LEC 5 Metabolism and Enzyme Function in Cells

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Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring in the cells of an organism. It is essential for managing the material and energy resources of the cell.

Metabolic pathways: Series of chemical reactions that transform molecules through specific steps, often regulated by enzymes.
Catabolic pathways: Break down complex molecules into simpler compounds, releasing energy. Example: Cellular respiration:
Anabolic pathways: Build complex molecules from simpler ones, requiring energy input. Example: Synthesis of proteins from amino acids.

Types of Energy in Biology

Kinetic energy: Energy of moving objects (e.g., heat, electricity).
Potential energy: Stored energy due to position or structure (e.g., chemical bonds).
Energy transformations: Energy can change forms, governed by the laws of thermodynamics.

Laws of Thermodynamics

First Law: Energy cannot be created or destroyed, only transformed.
Second Law: Every energy transfer increases the entropy (disorder) of the universe. Example: Some energy is lost as heat during metabolic processes.

Additional info: Entropy is a measure of disorder; living systems maintain order by expending energy.

Free Energy and Chemical Reactions

Free Energy (G)

Free energy () is the portion of a system's energy that can perform work under constant temperature and pressure.

Exergonic reactions: Release free energy; products have less free energy than reactants. Example: Cellular respiration.
Endergonic reactions: Require energy input; products have more free energy than reactants. Example: Synthesis of glucose in photosynthesis.

Energy Coupling

Cells use exergonic reactions to power endergonic reactions, a process called energy coupling.

Equilibrium in Chemical Reactions

Reactions at equilibrium have and do not perform work.
Cells maintain metabolic pathways far from equilibrium to sustain life.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate): Main energy carrier in cells.
Composed of adenine, ribose, and three phosphate groups.
Hydrolysis of ATP releases energy:
ATP powers cellular work by coupling exergonic and endergonic reactions.

Regeneration of ATP

ATP is regenerated from ADP and phosphate by catabolic pathways such as cellular respiration.

Enzymes and Their Function

Enzyme Properties

Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.
Enzymes are substrate-specific; each enzyme acts on a particular substrate.
The region where the substrate binds is called the active site.
Enzyme-substrate complex formation is essential for catalysis.

Enzyme Specificity and Induced Fit

Enzymes are highly specific due to the shape of their active site.
Induced fit: The active site changes shape slightly to fit the substrate more snugly.

Regulation of Enzyme Activity

Factors Affecting Enzyme Activity

Enzyme activity is affected by pH, temperature, and the presence of cofactors (non-protein helpers).
Cofactors: May be inorganic (e.g., metal ions) or organic (coenzymes).

Enzyme Inhibition

Competitive inhibitors: Bind to the active site, blocking substrate binding. Inhibition is reversible if the inhibitor is weakly bound.
Noncompetitive inhibitors: Bind elsewhere on the enzyme, changing its shape and reducing activity.

Feedback Inhibition

Feedback inhibition is a common regulatory mechanism in metabolic pathways. The end product of a pathway inhibits an enzyme involved earlier in the pathway.

Summary Table: Types of Enzyme Inhibition

Type	Binding Site	Effect
Competitive	Active site	Blocks substrate binding
Noncompetitive	Allosteric site	Changes enzyme shape, reduces activity

Additional info: Enzyme regulation is essential for cellular homeostasis and efficient metabolic control.