Chapter 8: An Introduction to Metabolism – Study Notes

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Chapter 8: An Introduction to Metabolism

Overview: The Energy of Life

Metabolism encompasses all chemical reactions within living cells, enabling the assembly and breakdown of molecules, energy extraction, and cellular work. These reactions are highly coordinated and responsive to environmental changes.

Metabolism: The sum of all chemical reactions in an organism.
Cells assemble small molecules into polymers and hydrolyze them as needed.
Cellular respiration extracts energy from sugars and fuels to perform work.
Metabolic reactions are efficient, coordinated, and responsive.

Concept 8.1: Metabolism Transforms Matter and Energy, Subject to the Laws of Thermodynamics

Metabolic Pathways

Metabolic pathways are sequences of chemical reactions, each catalyzed by a specific enzyme, that transform molecules within cells.

Catabolic pathways: Break down complex molecules, releasing energy (e.g., cellular respiration).
Anabolic pathways: Build complex molecules from simpler ones, consuming energy (also called biosynthetic pathways).
Energy released from catabolic pathways can drive anabolic pathways.
Bioenergetics: The study of how energy flows through living organisms.

Forms and Transformations of Energy

Energy: The capacity to cause change or do work.
Kinetic energy: Energy of motion (e.g., thermal energy, heat).
Potential energy: Stored energy due to position or structure (e.g., chemical energy in bonds).
Energy can be converted between forms (e.g., chemical to kinetic).

The Laws of Thermodynamics

Thermodynamics: Study of energy transformations.
System: Matter under study; surroundings: everything else.
Isolated system: No exchange of energy/matter with surroundings.
Open system: Energy/matter can be exchanged (organisms are open systems).
First Law of Thermodynamics: Energy can be transferred and transformed, but not created or destroyed (principle of conservation of energy).
Second Law of Thermodynamics: Every energy transfer or transformation increases the entropy (disorder) of the universe.
Entropy (S): Measure of disorder or randomness.
Spontaneous processes increase the entropy of the universe and can occur without energy input.

Example: Cellular respiration increases entropy by converting glucose and oxygen into carbon dioxide and water, releasing heat.

Concept 8.2: Free-Energy Change and Spontaneity of Reactions

Free Energy (G) and Spontaneity

Free energy is the portion of a system's energy available to do work at constant temperature and pressure. The change in free energy (ΔG) predicts whether a process is spontaneous.

Equation:
ΔH: Change in enthalpy (total energy); ΔS: Change in entropy; T: Temperature in Kelvin.
Spontaneous processes: ΔG < 0 (free energy decreases).
At equilibrium: ΔG = 0 (no work can be done).

Example: Cellular respiration is highly exergonic (ΔG = –686 kcal/mol for glucose breakdown).

Exergonic vs. Endergonic Reactions

Exergonic reaction: Releases free energy (ΔG negative); spontaneous.
Endergonic reaction: Absorbs free energy (ΔG positive); nonspontaneous.
Cells maintain metabolic disequilibrium by constant input/output of materials, preventing equilibrium (which would mean cell death).

Example: Photosynthesis is endergonic (ΔG = +686 kcal/mol), powered by light energy.

Concept 8.3: ATP Powers Cellular Work by Coupling Exergonic and Endergonic Reactions

ATP Structure and Function

ATP (Adenosine Triphosphate): Composed of ribose (sugar), adenine (nitrogenous base), and three phosphate groups.
ATP hydrolysis releases energy by breaking the terminal phosphate bond:

Standard ΔG for ATP hydrolysis: –7.3 kcal/mol (–30.5 kJ/mol); in cells, about –13 kcal/mol.
Energy is released due to the instability of the crowded negative charges on the phosphate groups.

Energy Coupling and Phosphorylation

ATP hydrolysis is coupled to endergonic reactions via phosphorylation (transfer of a phosphate group to another molecule).
The recipient molecule (phosphorylated intermediate) is more reactive.
ATP powers chemical, transport, and mechanical work in cells.

Example: Motor proteins "walk" along cytoskeletal tracks using ATP hydrolysis.

The ATP Cycle

ATP is regenerated from ADP and inorganic phosphate:

Regeneration is endergonic (requires energy input).
Catabolic pathways (e.g., cellular respiration) provide energy for ATP synthesis.
ATP is recycled rapidly (millions of molecules per cell per second).

Concept 8.4: Enzymes Speed Up Metabolic Reactions by Lowering Energy Barriers

Activation Energy and Catalysis

Enzyme: Macromolecule (usually protein) that acts as a catalyst, speeding up reactions without being consumed.
Every reaction requires an initial input of energy (activation energy, EA) to reach the transition state.
Enzymes lower EA, allowing reactions to proceed at cellular temperatures.
Enzymes do not change ΔG; they only speed up reactions that would occur eventually.

Enzyme Specificity and Mechanism

Substrate: The reactant an enzyme acts on.
Active site: Region on the enzyme where the substrate binds.
Induced fit: Enzyme changes shape to fit the substrate more closely.
Enzymes can catalyze thousands of reactions per second and are reusable.
Enzymes lower activation energy by:
- Orienting substrates correctly
- Straining substrate bonds
- Providing a favorable microenvironment
- Briefly forming covalent bonds with substrates

Factors Affecting Enzyme Activity

Enzyme activity is influenced by temperature, pH, and the presence of cofactors.
Each enzyme has optimal temperature and pH conditions.
Cofactors: Nonprotein helpers (inorganic ions like Zn2+, Fe2+, Cu2+ or organic coenzymes, often derived from vitamins).

Enzyme Inhibition

Competitive inhibitors: Resemble the substrate and compete for the active site; can be overcome by increasing substrate concentration.
Noncompetitive inhibitors: Bind elsewhere on the enzyme, changing its shape and reducing activity.
Many toxins, poisons, and antibiotics act as enzyme inhibitors (e.g., penicillin inhibits bacterial cell wall synthesis).
Cells use natural inhibitors to regulate metabolism.

Concept 8.5: Regulation of Enzyme Activity Helps Control Metabolism

Allosteric Regulation

Allosteric regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme activity (can inhibit or activate).
Most allosteric enzymes have multiple subunits and oscillate between active and inactive forms.
Cooperativity: Substrate binding to one active site increases activity at other sites (e.g., hemoglobin's oxygen binding).

Feedback Inhibition

End product of a metabolic pathway inhibits an enzyme early in the pathway, preventing overproduction.
Feedback inhibition conserves resources and maintains metabolic balance.

Localization of Enzymes

Enzymes may be organized into complexes or localized within specific cellular compartments, increasing metabolic efficiency.
Compartmentalization allows for regulation and coordination of metabolic pathways.

Table: Comparison of Exergonic and Endergonic Reactions

Feature	Exergonic Reaction	Endergonic Reaction
ΔG	Negative (< 0)	Positive (> 0)
Spontaneity	Spontaneous	Nonspontaneous
Energy Flow	Releases energy	Requires energy input
Example	Cellular respiration	Photosynthesis

Table: Types of Enzyme Inhibitors

Type	Binding Site	Effect	Overcome by Substrate?	Example
Competitive	Active site	Blocks substrate binding	Yes	Penicillin
Noncompetitive	Allosteric site	Changes enzyme shape	No	DDT

Additional info: The notes above expand on the original outline by providing definitions, examples, and context for key terms and processes, as well as summarizing the main equations and regulatory mechanisms relevant to metabolism.