Metabolism and Enzyme Activity Review

Metabolism

Metabolism encompasses all biochemical reactions in living cells, leading to the transformation of matter and energy, which is fundamental to life as an energy-driven process.

First Law of Thermodynamics

This law states the principle of conservation of energy, indicating that energy cannot be created or destroyed, only transformed.

Second Law of Thermodynamics

This law asserts that the total disorder (entropy) of a system and its surroundings always increases. Although living organisms decrease their own entropy during growth, they do so through energy transformations involving heat and chemical energy.

Free Energy Change (ΔG)

Free energy, denoted as ΔG, refers to the energy that can perform work at constant temperature and pressure. The equation relating free energy to enthalpy (ΔH), temperature (T), and entropy (ΔS) is:
$$
ΔG = ΔH - TΔS
$$
A negative ΔG indicates a spontaneous process, while a positive ΔG indicates a nonspontaneous process.

Exergonic and Endergonic Reactions

• Exergonic reactions are spontaneous (negative ΔG).
• Endergonic reactions require an input of energy (positive ΔG), such as the synthesis of sucrose.

Activation Energy Barrier

Activation energy (EA) is the energy required to initiate a reaction. It affects how quickly reactants can transform into products.

Energy Coupling

Energy coupling involves using the energy released by exergonic reactions to drive endergonic reactions, most commonly mediated by ATP hydrolysis. ATP acts as the primary energy currency in cells, releasing 7.3 kcal/mol upon hydrolysis:
$$
ATP + H2O → ADP + Pi
$$

Enzymes and Metabolic Reactions

Enzymes are typically catalytic proteins that speed up reactions by lowering the activation energy without altering ΔG. They stabilize the transition state and promote reactions through various mechanisms, such as altering charge environments and changing substrate shapes.

Cofactors and Coenzymes

Cofactors, which include metallic ions and small organic molecules (coenzymes), are often essential for enzyme activity. Deficiencies or mutations affecting cofactors can lead to metabolic disorders like phenylketonuria (PKU).

Enzyme Activity Regulation

Enzyme activity can be influenced by local conditions such as temperature, pH, and substrate concentration. Feedback mechanisms, including allosteric regulation, competitive, and noncompetitive inhibition, adjust enzyme activity to meet cellular needs.

SARS-CoV-2 and TMPRSS2

TMPRSS2 is a serine protease that plays a critical role in the viral entry of SARS-CoV-2 by cleaving the spike protein, allowing fusion with host cell membranes. Inhibiting TMPRSS2 may be considered for therapeutic approaches, but clinical trials have shown limited success.