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Study Guide - Smart Notes

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Metabolic Pathways and Enzyme Function

Definition and Organization of Metabolic Pathways

Metabolic pathways are organized sequences of chemical reactions occurring within a cell, each step catalyzed by a specific enzyme. These pathways convert substrates into products, facilitating cellular processes essential for life.

Metabolic pathway: A series of defined steps, each catalyzed by a specific enzyme, converting a specific reactant molecule into a specific product.
Enzyme specificity: Each enzyme catalyzes only one type of reaction or acts on a specific substrate.
Example: Glycolysis is a metabolic pathway that breaks down glucose into pyruvate through a series of enzyme-catalyzed steps.

Free Energy Changes in Chemical Reactions

The change in free energy (ΔG) during a reaction determines whether the reaction is energetically favorable. Reactions can be classified as exergonic (energy-releasing) or endergonic (energy-consuming).

Exergonic reactions: Release energy; ΔG is negative.
Endergonic reactions: Require energy input; ΔG is positive.
Coupled reactions: Cells often couple endergonic and exergonic reactions to drive necessary processes.
Equation:
Example: ATP hydrolysis is exergonic and can be coupled to endergonic reactions.

ATP Structure and Function

ATP as the Energy Currency of the Cell

Adenosine triphosphate (ATP) stores and provides energy for many cellular processes. Hydrolysis of ATP releases energy that can be used to drive endergonic reactions.

ATP structure: Composed of adenine, ribose, and three phosphate groups.
ATP hydrolysis:
Function: ATP hydrolysis "fuels" other reactions in the cell by providing free energy.
Example: Muscle contraction and active transport use ATP hydrolysis for energy.

Enzyme Specificity and Reaction Rates

Enzyme Function and Activation Energy

Enzymes are biological catalysts that lower the activation energy (Ea) required for reactions, increasing the rate at which reactions occur.

Activation energy (Ea): The energy barrier that must be overcome for a reaction to proceed.
Enzyme specificity: Enzymes bind specific substrates at their active sites, forming enzyme-substrate complexes.
Impact: Lowering Ea allows reactions to occur more rapidly and at lower temperatures.
Example: Catalase catalyzes the breakdown of hydrogen peroxide into water and oxygen.

Key Terms and Definitions

Term	Definition
Catabolic	Pathways that break down molecules, releasing energy.
Anabolic	Pathways that build complex molecules from simpler ones, requiring energy.
Thermodynamics	The study of energy transformations in biological systems.
Enzyme-substrate complex	Temporary association between an enzyme and its substrate during catalysis.
Allosteric regulation	Regulation of enzyme activity by binding of molecules at sites other than the active site.
Feedback inhibition	End product of a pathway inhibits an earlier step to regulate pathway activity.
Competitive inhibition	Inhibitor competes with substrate for binding at the active site.
Noncompetitive inhibition	Inhibitor binds elsewhere on the enzyme, altering its activity.

Spontaneous vs. Nonspontaneous Reactions

Classification of Chemical Reactions

Reactions are classified based on whether they occur without energy input (spontaneous) or require energy (nonspontaneous).

Spontaneous reactions: Occur naturally; ΔG is negative.
Nonspontaneous reactions: Require energy input; ΔG is positive.
Example: Cellular respiration is spontaneous; photosynthesis is nonspontaneous and requires energy from sunlight.

Factors Influencing Enzyme-Catalyzed Reaction Rates

Regulation and Inhibition

Several factors affect the rate of enzyme-catalyzed reactions, including substrate concentration, temperature, pH, and the presence of inhibitors.

Substrate concentration: Higher concentrations increase reaction rate up to a saturation point.
Temperature and pH: Each enzyme has optimal conditions for activity.
Inhibitors: Competitive and noncompetitive inhibitors decrease enzyme activity.
Allosteric regulation: Molecules bind to sites other than the active site to modulate enzyme activity.
Feedback inhibition: End product inhibits an earlier step in the pathway.

Comparison of Competitive and Noncompetitive Inhibition

Type of Inhibition	Mechanism	Effect on Enzyme
Competitive	Inhibitor binds to active site, blocking substrate	Can be overcome by increasing substrate concentration
Noncompetitive	Inhibitor binds to a different site, changing enzyme shape	Cannot be overcome by increasing substrate concentration

Additional info: Academic context and definitions have been expanded for clarity and completeness.