Rucete ✏ AP Biology In a Nutshell
5. Enzymes
This chapter covers the structure and function of enzymes, the environmental factors that affect them, how they lower activation energy, and how energy coupling supports metabolism in living organisms.
Enzyme Structure and Function
• Enzymes are biological catalysts—mostly proteins (some RNA-based ribozymes).
• They speed up chemical reactions by lowering activation energy.
• Enzymes have a specific 3D tertiary structure and an active site that binds the substrate.
• Substrate must match the shape and charges of the active site.
• Active sites with charged R-groups repel or attract specific substrates.
• Enzyme-substrate interaction follows a lock-and-key or induced fit model.
• Increasing substrate concentration increases reaction rate until enzymes are saturated.
Environmental Factors Affecting Enzymes
• Enzymes function best at their optimal temperature and pH.
• Low temperature: fewer collisions → slower reaction.
• High temperature: denatures enzyme (disrupts bonds and structure).
• Extreme pH: alters ionic environment, denatures enzyme.
• Denaturation = loss of structure and function; sometimes reversible.
Inhibitors
• Competitive inhibitors: – Resemble substrate, bind active site. – Can be outcompeted by more substrate.
• Noncompetitive (allosteric) inhibitors: – Bind elsewhere, not the active site. – Change enzyme shape and function. – Cannot be outcompeted by more substrate. – Used in feedback regulation mechanisms.
Cofactors and Coenzymes
• Cofactors: inorganic molecules (e.g., metals) that assist enzymes.
• Coenzymes: organic molecules (e.g., vitamins) that enhance substrate binding.
• Example: B vitamins help form NAD, important in cellular respiration.
Activation Energy
• Activation energy (Ea): the energy required to initiate a reaction.
• Enzymes lower activation energy by stabilizing the transition state.
• This increases the rate of product formation without being consumed in the reaction.
• Enzymes do not change the overall free energy (ΔG) of the reaction—only the pathway.
Metabolism and Energy Coupling
• Metabolism includes all chemical reactions in a cell: – Catabolic: break down molecules, release energy (e.g., cellular respiration). – Anabolic: build molecules, require energy (e.g., photosynthesis, protein synthesis).
• Coupled reactions: pair exergonic (energy-releasing) and endergonic (energy-consuming) reactions to drive processes forward.
• ATP is the universal energy currency—hydrolysis of ATP provides energy to drive cellular work.
ATP and the Phosphorylation Cycle
• ATP = adenosine triphosphate (adenine + ribose + 3 phosphate groups).
• ATP → ADP + Pi + energy (exergonic reaction).
• Cells use released energy to drive endergonic reactions via phosphorylation (adding a phosphate to a molecule).
• The ATP cycle continuously regenerates ATP from ADP and Pi using energy from catabolism.
Enzyme Evolution and Specificity
• Enzyme specificity is a result of protein folding and active site shape.
• Mutations can alter enzyme structure, changing function or leading to new functions (e.g., antibiotic resistance).
• Natural selection favors enzymes that improve survival and metabolic efficiency.
In a Nutshell
Enzymes catalyze essential biochemical reactions by lowering activation energy and enabling precise control of metabolic pathways. Their structure determines their specificity and activity, which are influenced by environmental conditions and inhibitors. Coupled reactions and the use of ATP allow cells to drive necessary endergonic processes, making enzymes vital for life.