An Introduction to Metabolism (Chapter 8): Study Notes

An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within an organism. These reactions are organized into metabolic pathways, where a specific molecule is modified in a series of steps, each catalyzed by a unique enzyme, to yield a final product.

• Metabolic pathway: A sequence of chemical reactions, each step catalyzed by a specific enzyme.

• Enzyme: A macromolecule (usually a protein) that acts as a catalyst to speed up a specific reaction.

• Reactants of one step are the products of the previous step.

Catabolic and Anabolic Pathways

• Catabolic pathways: Release energy by breaking down complex molecules into simpler compounds. Example: Cellular respiration (breakdown of glucose in the presence of O2).

• Anabolic pathways: Consume energy to build complex molecules from simpler ones. Example: Protein synthesis from amino acids.

Energy Transformations in Cells

• Cells transform energy from one form to another to perform work (e.g., chemical, transport, mechanical).

• Chemical energy from food is used for activities such as climbing.

• Kinetic energy (movement) can be converted to potential energy (position), and vice versa.

The Laws of Thermodynamics in Biology

The First Law of Thermodynamics

• Energy can be transferred and transformed, but it cannot be created or destroyed.

• Also known as the principle of conservation of energy.

• During energy transfers, some energy is lost as heat, making it unavailable to do work.

The Second Law of Thermodynamics

• Every energy transfer or transformation increases the entropy (disorder) of the universe.

• Living organisms increase the disorder of their surroundings through metabolism (e.g., breakdown of food releases heat and CO2).

• Spontaneous processes: Increase entropy and occur without energy input.

• Nonspontaneous processes: Decrease entropy and require energy input.

Free Energy and Metabolic Reactions

• Free energy change (ΔG): The difference in free energy between the final state (products) and the initial state (reactants).

• Catabolic (exergonic) reactions: Energy is released, ΔG < 0, spontaneous.

• Anabolic (endergonic) reactions: Energy is consumed, ΔG > 0, nonspontaneous.

Anabolic	Catabolic
Build up (decreases entropy)	Break down (increases entropy)
Requires energy	Releases energy
Non-spontaneous	Spontaneous
ΔG > 0 (Endergonic)	ΔG < 0 (Exergonic)

ATP: The Energy Currency of the Cell

ATP Structure and Hydrolysis

• ATP (adenosine triphosphate): Composed of ribose (sugar), adenine (nitrogenous base), and three phosphate groups.

• Energy is released from ATP when the terminal phosphate bond is broken by hydrolysis (addition of water).

• The energy comes from the chemical change to a state of lower free energy, not directly from the phosphate bonds.

ATP and Cellular Work

• ATP hydrolysis powers cellular work by coupling exergonic reactions (ATP hydrolysis) to endergonic reactions (cellular work).

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making the recipient molecule more reactive (less stable, with more free energy).

• ATP hydrolysis causes changes in protein shape and binding ability, essential for transport and mechanical work.

The ATP Cycle

• ATP is regenerated by the addition of a phosphate group to ADP (adenosine diphosphate).

• Free energy needed to phosphorylate ADP comes from exergonic breakdown reactions (catabolism).

• The ATP cycle couples energy-yielding processes to energy-consuming ones.

Enzymes and Metabolic Reactions

Enzymes as Catalysts

• Enzyme: A macromolecule (usually protein) that acts as a catalyst to speed up a specific reaction without being consumed.

• Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

The Activation Energy Barrier

• Every chemical reaction involves bond breaking and forming.

• Reactant molecules must reach a highly unstable state (transition state) by absorbing energy (activation energy, EA).

• Enzymes lower the activation energy barrier, allowing reactions to occur at moderate temperatures.

Substrate Specificity and the Active Site

• The reactant an enzyme acts on is called its substrate.

• The enzyme binds to its substrate, forming an enzyme-substrate complex.

• The active site is the region on the enzyme where the substrate binds, often a pocket or groove.

• Induced fit: The active site changes shape to fit the substrate more closely, enhancing catalysis.

• Most enzyme names end in -ase (e.g., sucrase, lactase).

Mechanisms of Enzyme Action

• Enzymes lower EA by:

  • Orienting substrates correctly

  • Straining substrate bonds

  • Providing a favorable microenvironment

  • Directly participating in the reaction via amino acid side chains

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature for maximum activity (e.g., human enzymes ~37°C, thermophilic bacteria ~75°C).

• pH: Each enzyme has an optimal pH (e.g., pepsin in the stomach pH 2, trypsin in the intestine pH 8).

Cofactors and Coenzymes

• Cofactors: Nonprotein helpers required for enzyme function; may be inorganic (metal ions like Zn2+, Fe2+, Cu2+) or organic.

• Coenzymes: Organic cofactors, often derived from vitamins.

Enzyme Inhibitors

• Competitive inhibitors: Resemble the substrate and bind to the active site, blocking substrate binding.

• Noncompetitive inhibitors: Bind elsewhere on the enzyme, causing a conformational change that reduces active site effectiveness.

Regulation of Metabolic Pathways

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

• Compartmentalization: Cellular structures help organize and regulate metabolic pathways by localizing enzymes and substrates.

Summary Table: Key Concepts in Metabolism

Concept	Definition/Role	Example
Catabolic Pathway	Breaks down molecules, releases energy	Cellular respiration
Anabolic Pathway	Builds molecules, consumes energy	Protein synthesis
ATP	Main energy currency of the cell	Drives cellular work
Enzyme	Catalyst for biochemical reactions	Sucrase
Cofactor	Nonprotein enzyme helper	Zinc ion, NAD+
Competitive Inhibitor	Blocks active site	Sulfa drugs (antibiotics)
Noncompetitive Inhibitor	Changes enzyme shape	Heavy metals

Key Equations

• Cellular respiration: $\mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{O}_2 \rightarrow 6\mathrm{CO}_2 + 6\mathrm{H}_2\mathrm{O}$

• Free energy change: $\Delta G = G_{\text{final state}} - G_{\text{initial state}}$

• ATP hydrolysis: $\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

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