Enzymes and Metabolic Reactions

Spontaneous Reactions

Spontaneous reactions are chemical processes that occur without the need for added energy. However, even though they are energetically favorable, they may proceed very slowly unless assisted by catalysts.

• Definition: Spontaneous reactions do not require added energy to occur.

• Important Note: These reactions can still be very slow or imperceptibly slow without assistance.

Catalysts

Catalysts are substances that increase the rate of a chemical reaction without being consumed in the process.

• Definition: A catalyst is a chemical agent that speeds up a reaction without being consumed.

• Example: Enzymes are biological catalysts.

Enzymes: Biological Catalysts

Definition and Function

An enzyme is a macromolecule, usually a protein, that acts as a biological catalyst to speed up specific chemical reactions in living organisms.

• Function: Enzymes speed up specific chemical reactions in the body or cell.

• Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Example Reaction:

• Adding sucrase to a sucrose solution at room temperature results in rapid hydrolysis of sucrose into glucose and fructose.

Equation:

The Activation Energy Barrier

Chemical Reactions and Bonds

All chemical reactions involve breaking existing bonds and forming new ones between molecules.

• Key Point: Every chemical reaction involves both breaking existing bonds and forming new ones.

Activation Energy (EA)

Activation energy is the initial energy required to start a chemical reaction.

• Definition: Activation energy is the initial investment of energy needed to start a reaction.

• Purpose: It pushes reactants over an energy barrier, allowing the energy-releasing portion of the reaction to begin.

• Supplied By: Often provided by heat (thermal energy) from the surroundings.

Transition State

During a reaction, molecules absorb energy and reach an unstable, high-energy state called the transition state.

• Significance: At the transition state, bonds can break and the reaction can proceed.

Energy Profile of a Reaction

Consider a hypothetical exergonic reaction:

• The activation energy () is represented by the uphill portion of the energy graph.

• At the summit (transition state), reactants are activated and their bonds can be broken.

• The downhill portion represents the formation of new bonds and the release of free energy.

How Enzymes Speed Up Reactions

Role of Enzymes

Enzymes catalyze reactions by lowering the activation energy barrier, allowing reactions to occur at moderate temperatures.

• Organisms carry out catalysis: Catalysts selectively speed up reactions without being consumed.

• Enzyme function: Enzymes lower enough for reactions to occur at moderate temperatures.

• Limits: Enzymes cannot change (free energy change); they only speed up reactions that would occur anyway.

Equation:

• is unaffected by enzyme; is lowered.

Substrate Specificity of Enzymes

Enzyme-Substrate Complex

Enzymes are highly specific for their substrates, forming an enzyme-substrate complex during catalysis.

• Substrate: The reactant that an enzyme acts upon.

• Enzyme-substrate complex: The temporary association formed when an enzyme binds its substrate.

• Product formation: The enzyme converts substrate to product while bound.

• Naming: Most enzyme names end in -ase (e.g., sucrase).

Active Site of Enzyme

The active site is the region on the enzyme where substrate binding and catalysis occur.

• Substrate recognition: Enzymes recognize specific substrates, even among closely related compounds.

• Active site: A pocket or groove on the enzyme that binds the substrate and is the site of catalysis.

• Complementary fit: The shape of the active site matches the substrate, ensuring enzyme specificity.

Additional info: The notes above are expanded and clarified for academic completeness, including definitions, examples, and equations relevant to General Biology students.