Lesson 2 – Fundamentals of Cellular Energy and the Role of Biological Catalysts (Enzymes)

We begin by understanding energy as the ability to do work or cause change. Managing metabolic energy is crucial for the cell, which operates as an open system.

Energy Flow: We discuss how energy continuously flows throughout the biosphere, distinguishing organisms by their energy sources (phototrophs use sunlight; chemotrophs use chemical compounds) and nutrition sources (autotrophs and heterotrophs).

Thermodynamics: We explain the fundamental laws of energy. The First Law states that the universe’s total energy is conserved – it can be transformed but neither created nor destroyed. The Second Law states that the universe’s overall disorder (entropy) tends to always increase.

Favored Reactions: Typically, spontaneous reactions move toward minimal energy and/or maximum entropy (disorder). The Gibbs Free Energy concept helps us predict the energetic outcome of a chemical reaction, combining changes in enthalpy and entropy.

Energy Coupling: To drive energetically unfavorable (non-spontaneous) reactions, cells couple them to spontaneous ones, using ATP (adenosine triphosphate) as a short-term energy reserve.

Enzymes: Speed Catalysts

Even a spontaneous reaction may proceed slowly because it needs Activation Energy to convert substrate to product. This is where enzymes act.

Catalyst Role: Enzymes are biological catalysts that drastically speed up reactions. Importantly, they do not alter the reaction’s equilibrium or thermodynamics.

Mechanism: Enzymes facilitate reactions by lowering the energy needed to reach the Transition State. The enzyme binds its substrate in a specialized region called the Active Site, isolating it from the environment.

Characteristics: Enzymes are highly substrate-specific and sensitive to environmental conditions like temperature and pH. Other molecules, such as coenzymes, aid enzymes and enhance activation energy reduction.

Enzyme Regulation

Enzymatic reaction speed is finely regulated inside the cell.

• Main Regulation Mechanisms:

- Competition for the active site (Competitive Inhibition)

- Allosteric Regulation, where activity changes due to molecules binding sites other than the active site (these can activate or inhibit the enzyme)

- Phosphorylation and Dephosphorylation: Here, the addition or removal of phosphate groups (via kinase and phosphatase proteins) changes the enzyme’s structure, activating or deactivating it. Phosphorylation is key in enzymatic cascades for amplifying biological responses.

- Proteolytic Cleavage.