Alberts - Essential Cell Biology (4th ed.)

Chapter 3: Energy, Catalysis, and Biosynthesis

Oxidation and Reduction in Organic Molecules

Polar Covalent Bonds:

• When carbon forms a polar covalent bond, it can acquire a partial positive charge (δ+), indicating oxidation. This occurs when carbon is bonded to more electronegative atoms, leading to an unequal sharing of electrons.

• Conversely, carbon in a carbon-hydrogen (C–H) bond, which has a relatively equal share of electrons, acquires a partial negative charge (δ–), indicating reduction.

• The process of hydrogenation (adding hydrogen) results in reductions; dehydrogenation (removal of hydrogen) leads to oxidation, showcasing the dynamic nature of redox reactions in organic chemistry.

C–H Bond Counting:

• Reduction is marked by an increase in C–H bonds, which signifies the addition of electrons to a molecule, making it more energy-rich.

• Oxidation occurs with a decrease in C–H bonds, which signifies the loss of electrons, thereby releasing energy stored in chemical bonds.

Role of Enzymes in Energy and Reactions

Energetically Favorable Reactions:

• Enzymes obey the second law of thermodynamics, speeding up reactions that result in an increase in disorder (entropy). They do not drive energetically unfavorable reactions independently; however, they are essential for coupling them with favorable ones, which is crucial for cellular growth and survival.

Activation Energy:

• Enzymes significantly lower the activation energy required for reactions, allowing reactions to proceed at a faster pace and with less energy input.

• Activation energy is the initial energy required to start a reaction, which is often provided by molecular collisions, thereby making enzyme efficiency critical for sustaining life processes.

Free Energy and Catalysis

Free Energy (ΔG):

• Free energy (ΔG) represents the energy available to perform work in a system. Reactions proceed downhill (spontaneously) when they result in a loss of free energy.

• Energetically favorable reactions are characterized by a negative ΔG, indicating an increase in entropy and spontaneous occurrence. In these cases, free energy is lost as reactants convert to products, meaning reactants have higher free energy than products.

Chemical Reactions:

• Exothermic Reactions: These reactions release energy, resulting in an increase in disorder (e.g., combustion reactions like paper burning).

• Endothermic Reactions: Require energy input to proceed and do not occur without being coupled to a favorable reaction.

• Cells continuously exchange materials and energy to maintain metabolic activity and prevent reaching equilibrium where ΔG = 0. This constant flux is vital for sustaining cellular life.

Enzyme Characteristics

Catalysis:

• Enzymes are exceedingly selective and bind specific substrates to catalyze reactions, leading to enhanced reaction rates without being consumed in the process.

• They remain unchanged post-reaction, allowing them to be reused multiple times, which is essential for metabolic efficiency.

• The efficiency of enzymes can be measured by determining Vmax (maximal velocity of product formation) and KM (Michaelis constant), reflecting substrate concentration and affinity, respectively.

Activated Carriers and Biosynthesis

ATP:

• ATP (adenosine triphosphate) is the most common activated carrier and serves as a primary energy source for various cellular reactions. The hydrolysis of ATP releases energy that can drive energetically unfavorable reactions, making it central to cellular metabolism.

NADH/NADPH:

• NADH and NADPH are crucial electron carriers involved in oxidation-reduction reactions essential for biosynthesis.

• NADH is primarily involved in catabolic reactions, facilitating energy release from macromolecules. In contrast, NADPH mainly participates in anabolic processes, supplying reducing power for biosynthetic pathways.

Acetyl CoA:

• Acetyl CoA is another important activated carrier that transfers acetyl groups for biosynthesis, particularly significant in fatty acid and energy metabolism. It provides the necessary moieties for building complex biomolecules.

Polymers Synthesis and Energetics

Polymer Formation:

• Polymers, such as nucleic acids, proteins, and polysaccharides, are constructed through condensation reactions that require energy input from ATP hydrolysis. These polymers form the foundation of cellular structure and function.

• Each biosynthesis pathway utilizes the high-energy bonds from activated carriers to facilitate the formation of these essential biomolecules, underscoring the intricate relationship between energy flow and molecular assembly.

Conclusion: Essential Concepts

• Living organisms continuously require energy input for metabolic processes to sustain life. This dynamic interplay between energy capture, conversion, and storage is crucial for maintaining cellular functionality.

• Enzymes serve as crucial catalysts in various biochemical reactions, and their activity is tightly regulated to meet the needs of the cell. Understanding changes in free energy and how reactions are coupled offers valuable insight into the complex nature of cellular metabolism and energy management.