Energy & Enzymes

Introduction to Energy in Biological Systems

Energy is fundamental to all biological processes, driving chemical reactions that sustain life. In cells, energy transformations are tightly regulated and often require the assistance of enzymes to proceed efficiently.

• Energy is the capacity to do work or cause change, especially in chemical reactions.

• Enzymes are biological catalysts that accelerate chemical reactions without being consumed.

• Most cellular reactions would not occur at appreciable rates without enzymes.

Glucose Combustion and Cellular Respiration

Spontaneity and Activation Energy

The combustion of glucose is a highly exergonic reaction, releasing energy. However, in biological systems, this reaction does not occur spontaneously due to the requirement for activation energy.

• Combustion Reaction:

• Free Energy Change: kcal/mol (highly exergonic)

• The reaction is not spontaneous unless an enzyme is present to lower the activation energy.

• Energy is required for reactants to reach the transition state.

• Phosphorylation of sugar increases its potential energy, facilitating the reaction.

Activation Energy and Reaction Progress

Understanding Activation Energy

Activation energy () is the minimum energy required for reactants to undergo a chemical transformation. Enzymes lower this barrier, allowing reactions to proceed at physiological temperatures.

• Transition State: The high-energy state reactants must reach for the reaction to occur.

• Activation Energy (): The energy difference between reactants and the transition state.

• Free Energy Change (): The difference in energy between reactants and products; determines spontaneity.

Example: In the absence of an enzyme, burning sugar requires high heat to overcome activation energy. In cells, enzymes lower , enabling controlled energy release.

Factors Affecting Reaction Rates

Temperature and Concentration

Reaction rates can be increased by raising temperature or reactant concentration, both of which enhance the frequency and energy of molecular collisions.

• Temperature: Higher temperatures increase kinetic energy, leading to more frequent and energetic collisions.

• Concentration: Higher reactant concentrations result in more collisions per unit time.

Equation: The rate of a bimolecular reaction is given by:

Example: Experimental data show that increasing temperature or concentration increases reaction rates.

Enzyme Structure and Function

Active Site and Catalysis

The active site of an enzyme is the region where substrate molecules bind and undergo a chemical reaction. Enzyme specificity and catalytic efficiency are determined by the structure of the active site.

• Active Site: Region involved in the catalytic reaction of the enzyme.

• Can bind substrates, competitive inhibitors, and allosteric regulators.

• Shape changes during catalysis facilitate the reaction.

Example: Enzymes do not change the overall of a reaction, but they lower the activation energy (), increasing the rate.

Enzyme Mechanism

Stages of Enzyme Action

Enzyme-catalyzed reactions proceed through several stages, including substrate binding, transition state formation, and product release.

• Initiation: Substrates bind to the active site.

• Transition State Facilitation: Enzyme stabilizes the transition state, lowering .

• Termination: Products are released, and the enzyme is free to catalyze another reaction.

Enzyme Denaturation

Effect of Temperature on Enzyme Structure

Enzymes are proteins that can lose their functional shape (denature) at high temperatures, rendering them inactive.

• Denaturation: Loss of three-dimensional structure due to heat, pH, or chemicals.

• Human enzymes, such as trypsin, denature at temperatures above normal body temperature (37°C).

• At 70°C, most human enzymes are denatured and inactive.

Enzyme Regulation

Types of Enzyme Inhibition

Enzyme activity can be regulated by inhibitors and covalent modifications, allowing cells to control metabolic pathways.

• Competitive Inhibition: Inhibitor competes with substrate for the active site, blocking substrate binding.

• Allosteric Regulation: Regulatory molecule binds at a site other than the active site, causing a conformational change that affects enzyme activity.

• Covalent Modification: Addition or removal of chemical groups (e.g., phosphorylation) can activate or deactivate enzymes.

Phosphorylation and Enzyme Activity

Phosphorylation is a common covalent modification that regulates enzyme activity by adding a phosphate group to specific amino acids.

• Phosphorylation: Addition of a phosphate group (PO4) to serine, threonine, or tyrosine residues.

• Can switch enzymes between active and inactive forms.

• May alter enzyme shape, location, stability, or pH optimum.

Example: Unphosphorylated enzyme is inactive; phosphorylation activates the enzyme.

Metabolic Pathways and Feedback Inhibition

Pathway Regulation by End Products

Metabolic pathways are sequences of enzyme-catalyzed reactions. Feedback inhibition occurs when the end product inhibits an enzyme earlier in the pathway, preventing overproduction.

• Feedback Inhibition: End product binds to and inhibits an enzyme, often at an allosteric site.

• Prevents accumulation of intermediates and conserves resources.

• Example: If arginine inhibits Enzyme 3, precursor and ornithine build up, and pathway slows.

Summary Table: Enzyme Regulation Mechanisms

Additional info: These notes expand on the original slides by providing definitions, examples, and context for key concepts in enzyme function and regulation, suitable for General Biology students.