Introduction to Enzymes

• Enzymes are biological catalysts
• Enzymes speed up chemical reactions without being consumed in the process
• Most enzymes consist of proteins
• Approximately 4000 different enzymes exist within each living cell

Enzyme Specificity

• Each enzyme has a unique 3D shape, which determines the reaction it can catalyze
• Enzymes catalyze reactions for a specific type of molecule or a group of closely related molecules
• An enzymes structure determines it's function

Enzyme Structures

• The substrate is recognized by and binds to the enzyme
• The active site is a small section of the enzyme, where the substrate interacts
• The active site forms a pocket or groove within the 3D enzyme shape

Substrate-Enzyme Complex

• A substrate-enzyme complex is formed when a substrate attaches to the active site of an enzyme

Allosteric Site

• An enzyme can have an allosteric site, where molecules bind to activate or inhibit enzymatic activity
• The allosteric site is different from the active site
• Substances binding at the allosteric site can affect the enzyme and active site's shape

Other Factors

• A cofactor is a non-protein group that binds to an enzyme to assist in catalytic activity
• A coenzyme is an organic molecule that works as a cofactor for an enzyme, often shuttling molecules between enzymes

Chemical Reaction Rate Increase Factors

• The initial energy required to start a chemical reaction is termed activation energy (Ea)
• Molecules gain the necessary energy (Ea) and enter the transition state, where bonds become unstable and ready to break
• Enzymes accelerate chemical reactions by lowering this activation energy
• Enzymes do not supply energy to a reaction, they only reduce the amount of energy required
• Enzymes can increase the reaction rate by straining or weakening bonds in the substrate molecule, or by proper molecules positioning

Lock and Key Model

• The lock and key model suggests the substrate and the active site contain complementary geometric shapes
• The model's shapes allow them to fit together like a lock and key
• The model does not include enzyme flexibility to stabilize the transition state

Induced-Fit Hypothesis

• In the induced-fit hypothesis enzymes are flexible
• Before a substrate binds to an enzyme, the enzyme undergoes a conformational change
• The conformational change allows the enzyme to better accommodate the precise shape of the substrate

Factors Affecting Enzyme Activity

• Enzyme concentration
• Substrate concentration
• Temperature
• pH

Effect of Enzyme Concentration

• With excess substrate, the reaction rate is proportional to the enzyme concentration
• The amount of enzymes limits the reaction rate
• With an increased enzyme concentration, the rate of reaction will consequently increase

Effect of Substrate Concentration

• As the amount of the substrate increases, the reaction rate will increase until it reaches a saturation level, given a constant intermediate concentration of enzyme
• This saturation occurs when the enzymes reach the maximal reaction rate
• After saturation, increased substrate concentration has a reduced effect

Effect of pH Levels

• Enzymes have an optimal pH for enzyme efficiency
• Structure and active site function is impacted if an enzymes pH level is not optimal
• An increase or decrease in pH from the optimal decreases reaction rate
• Optimal pH is near the contents of a cells pH (pH = 7) with variances based on enzyme location

Effect of Temperature

• An increase in temperature will result in higher kinetic motion of molecules and therefore will increase the reactions rate
• Proteins (enzymes) are impacted by temperature
• Increased temperature will increase kinetic motion of the enzymes amino acid chains, therefore increasing the rate of reactivity
• If temperature passes critical point, the enzyme will denature, reducing enzyme activity

Enzyme Inhibition

• Enzyme inhibition reduces the rate at which an enzyme catalyzes a reaction
• This occurs when a competitor substance attached to an enzyme decreases activity
• Two main types of enzyme inhibition: competitive and non-competitive inhibition

Competitive Inhibition

• In competitive inhibition, a competitor substance resembles the substrate and binds to the active site, preventing normal substrate enzyme activity
• The competitor substance competes with the substrate for access to the enzyme's active site

Non-Competitive Inhibition

• In non-competitive inhibition, the competitor substance binds to the enzyme at a location other than the active site
• With non-competitive inhibition, enzyme shape changes, reducing the substrate’s ability to bind efficiently

Allosteric Activation

• In allosteric activation, an allosteric activator molecule binds to the allosteric site
• Molecule binding causes the enzyme's active site to increase the affinity for the substrate

Allosteric Inhibition

• In allosteric inhibition, an allosteric inhibitor molecule binds to the allosteric site
• Molecule binding causes the enzyme to change shape and releasing the substrate from its active site

Allosteric Feedback Inhibition

• Feedback inhibition regulates pathways using pathway products
• If excess product accumulates, it will allosterically inhibit a previous reaction enzyme
• This leads to homeostasis/regulation