Energy, Metabolism, and Enzymes

Thermodynamics in Biological Systems

Thermodynamics is the study of energy transformations. In biological systems, understanding the laws of thermodynamics is essential for explaining how organisms obtain, use, and transfer energy.

• First Law of Thermodynamics: Energy cannot be created or destroyed; it can only be transformed or transferred from one form to another.

• Second Law of Thermodynamics: The entropy (disorder) of the universe is always increasing. This means that energy transformations are not 100% efficient, and some energy is always lost as heat.

• Application to Biology: Living organisms must constantly obtain energy to maintain order and carry out life processes, counteracting the natural tendency toward increased entropy.

Free Energy and Chemical Reactions

Free energy (G) is the portion of a system's energy that can perform work. The change in free energy (ΔG) determines whether a reaction is spontaneous and how it relates to cellular processes.

• Definition: ΔG is the difference in free energy between products and reactants.

• Equation:

• Spontaneous Reactions: Occur without input of energy; characterized by ΔG < 0 (exergonic).

• Nonspontaneous Reactions: Require energy input; characterized by ΔG > 0 (endergonic).

• ΔG = 0: The system is at equilibrium; no net change occurs.

• Stability: Lower free energy means greater stability.

• Capacity to Do Work: Only processes with a negative ΔG can perform work spontaneously.

Example Calculation: If Ginitial = 300 kcal/mol and Gfinal = 400 kcal/mol:

• kcal/mol$

• This is an endergonic (nonspontaneous) reaction.

Types of Reactions: Anabolic vs. Catabolic; Exergonic vs. Endergonic

• Anabolic Reactions: Build complex molecules from simpler ones; require energy input (endergonic).

• Catabolic Reactions: Break down complex molecules into simpler ones; release energy (exergonic).

• Exergonic Reactions: Release free energy (ΔG < 0); spontaneous.

• Endergonic Reactions: Absorb free energy (ΔG > 0); nonspontaneous.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) is the primary energy carrier in cells. Its hydrolysis powers many cellular processes.

• Role of ATP: Provides energy for endergonic reactions by coupling its hydrolysis (an exergonic process) to them.

• ATP Hydrolysis:

• Energy Coupling: The use of energy released from exergonic ATP hydrolysis to drive endergonic reactions.

• Reactions Producing ATP: Typically catabolic (e.g., cellular respiration).

• Reactions Requiring ATP: Typically endergonic (e.g., biosynthesis, active transport).

Example: The synthesis of glutamine from glutamate and ammonia is endergonic but can proceed when coupled to ATP hydrolysis.

Coupling Criteria: Reactions with ΔG values less than or equal to the energy released by ATP hydrolysis can be coupled to it.

Enzymes and Metabolic Reactions

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy required.

• Catalyst: A substance that increases the rate of a reaction without being consumed.

• Enzyme Function: Enzymes bind substrates, stabilize the transition state, and reduce activation energy.

• Transition State: A high-energy, unstable state that reactants must reach for a reaction to proceed.

• Induced Fit: The enzyme changes shape slightly to fit the substrate more snugly, enhancing catalysis.

• Effect on Stability: Enzymes do not change the free energy of reactants or products; they only lower the activation energy.

• Enzyme Reusability: Enzymes are not changed or consumed in the reaction.

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

Environmental Effects on Enzyme Activity

Enzyme activity is influenced by environmental factors such as temperature and pH.

• Optimal Temperature: Each enzyme has a temperature at which it functions most efficiently. Too low slows activity; too high can denature the enzyme.

• Optimal pH: Each enzyme has a pH range where it is most active. Deviations can lead to denaturation or reduced activity.

• Denaturation: Extreme temperatures or pH values can disrupt the enzyme's structure, rendering it inactive.

• Enzyme Specificity: Not all enzymes in an organism have the same optimal pH or temperature. For example, pepsin (stomach) works best at low pH, while trypsin (intestine) works best at higher pH.

Enzyme Inhibition: Competitive vs. Noncompetitive

Enzyme inhibitors reduce or block enzyme activity. There are two main types:

• Competitive Inhibition: Inhibitor binds to the active site, competing with the substrate. Can be overcome by increasing substrate concentration.

• Noncompetitive Inhibition: Inhibitor binds to a site other than the active site (allosteric site), changing the enzyme's shape and function. Cannot be overcome by increasing substrate concentration.

Summary Table: Key Concepts in Energy and Enzymes

Additional info: Figures referenced (e.g., Fig 8.14, Fig 8.17) typically show energy diagrams and enzyme activity curves. In general, energy diagrams illustrate the difference in free energy between reactants and products, and enzyme activity curves show optimal temperature or pH as a peak in the graph.