Unit 3: Chapter 8 – Energy and Enzymes: An Introduction to Metabolism

Overview

This chapter introduces the fundamental concepts of energy in biological systems, the role of enzymes in chemical reactions, and the principles of metabolism. Students will learn how energy is transformed and conserved, how enzymes catalyze reactions, and how metabolic pathways are regulated.

Energy in Chemical Reactions

Types of Energy

Energy is essential for all biological processes. There are two main types of energy relevant to chemical reactions:

• Kinetic energy: The energy of motion. Example: Thermal energy is the kinetic energy of molecules moving.

• Potential energy: The energy stored due to position or configuration. Example: Chemical energy is stored in chemical bonds.

Energy Transformations

Energy can be transformed from one type to another, but it cannot be created or destroyed. This principle is described by the first law of thermodynamics.

• First Law of Thermodynamics: Energy is conserved; it can only be transferred or transformed.

• Examples of energy transformations: Chemical energy in food is converted to kinetic energy for movement, or to thermal energy for heat.

Chemical Energy in Bonds

Potential Energy in Chemical Bonds

The potential energy stored in chemical bonds depends on the configuration and position of shared electrons:

• Weaker bonds with equally shared electrons (nonpolar) have high potential energy.

• Stronger bonds with unequally shared electrons (polar) have low potential energy.

Energy Transformations in Chemical Reactions

Chemical reactions involve breaking and forming bonds, which transforms energy:

• Nonpolar bonds (e.g., C-H) have high potential energy.

• Polar bonds (e.g., O-H) have low potential energy.

• As bonds change from nonpolar to polar, potential energy decreases and energy is released as heat or light.

Thermodynamics in Biology

Enthalpy (H)

Enthalpy is the total energy in a molecule, including potential energy and the effect on surrounding pressure and volume.

• Change in enthalpy (): Difference in potential energy between products and reactants.

• (negative): Products have lower potential energy than reactants (energy released).

• (positive): Products have higher potential energy than reactants (energy absorbed).

Entropy (S)

Entropy is a measure of disorder in a system.

• When products of a reaction are less ordered than reactants, entropy increases ().

• Second Law of Thermodynamics: Total entropy always increases in a system; energy transformations increase the disorder of the universe.

Gibbs Free Energy (G)

Gibbs free energy determines whether a reaction is spontaneous (occurs without added energy).

• Equation: Where is temperature in Kelvin.

• : Reaction is exergonic (spontaneous, energy released).

• : Reaction is endergonic (not spontaneous, energy required).

Reaction Rates and Activation Energy

Factors Affecting Reaction Rates

For chemical reactions to proceed, reactants must collide in the correct orientation and with sufficient energy to break bonds.

• Higher concentration and higher temperature increase the number of collisions, thus increasing reaction rate.

Activation Energy ()

Activation energy is the minimum energy required for reactants to reach the transition state and form products.

• Enzymes lower the activation energy, making reactions proceed faster.

Enzymes: Biological Catalysts

Enzyme Structure and Function

Enzymes are proteins that act as catalysts, speeding up chemical reactions without being consumed.

• Enzymes bring reactants (substrates) together in precise orientations.

• Enzymes are specific for a single type of reaction.

• Substrates bind to the enzyme's active site, forming an enzyme-substrate complex.

• Enzymes may undergo a conformational change called induced fit when substrates bind.

Enzyme Mechanism

• Substrates bind via hydrogen bonding or other interactions with amino acids in the active site.

• An unstable transition state is formed, requiring activation energy ().

• Enzymes stabilize the transition state and lower .

• Enzymes are not consumed during the reaction.

Enzyme Saturation and Reaction Rate

Enzyme-catalyzed reaction rates increase with substrate concentration but level off at high concentrations (saturation).

• Saturation: All enzyme molecules are occupied; maximum reaction rate is reached.

Enzyme Regulation

Cofactors, Coenzymes, and Prosthetic Groups

• Cofactors: Inorganic ions (e.g., Zn2+, Mg2+, Fe2+) that reversibly interact with enzymes.

• Coenzymes: Organic molecules (e.g., NADH, FADH2) that interact with enzymes.

• Prosthetic groups: Non-amino acid atoms or molecules permanently attached to proteins.

Enzyme Inhibition and Activation

• Competitive inhibition: A molecule competes with the substrate for the active site, blocking substrate binding.

• Allosteric regulation: A molecule binds at a site other than the active site, causing a change in enzyme shape that can activate or inhibit the enzyme.

• Covalent modification: Enzyme activity can be regulated by covalent changes, such as phosphorylation (addition of phosphate group) or cleavage of peptide bonds.

Metabolic Pathways

Anabolic and Catabolic Pathways

• Anabolic pathways: Small molecules are assembled into large ones; energy is required.

• Catabolic pathways: Large molecules are broken down into small ones; energy is released.

Feedback Inhibition

• Feedback inhibition: The end product of a metabolic pathway inhibits an enzyme involved earlier in the pathway, preventing overproduction.

• This regulation helps maintain homeostasis in the cell.

Summary Table: Key Terms and Concepts

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