Overview
This lecture explains the essential role of enzymes in living organisms, focusing on how they accelerate chemical reactions, the principles of enzyme kinetics, different types of enzyme inhibition, and the regulation of enzyme activity through allosteric mechanisms.
Enzyme Overview
- Enzymes are biological catalysts that significantly speed up chemical reactions in the body without being consumed or permanently altered in the process. If they are changed during the reaction, they are regenerated by the end.
- Most enzymes are proteins, though a few RNA molecules can also have catalytic properties.
- Enzymes are crucial for sustaining life, as they enable biochemical reactions to occur at rates fast enough to meet the needs of living organisms.
- Each enzyme is highly specific, typically acting only on particular substrates and catalyzing specific reactions.
Enzyme Kinetics and Catalysis
- Many chemical reactions in the body can occur spontaneously, but at rates too slow to support life.
- Increasing temperature can speed up reactions, but in humans, even a small rise above the normal body temperature (37°C) can denature proteins and enzymes, leading to system failure and death at around 42°C.
- Enzymatic catalysis is the primary way organisms increase reaction rates safely, often accelerating reactions by factors of 10⁵ to 10⁷, without increasing the total amount of product formed.
- Enzymes lower the activation energy (ΔG‡) required for reactants to reach the transition state, making it easier and faster for products to form.
- The presence of an enzyme does not change the equilibrium position or the total amount of product formed; it only increases the speed at which equilibrium is reached.
- Enzyme active sites are specialized regions where substrates bind, allowing the enzyme to facilitate their conversion into products. For example, an enzyme can bind a disaccharide at its active site and help break it down into two monosaccharides for energy use.
Enzyme Inhibition
- Inhibitors are substances that decrease or prevent enzyme activity by binding to the enzyme, reducing the rate of the catalyzed reaction.
- Enzyme inhibition can be reversible or irreversible, depending on the nature of the inhibitor. Many drugs and toxins act as enzyme inhibitors, sometimes leading to system failure if critical reactions are blocked.
Types of Reversible Inhibition
- Competitive inhibition: The inhibitor resembles the substrate and competes for binding at the enzyme's active site. Only one (inhibitor or substrate) can bind at a time, so the outcome depends on their relative concentrations. If the inhibitor binds, the reaction is blocked; if the substrate binds, the reaction proceeds.
- Pure non-competitive inhibition: The inhibitor binds to a site other than the active site, far from where the substrate binds. This binding changes the enzyme's conformation, reducing or stopping its catalytic activity, regardless of substrate concentration.
- Mixed non-competitive inhibition: The inhibitor binds near the active site, affecting the enzyme's affinity for the substrate. This may not completely stop the enzyme from working but reduces its efficiency by altering how well the substrate binds.
- Uncompetitive inhibition: The inhibitor binds only to the enzyme-substrate complex (after the substrate has already bound to the enzyme), preventing the reaction from proceeding and blocking product formation.
Allosteric Enzymes
- Allosteric enzymes are composed of multiple subunits and have both active sites (where substrates bind) and one or more regulatory sites (where effector molecules bind).
- Effector molecules can be either activators (which increase enzyme activity) or inhibitors (which decrease activity). They bind to regulatory sites, not the active site.
- Binding of an activator to the regulatory site increases the enzyme's affinity for its substrate, allowing the reaction to proceed more efficiently and at a faster rate.
- Binding of an inhibitor to the regulatory site causes a conformational change in the enzyme, reducing its ability to bind the substrate and slowing or stopping the reaction.
- Allosteric enzymes do not follow typical Michaelis-Menten kinetics; their activity is regulated by the presence of effectors, which can shift the rate of product formation up or down.
Key Terms & Definitions
- Enzyme: A biological molecule, usually a protein, that acts as a catalyst in biochemical reactions.
- Substrate: The specific reactant that an enzyme acts upon.
- Active site: The region on an enzyme where the substrate binds and the reaction occurs.
- Activation energy (ΔG‡): The energy required to convert reactants to products via the transition state.
- Inhibitor: A substance that decreases or prevents enzyme activity.
- Allosteric enzyme: An enzyme regulated by molecules binding at sites other than the active site.
- Effector molecule: A molecule that binds to a regulatory site on an allosteric enzyme to modify its activity, either activating or inhibiting the enzyme.
Action Items / Next Steps
- Review specific examples of each type of enzyme inhibition to reinforce understanding.
- Prepare for the next lecture, which will cover the cell as the basis of life, starting with cell theory and cell size.