Energy, Enzymes, and Metabolic Pathways in General Biology

Energy and Thermodynamics in Biological Systems

Two Laws of Thermodynamics

Biological processes are governed by the laws of thermodynamics, which describe how energy is transferred and transformed in living organisms.

• First Law (Law of Conservation of Energy): Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law: Entropy (disorder) tends to increase over time; energy spreads out, and less energy is available to do work. Spontaneous processes increase entropy.

Systems naturally progress from stable (ordered) states to unstable (disordered) states.

Gibbs Free Energy and Spontaneity

The change in Gibbs free energy ($\Delta G$) determines whether a reaction is spontaneous.

• Formula:

$\Delta G = \Delta H - T \cdot \Delta S$

• $\Delta G$ = Gibbs free energy

• $\Delta H$ = Change in enthalpy (heat content)

• $\Delta S$ = Change in entropy (disorder)

• $T$ = Temperature in Kelvin

• Spontaneous reactions: $\Delta G < 0$ (negative), indicating a loss of free energy and an increase in entropy.

• Exergonic reactions: Release energy ($\Delta H$ negative), often spontaneous.

• Endergonic reactions: Require energy input ($\Delta H$ positive), non-spontaneous.

Metabolic Pathways

Catabolism and Anabolism

Metabolism consists of two major types of pathways: catabolic and anabolic.

• Catabolism: Breakdown of large, complex molecules into smaller, simpler ones, releasing energy. Example: Cellular respiration breaks down glucose into ATP and smaller molecules.

• Anabolism: Synthesis of large, complex molecules from smaller, simpler ones, consuming energy. Example: Protein synthesis uses ATP to build proteins from amino acids.

Catabolic reactions are generally spontaneous due to energy release, lowering the system's total energy.

Types of Energy in Biology

• Potential Energy: Stored energy, capacity to do work (e.g., chemical bonds, position).

• Kinetic Energy: Energy of movement (e.g., molecules in motion).

• Chemical Energy: Energy stored in chemical bonds of molecules such as carbohydrates, lipids, and proteins.

Most energy obtained from food is lost as heat due to the second law of thermodynamics.

ATP Hydrolysis and Energy Coupling

ATP Structure and Hydrolysis

ATP (adenosine triphosphate) is the primary energy currency of the cell. Hydrolysis of ATP releases energy for cellular work.

• Hydrolysis Reaction: Addition of water breaks the bond between two phosphate groups.

• Equation:

$\text{ATP} \rightarrow \text{ADP} + \text{P}_i$

• Breaking the unstable phosphate bond releases energy.

• Energy released can phosphorylate intermediates to drive endergonic reactions.

• ATP is regenerated by phosphorylation of ADP during catabolic processes (e.g., cellular respiration).

Coupled Reactions

Cells couple exergonic reactions (energy-releasing) with endergonic reactions (energy-consuming) to drive essential processes.

• Example: Glucose breakdown (exergonic) is coupled with protein synthesis (endergonic).

Enzymes and Activation Energy

Role of Enzymes

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions by lowering activation energy without being consumed.

• Catalyst: Substance that increases reaction rate without being used up.

• Enzymes do not affect the overall free energy change ($\Delta G$) of a reaction.

• Enzymes are specific to their substrates due to unique three-dimensional active sites.

Activation Energy and Transition State

• Activation Energy ($E_A$): The energy required to initiate a chemical reaction.

• Enzymes lower $E_A$ by stabilizing the transition state.

Enzyme Structure and Function

• Active Site: Region on the enzyme where substrate binds; highly specific due to shape and chemical environment.

• Induced Fit: Binding of substrate induces a change in the enzyme's active site, enhancing substrate binding.

• Allosteric Site: Site distinct from the active site where regulators (activators or inhibitors) bind, affecting enzyme activity.

• Allosteric Activator: Enhances substrate binding.

• Allosteric Inhibitor: Reduces substrate binding efficiency.

• Cooperativity: Binding of one substrate affects binding affinity of others (common in multimeric enzymes).

• Feedback Inhibition: End product of a pathway inhibits an enzyme earlier in the pathway, regulating metabolic flow.

• Cofactors: Non-protein molecules (inorganic ions or organic coenzymes) required for enzyme function.

Enzyme Kinetics and Inhibition

• Competitive Inhibitors: Bind to the active site, competing with the substrate.

• Noncompetitive Inhibitors: Bind to a different site (often allosteric), reducing the number of functional enzymes and lowering maximum reaction rate (Vmax).

• Enzyme-catalyzed reactions reach a maximum rate (Vmax) when all enzymes are saturated with substrate.

Signal Transduction and Receptor Types

Signal Amplification, Diversity, and Regulation

Cells use signal transduction pathways to respond to external signals, often amplifying the response and regulating it through various mechanisms.

• Signal Amplification: One signal molecule can activate many downstream molecules, increasing the response.

• Diversity: The same signal can trigger different pathways depending on the receptor type.

• Regulation: Pathways are regulated by degradation of secondary messengers, GTP hydrolysis, and dephosphorylation by phosphatases.

G Protein-Coupled Receptors vs. Ligand-Gated Ion Channels

These are two major classes of cell surface receptors with distinct mechanisms and functions.

Feature	G Protein-Coupled Receptor (GPCR)	Ligand-Gated Ion Channel
Ligands	Hormones, lipids, proteins	Small neurotransmitters
Mechanism	Ligand binding activates G protein and secondary messenger	Ligand binding causes direct conformational change, opening ion channel
Response Speed	Slower (multiple steps)	Fast (direct signal delivery)
Functional Outcome	Amplification, long-term changes	Rapid, short-term changes (membrane potential)
Duration	Prolonged response	Milliseconds
Common Location	Endocrine system, sensory cells, various tissues	Nervous system synapses

Summary Table: Key Concepts

Concept	Definition	Example/Application
Catabolism	Breakdown of complex molecules, releases energy	Cellular respiration
Anabolism	Synthesis of complex molecules, consumes energy	Protein synthesis
ATP Hydrolysis	ATP → ADP + Pi, releases energy	Drives endergonic reactions
Enzyme	Biological catalyst, lowers activation energy	Hexokinase in glycolysis
Allosteric Regulation	Regulation by binding at a site other than active site	Feedback inhibition in metabolic pathways

Additional info:

• Some organisms have enzymes adapted to different temperatures (e.g., ice worms vs. thermophiles).

• Enzyme kinetics graphs show how inhibitors affect reaction rates and Vmax.

• Phosphorylation and dephosphorylation are key regulatory mechanisms in signal transduction and metabolism.