Energy and Enzymes

1. Importance of Enzymes

Chemical Reactions and Temperature: Life cannot survive at high temperatures because proteins, especially enzymes, denature (lose their functional shape). Instead, it relies on catalysts (enzymes) that speed up chemical reactions without temperature increase.
Enzyme Efficiency: Richard Wolfenden's research at the University of North Carolina revealed how enzymes significantly speed up biochemical reactions.
- Example: Enzyme-catalyzed removal of phosphate group takes $\sim10$ milliseconds versus an uncatalyzed reaction that would take over 1 trillion ( $10^{12}$ ) years.
- This difference represents a factor of 21 orders of magnitude ( $10^{21}$ ).
Central Role in Life: Enzymes facilitate metabolic reactions crucial for life functions like growth, reproduction, and energy utilization.

2. Energy and Thermodynamics

Definition of Energy: Energy is the capacity to do work or be transferred as heat. In biological systems, work includes mechanical work (e.g., muscle contraction), transport work (e.g., pumping ions), and chemical work (e.g., synthesizing molecules).
Different Forms of Energy: Energy exists in forms such as chemical, electrical, and mechanical.
Types of Energy:
- Kinetic Energy: Energy of motion (e.g., waves, moving objects).
- Potential Energy: Stored energy (e.g., position in gravitational field, chemical bonds). Chemical energy, stored in molecular bonds, is a crucial form of potential energy for life.

2.1 Laws of Thermodynamics

Thermodynamics: Branch of science that studies energy changes.
System Definitions:
- Isolated System: No exchange of matter/energy (e.g., the universe).
- Closed System: Exchanges energy but not matter (e.g., Earth).
- Open System: Exchanges both energy and matter (e.g., oceans).
First Law of Thermodynamics: Energy cannot be created or destroyed but can change forms or be transferred.
Second Law of Thermodynamics: Energy disperses; systems move towards greater entropy (disorder). This means that every energy transfer or transformation increases the entropy of the universe, often as heat.
Entropy (S): Measure of disorder; systems tend to spread energy and matter. A higher entropy state means that energy is more widely dispersed.

3. Free Energy and Spontaneous Processes

Spontaneous Processes: Reactions that occur without sustained energy input; not necessarily fast. A spontaneous process means that it will proceed to completion on its own, eventually.
Enthalpy and Entropy:
- Enthalpy (∆H): Heat content of a system influencing reaction spontaneity.
- Entropy Change (∆S): Influences spontaneity; processes tend to be spontaneous if entropy of products exceeds that of reactants.
Gibbs Free Energy (G): Determines if a reaction is spontaneous:
- $\text{∆G} = \text{∆H} - T \text{∆S}$
- $\Delta G$ represents the maximum amount of work that can be extracted from a reaction at constant temperature and pressure.
Spontaneous Reaction: Negative $\Delta G$ (exergonic). These reactions release free energy.
Nonspontaneous Reaction: Positive $\Delta G$ (endergonic). These reactions require an input of free energy.

3.1 Reactions and Equilibrium

Reactions reach a state of equilibrium where the forward and reverse rates are equal; free energy ( $\Delta G$ ) approaches zero.

4. Thermodynamics and Life

Life's Energy Utilization: Life harnesses energy from the environment to maintain lower entropy states while increasing the entropy of their surroundings.
Consumption of Energy: Continuous energy intake is necessary to counteract system breakdown and maintain organized state.

5. Metabolism Overview

Metabolism Definition: All chemical reactions within cells.
Pathways: Two types - catabolic (break down) and anabolic (build up).
- Catabolic = exergonic, releases energy (e.g., cellular respiration).
- Anabolic = endergonic, requires energy (e.g., biosynthesis).

6. ATP Hydrolysis

ATP as Energy Currency: ATP undergoes hydrolysis to release energy. The energy is primarily stored in the bonds between the phosphate groups, particularly the terminal phosphate bond.
- $\text{ATP} + \text{H}_2\text{O} \longrightarrow \text{ADP} + \text{Pi} \text{, ∆G} = -7.3 \text{ kcal/mol}$
Energy Coupling: Connecting exergonic reactions to drive endergonic reactions (e.g., synthesis of glutamine).

7. Role of Enzymes

Enzymes: Biological catalysts that speed reactions by lowering activation energy ( $Ea$ ) without changing the reaction’s $\Delta G$ . Enzymes achieve this through specific interactions with their substrates at unique active sites.
Mechanisms:
- Bring reactants closer together in the correct orientation.
- Alter charge environments favoring reactions.
- Strain substrate bonds, lowering activation energy.
- Induce a conformational change in the enzyme upon substrate binding, known as the "induced fit" model, which often enhances catalytic activity.

8. Factors Affecting Enzyme Activity

Enzyme Concentration: Increased enzyme concentration raises rates in the presence of excess substrates up to saturation level.
Substrate Concentration: Rates increase with substrate concentration until the enzyme is saturated.
Temperature and pH: Each enzyme operates optimally within a specific temperature and pH range. Deviations from this optimum can lead to denaturation, where the enzyme loses its three-dimensional structure and thus its function.

8.1 Enzyme Regulation

Competitive Inhibition: Molecules compete for the active site; can be overcome with high substrate concentration.
Noncompetitive Regulation: Molecules bind at allosteric sites, altering enzyme function—either activating or inhibiting catalysis. Allosteric regulators cause a change in the active site's shape, making it either more or less receptive to the substrate.
Feedback Inhibition: Products inhibit their own synthesis path.

Summary

Enzymes are crucial for metabolism, allowing endergonic reactions by coupling with exergonic processes.
Energy transformations follow thermodynamic laws, supporting life in an open system.
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