Enzymes and Energy in Biological Systems

Metabolic Pathways and Enzyme Specificity

Metabolic pathways are sequences of chemical reactions occurring within a cell, each step catalyzed by a specific enzyme. Enzyme specificity ensures that each enzyme only catalyzes a particular reaction, converting a specific reactant (substrate) into a specific product.

• Metabolic pathway: A series of enzyme-catalyzed reactions transforming molecules stepwise.

• Enzyme specificity: The property by which an enzyme selects and acts on a particular substrate.

• Example: Glycolysis is a metabolic pathway where glucose is converted to pyruvate through multiple enzyme-catalyzed steps.

Free Energy Changes in Chemical Reactions

Free energy changes determine whether a chemical reaction is energetically favorable. Reactions can be exergonic (release energy) or endergonic (require energy input).

• Exergonic reaction: Releases free energy; spontaneous.

• Endergonic reaction: Requires energy input; nonspontaneous.

• Equation:

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP: Structure, Function, and Hydrolysis

Adenosine triphosphate (ATP) is the primary energy carrier in cells. Energy is stored in the high-energy phosphate bonds and released upon hydrolysis, fueling cellular processes.

• Structure: ATP consists of adenine, ribose, and three phosphate groups.

• ATP hydrolysis:

• Function: Provides energy for cellular work, such as muscle contraction and active transport.

Enzyme Specificity and Mechanism

Enzymes lower the activation energy () of reactions, increasing reaction rates. They achieve specificity through their unique active sites, which bind substrates precisely.

• Activation energy (): The energy required to initiate a reaction.

• Enzyme-substrate complex: Temporary association between enzyme and substrate.

• Impact: Enzymes accelerate reactions without being consumed.

Energy Types and Thermodynamics in Biology

Cells utilize various forms of energy and obey the laws of thermodynamics. Energy transformations are central to metabolism.

• Types of energy: Kinetic, thermal, potential, chemical.

• First law of thermodynamics: Energy cannot be created or destroyed.

• Second law of thermodynamics: Entropy (disorder) increases in spontaneous processes.

• Coupled reactions: Endergonic reactions are powered by exergonic ones (e.g., ATP hydrolysis).

Enzyme Function: Substrate Binding and Regulation

Enzymes bind substrates at their active sites, forming enzyme-substrate complexes. Regulation occurs via allosteric sites, feedback inhibition, and other mechanisms.

• Active site: Region on enzyme where substrate binds.

• Transition state: High-energy state during reaction; stabilized by enzyme.

• Allosteric regulation: Enzyme activity modulated by molecules binding at sites other than the active site.

• Feedback inhibition: End product inhibits an earlier step in the pathway.

Spontaneous vs. Nonspontaneous Reactions

Spontaneous reactions occur without energy input, while nonspontaneous reactions require energy.

• Spontaneous:

• Nonspontaneous:

• Example: Diffusion is spontaneous; active transport is nonspontaneous.

Factors Influencing Enzyme-Catalyzed Reaction Rates

Several factors affect how quickly enzymes catalyze reactions, including substrate concentration, temperature, pH, and presence of inhibitors.

• Substrate concentration: Higher concentration increases rate up to saturation.

• Temperature: Rate increases with temperature until enzyme denatures.

• pH: Each enzyme has an optimal pH.

• Inhibitors: Molecules that decrease enzyme activity.

Enzyme Inhibition: Competitive vs. Noncompetitive

Enzyme inhibitors reduce activity by different mechanisms. Competitive inhibitors bind the active site, while noncompetitive inhibitors bind elsewhere, altering enzyme function.