Understanding Enzymes

• Enzymes are efficient catalysts for biochemical reactions.
• They speed up reactions by providing an alternative pathway with lower activation energy.
• Enzymes participate in reactions but remain unchanged at the end.
• They alter reaction rates but not equilibrium positions.
• Most chemical catalysts are not selective, but enzymes are highly selective.
• Enzyme specificity is due to the shapes of enzyme molecules.
• Many enzymes consist of a protein and a non-protein (cofactor).
• Enzyme proteins are usually globular.
• Enzyme activity is sensitive to pH and temperature due to bond disruptions in protein structures.
• Cofactors can be prosthetic groups, activators (positively charged metal ions), or coenzymes (organic molecules).
• Reacting molecules must collide with correct orientation and sufficient energy to overcome activation energy.
• Enzymes provide an active site conducive to binding reacting molecules (substrates).
• Enzyme-substrate complexes form intermediates with lower activation energy routes.
• Amylase is a digestive enzyme that breaks down starches.
• Aspartate aminotransaminase (AST) and alanine aminotransaminase (ALT) help the liver convert sugar into energy.
• Creatine kinase (CK) helps muscles create energy.
• DNA polymerase facilitates growth by allowing DNA to duplicate.
• Glucose-6-phosphate dehydrogenase keeps red blood cells healthy by preventing damage to the cell.
• Lipase is a digestive enzyme that breaks down fats and oils
• Protease is a digestive enzyme that breaks down proteins.
• Thrombin causes blood to clot.

Models of Enzyme Action

• The active site is part of the molecule that has just the right shape and functional groups to bind to one of the reacting molecules.
• The substrate is the reacting molecule that binds to the enzyme.
• The Lock and Key Hypothesis says substrates fit into the enzyme's active site to form an intermediate.
• The Induced Fit Hypothesis says enzymes change shape when substrates approach, induced by substrate molecules.

Factors Affecting Catalytic Activity of Enzymes

• Optimal enzyme activity is usually around 37.5°C for human enzymes but higher temperatures increase kinetic energy and collision rates.
• Enzymes denature at temperatures above optimal as bonds break.
• Enzymes work within a narrow pH range for optimal activity.
• pH changes can make or break bonds, altering enzyme shape and effectiveness.
• The rate of enzyme catalyzed reactions depends on enzyme and substrate concentrations.
• Higher substrate concentration increases rate until active sites are saturated.
• With constant substrate concentration, reaction rate is proportional to enzyme concentration.
• Inhibitors reduce or stop enzyme activity by blocking or distorting the active site.
• Competitive inhibitors occupy the active site.
• The inhibitor (molecule) has a structural and chemical similarity to the substrate.
• Non-competitive inhibitors attach elsewhere (allosteric site) and alter enzyme shape.
• When the inhibitor binds to the allosteric site, the enzyme's active site undergoes structural shift.
• The active site and substrate no longer share affinity as a result of this alteration, preventing substrate binding.
• Uncompetitive inhibition binds only to the substrate-enzyme complex.
• Non-competitive inhibition can occur with or without the presence of the substrate, whereas uncompetitive inhibition requires the formation of an enzyme-substrate complex.