaction.

  • Enzyme-Substrate Complex: The intermediate structure formed when a substrate binds to the active site of an enzyme.

  • Product: The resulting molecule(s) from an enzyme-catalyzed reaction.

Enzyme Structure

  • Enzymes are composed of chains of amino acids folded into a specific 3D shape.

  • This shape is maintained by:

    • Hydrogen bonds

    • Disulfide bridges

  • The 3D structure, particularly the active site, is crucial for enzyme function.

  • The active site's shape is highly specific to its substrate, enabling enzymes to catalyze only particular reactions.

Enzyme Reactions: Anabolic and Catabolic

  • When a substrate binds to an enzyme's active site, chemical reactions occur, resulting in the formation of products.

  • Two main types of reactions:

    • Anabolic Reactions: Enzymes join smaller substrates to form larger products (building up).

      • Example: Photosynthesis, where plants create glucose from water and carbon dioxide.

    • Catabolic Reactions: Enzymes break down large substrates into smaller products (breaking down).

      • Example: Cellular respiration, where glucose is broken down to release energy.

How Enzymes Speed Up Reactions

  • Chemical reactions require energy, known as activation energy.

  • Enzymes function by lowering the activation energy required for a reaction to occur.

  • Without enzymes, many biological reactions would be too slow to sustain life.

Models of Enzyme Activity

  • Two models explain how enzymes interact with substrates:

    • Lock and Key Model:

      • The active site and substrate have perfectly matching shapes.

      • The enzyme and substrate bind to form an enzyme-substrate complex.

      • Chemical bonds are formed or broken, leading to product formation.

    • Induced Fit Model:

      • Enzymes are flexible and do not initially have the exact shape of the substrate.

      • The enzyme molds itself to fit the substrate upon binding, forming an enzyme-substrate complex.

      • After product release, the enzyme returns to its original shape.

Comparison of Lock and Key vs. Induced Fit Models

  • Both models agree that enzymes have a specific shape that corresponds to a particular substrate.

  • Key Difference:

    • Lock and Key: The enzyme's shape is rigid and does not change.

    • Induced Fit: The enzyme is flexible and changes shape to fit the substrate.

Factors Affecting Enzyme Activity

  • Enzymes operate optimally under specific conditions. Deviations can impair function.

Temperature

  • Enzymes have a unique temperature range for function, with an optimum temperature at which the reaction rate is highest.

  • Low Temperatures:

    • Molecules move slowly, reducing successful collisions between enzymes and substrates.

    • The reaction rate slows down or stops.

  • Slightly Increased Temperatures:

    • Enzymes and substrates move faster, increasing collision frequency.

    • The reaction rate increases up to a point.

  • High Temperatures:

    • Bonds holding the enzyme's shape are disrupted and break.

    • The substrate can no longer fit into the active site.

    • The enzyme is denatured.

  • Denaturation:

    • Breaking of bonds causes the enzyme to unfold and lose its shape.

    • Denaturation due to extreme temperatures is usually permanent.

pH

  • Enzymes have a specific optimum pH at which they are most active.

  • Example:

    • Catalase (optimum pH 7)

    • Pepsin (optimum pH around 1.5)

  • Slight pH Changes:

    • Can temporarily alter the shape of the active site, reducing the reaction rate.

    • The enzyme is not denatured, and the effect is reversible if pH returns to optimum.

  • Extreme pH Changes:

    • Can denature the enzyme by disrupting hydrogen bonds, causing a permanent change in shape of the active site.

    • The enzyme loses its ability to bind to the substrate and catalyze the reaction.

Enzyme Concentration

  • Increased enzyme concentration generally increases the reaction rate, as there is a higher likelihood of substrate binding.

  • The rate of reaction plateaus when all available substrate is bound, and adding more enzyme has no further effect.

Substrate Concentration

  • Increasing substrate concentration increases the reaction rate, as more substrates successfully combine with the enzyme's active site.

  • The reaction rate plateaus when the enzyme's active sites are saturated with substrate (saturation point).

Inhibitors

  • Inhibitors are molecules that interfere with enzymes, reducing enzyme activity.

  • Examples: Heavy metals (lead, mercury, cadmium) and poisons.

  • Two types:

    • Competitive Inhibitors: Bind to the active site, preventing substrate binding.

    • Non-Competitive Inhibitors: Bind to the enzyme outside the active site, causing a change in the active site's shape.

Co-factors/Co-enzymes

  • Some enzymes require co-factors or co-enzymes to function.

  • They bind to the enzyme's active site, altering its shape to allow substrate binding.

  • Co-factors: Inorganic molecules (e.g., metal ions).

  • Co-enzymes: Organic molecules (e.g., vitamins).

Limiting Factors

  • The rate of an enzyme reaction is limited by the factor that is in least supply or least optimal condition.

  • If substrate is abundant but enzyme concentration, temperature, or pH are not optimal, the reaction rate will be limited by the non-optimal factor.