Enzyme Function & Regulation

Overview of Enzyme Activity

Enzymes are biological catalysts that accelerate chemical reactions in living organisms by lowering the activation energy required for the reaction to proceed. Their activity is influenced by several factors, which determine how efficiently they catalyze reactions.

• Enzyme specificity: Each enzyme is specific to its substrate due to the unique shape of its active site.

• Substrate concentration: Increasing substrate concentration increases reaction rate until the enzyme becomes saturated.

• Enzyme concentration: More enzyme molecules can increase the rate of reaction if substrate is available.

• Temperature: Enzyme activity increases with temperature up to an optimal point, after which activity declines due to denaturation.

• pH: Each enzyme has an optimal pH; deviations can reduce activity or denature the enzyme.

• Water activity: Water is essential for enzyme function and substrate interaction.

• Mixing intensity: Proper mixing ensures substrate and enzyme interaction.

• Inhibiting substances: Certain molecules (e.g., fat, sugar) can inhibit enzyme activity.

Metabolism and Energy

Metabolic Reactions: Anabolism and Catabolism

Metabolism encompasses all chemical reactions in an organism, divided into two main types:

• Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. These are endergonic reactions (ΔG > 0).

• Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. These are exergonic reactions (ΔG < 0).

Example: Photosynthesis (anabolic) and cellular respiration (catabolic) are linked processes in metabolism.

Energy in Biological Systems

• Potential energy: Stored energy in chemical bonds, gradients, or charge imbalances.

• Kinetic energy: Energy of movement, such as heat or motion.

Thermodynamics in Biology

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed.

• Second Law: Disorder (entropy) tends to increase; some energy is lost as heat in biological processes.

Activation Energy and Reaction Progress

Energy Diagrams and Activation Energy

Chemical reactions require an initial input of energy, called activation energy (EA), to reach the transition state. The net change in free energy (ΔG) determines whether a reaction is exergonic or endergonic.

• Exergonic reactions: Release energy; products have less free energy than reactants (ΔG < 0).

• Endergonic reactions: Require energy input; products have more free energy than reactants (ΔG > 0).

Enzyme Catalysis

How Enzymes Work

Enzymes lower the activation energy required for reactions, allowing them to proceed faster. They do not change the net free energy (ΔG) of the reaction.

• Catalyst: A substance that speeds up a reaction without being consumed.

• Enzyme: A biological macromolecule (usually a protein) that acts as a catalyst.

• Activation energy: The energy required to initiate a reaction.

Enzyme-Substrate Interaction: Induced Fit Model

Enzymes bind substrates at their active site, forming an enzyme-substrate complex. The active site changes shape slightly to fit the substrate tightly, a process called induced fit.

• Active site: The region of the enzyme where substrate binds.

• Substrate: The reactant molecule acted upon by the enzyme.

• Induced fit: The enzyme changes shape to better accommodate the substrate.

Mechanisms of Enzyme Action

Enzymes facilitate reactions by:

• Inducing strain on substrate bonds (exergonic reactions).

• Orienting substrates for optimal interaction (endergonic reactions).

• Providing a conducive microenvironment for the reaction.

• Involving side chains (R groups) in catalysis.

Enzyme Kinetics

Reaction Rate and Saturation

The rate of enzyme-catalyzed reactions increases with substrate concentration until the enzyme becomes saturated (Vmax).

• Saturation: All enzyme molecules are bound to substrate; maximum reaction rate is reached.

Factors Affecting Enzyme Activity

Temperature and pH

Enzymes have optimal temperature and pH ranges. Deviations can reduce activity or cause denaturation.

• Optimal temperature: Human enzymes typically function best at 37°C; thermophilic enzymes at higher temperatures.

• Optimal pH: Enzymes like pepsin (stomach) function at low pH, while trypsin (intestine) functions at higher pH.

Denaturation

Extreme temperature or pH can denature enzymes, disrupting their structure and function. Example: Cooking an egg denatures its proteins, changing its texture.

Enzyme Regulation

Allosteric Regulation

Enzymes can be regulated by molecules binding to sites other than the active site (regulatory binding sites), causing conformational changes that affect activity.

• Allosteric regulation: Regulation of enzyme activity by binding of regulatory molecules at allosteric sites.

• Allosteric activator: Turns enzyme activity "on".

• Allosteric inhibitor: Turns enzyme activity "off".

Cofactors and Coenzymes

Some enzymes require additional molecules for activity:

• Cofactors: Inorganic ions (e.g., zinc, iron) that assist enzyme function.

• Coenzymes: Organic molecules (e.g., vitamins) that assist enzyme function.

Enzyme Inhibition

Types of Inhibition

• Irreversible inhibition: Inhibitor binds covalently to the enzyme, permanently inactivating it.

• Reversible inhibition: Inhibitor binds non-covalently and can dissociate.

• Competitive inhibition: Inhibitor binds to the active site, blocking substrate binding.

• Noncompetitive inhibition: Inhibitor binds to a different site, altering enzyme shape and function.

Metabolic Pathways

Linear and Cyclic Pathways

Metabolic pathways are series of enzyme-catalyzed reactions.

• Linear pathway: Substrate is converted to end product through a series of steps.

• Cyclic pathway: Substrate is regenerated at the end of the cycle.

Feedback Inhibition

The end product of a pathway can inhibit an enzyme earlier in the pathway, regulating the pathway's activity. Example: Feedback inhibition is common in amino acid biosynthesis.

Terminology

Key Terms

• Activation energy

• Active site

• Allosteric regulation

• Allosteric activator

• Allosteric inhibitor

• Catalyst

• Coenzyme

• Cofactor

• Competitive inhibitor

• ΔG

• Energy barrier

• Enzyme

• Enzyme-substrate complex

• Feedback inhibition

• Induced fit

• Irreversible inhibition

• Metabolic pathway

• Noncompetitive inhibitor

• Products

• Regulatory binding site

• Regulatory molecule

• Reversible inhibition

• Ribozyme

• Saturated

• Substrates

• Transition state

ATP & Redox Reactions

ATP Structure and Function

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• ATP consists of adenosine, ribose, and three phosphate groups.

• Formation of ATP from ADP + Pi is an endergonic reaction (requires energy input).

Redox Reactions

• Oxidation: Loss of electrons or hydrogen atoms.

• Reduction: Gain of electrons or hydrogen atoms.

• In a redox reaction, the oxidizing agent accepts electrons, and the reducing agent donates electrons.

Example: In Na + Cl → Na+ + Cl-, sodium is oxidized (reducing agent), chlorine is reduced (oxidizing agent).

Summary Table: Types of Enzyme Regulation

Summary Table: Anabolic vs. Catabolic Pathways

Key Equations

Gibbs Free Energy

• ΔG = ΔH - TΔS

ATP Formation

• ADP + Pi + energy → ATP

Redox Reaction Example

Additional info:

• Enzyme regulation is essential for maintaining homeostasis and metabolic balance.

• Feedback inhibition prevents overproduction of metabolic products.

• Enzyme denaturation is irreversible and results in loss of function.