Sept. 24 - BIOL 1000

Energy and Enzymes Chapter 8

PASS Peer Assisted Study Sessions

• Leader: Ruth

• Schedule:

  • Mondays: 3:00 PM - 4:30 PM in Bethune College (BC 202)

  • Wednesdays: 12:00 PM - 1:30 PM in BC 202

Metabolism

• Defined as the chemical reactions of life

• Key characteristics:

  • Require energy

  • Involve energy transformations

  • Involve enzymes

Energy

• Definition of Energy: The capacity to do work.

• Types of Energy:

  • Potential Energy: Stored energy due to location or structure.

  • Chemical Potential Energy: Found in chemical bonds.

  • Kinetic Energy: Energy of motion.

Potential Energy in Chemical Bonds

• Potential energy in a covalent bond depends on the position of the shared electrons:

  • If shared electrons are far from the nuclei of both atoms, the bond has higher potential energy.

  • Equal sharing of electrons leads to weaker bonds and higher energy.

  • Unequal sharing results in stronger and lower energy.

• Electronegativity values for common elements:

  • H: 2.2, C: 2.55, N: 3.0, O: 3.44

Kinetic Energy

• Definition: Energy of motion.

• Conversion: Energy can be converted from one form to another.

Laws of Thermodynamics

• Two major laws.

  • First Law of Thermodynamics (Conservation of Energy):

  1. Energy cannot be created or destroyed, only converted from one form to another.

  2. Energy is conserved and can be transferred or transformed.

  • Second Law of Thermodynamics:

  • The total entropy of a system and its surroundings will increase over time.

  • Energy tends to disperse or spread out if not hindered.

Enthalpy and Entropy

• Enthalpy (H): Refers to the total energy in a molecule.

  • Changes in Enthalpy are denoted as ΔH (in living systems, ΔH approximates changes in potential energy).

• Entropy (S): Refers to the amount of disorder.

  • Energy tends to disperse or spread out, leading to increased entropy.

Examples of Increasing Entropy

1. Increase in Volume (e.g., phase change).

2. Increase in Number (e.g., breakdown of a macromolecule).

3. Increase in Molecular Random Motion.

Gibbs Free Energy

• Formula: $(ΔG = ΔH - TΔS)$

• Definition of Gibbs Free Energy (G): Portion of a system’s energy available for work.

• Changes in free energy are important in chemical reactions in living systems.

• Exergonic Reactions:

  • Characterized by negative ΔG (-ΔG).

  • Spontaneous and do not require energy input.

  • Tends to occur when products have lower enthalpy or higher entropy.

• Endergonic Reactions:

  • Characterized by positive ΔG (+ΔG).

  • Require energy input and are non-spontaneous.

  • Typically result in products with higher enthalpy and lower entropy.

Chemical Reactions in Living Systems

• Combustion of Glucose: Involves changes in both enthalpy and entropy, and is an example of a spontaneous reaction.

• Gibbs Free Energy Changes:

  • The equation $(ΔG = ΔH - TΔS)$ is fundamental in understanding metabolic reactions.

Types of Reactions in Metabolism

• Catabolic Reactions:

  • Break down complex molecules into simpler compounds.

  • Release energy (exergonic).

  • Example: Starch converts to glucose.

• Anabolic Reactions:

  • Build more complex molecules from simple ones.

  • Use energy (endergonic).

  • Example: Protein synthesis from amino acids.

Coupling of Reactions

• Endergonic reactions are powered by coupling them with exergonic reactions.

• Energy released from catabolic reactions is utilized to power anabolic reactions.

• Mechanism of Energy Transfer:

  • Energy is transferred via phosphate groups or electrons.

• Active carrier molecules:

  • ATP (Adenosine Triphosphate)

  • FADH2

  • NADH

ATP (Adenosine Triphosphate)

• Function: Stores a large amount of potential energy.

• Structure:

  • Composed of three phosphate groups, ribose, and adenine.

  • Structural representation: O-P-O-P-O-P-O-CH2

• Hydrolysis of ATP:

  • Process:
    $ATP + H<em>2O ightarrow ADP + P</em>i + energy$

  • Releases approximately -30.5 kJ/mol or -7.3 kcal/mol of energy.

Example of ATP Function: Glutamine Synthesis

• Glutamic Acid to Glutamine Reaction:

  • Reaction:
    $NH<em>3 + ext{Glu} ightarrow ext{Gln} + ext{P}</em>{i} + energy$

  • Positive ΔG for glutamic acid conversion to glutamine: ΔG = +3.4 kcal/mol.

• Coupling Reaction:

  • Attributed to ATP hydrolysis,

  • Net ΔG: -3.9 kcal/mol makes the pathway exergonic.

Summary of a Coupled Reaction

• Energy released from ATP hydrolysis is transferred to substrates by:

  • Phosphorylation (adding a phosphate group).

  • This usually causes a change in shape, increasing the free energy and reactivity of the substrate, making it less stable and more reactive.

• Key Insight: Exergonic phosphorylation reactions are coupled to endergonic reactions, facilitating metabolic processes.