Bioenergetics I: Enzymes and Biochemical Reactions

Bioenergetics I: Enzymes and Biochemical Reactions

Overarching Concept

Organisms use energy for cellular processes, and this energy is acquired from molecular bonds. Understanding how energy flows through biological systems is essential for grasping metabolism and cellular function.

Energy Principles

Forms and Laws of Energy

• 1st Law of Thermodynamics: Energy is neither created nor destroyed; it only changes forms.

• 2nd Law of Thermodynamics: When energy is converted from one form to another, some energy becomes unavailable to do work (often lost as heat).

In biochemical reactions, the products typically have less potential energy than the reactants, reflecting the transformation and release of energy.

Energy in Biology

Form of Energy	Example in Biology
Chemical: Stored in bonds	Chemical energy is released during the hydrolysis of polymers
Electrical: Separation of charges	Electrical gradients across cell membranes help drive the movement of ions through channels
Heat: Transfer due to temperature difference	Heat can be released by chemical reactions
Light: Electromagnetic radiation stored as photons	Light energy is captured by pigments in the eye
Mechanical: Energy of motion	Mechanical energy is used in muscle movements

Metabolism

Definition and Pathways

• Metabolism: The sum of all biochemical reactions in an organism, including both catabolic (breakdown) and anabolic (building) pathways.

• Catabolic pathways release energy by breaking down molecules; anabolic pathways consume energy to build molecules.

Biochemical Reaction Types

Spontaneous vs. Nonspontaneous Reactions

• Exergonic (Spontaneous) Reactions: $\Delta G < 0$; energy is released.

• Endergonic (Nonspontaneous) Reactions: $\Delta G > 0$; energy is consumed.

Potential Energy in Bonds

• Potential energy in molecules depends on bond type:

• Nonpolar bonds (e.g., C–H) are longer and weaker, containing more potential energy.

• Polar bonds (e.g., O–H) are shorter and stronger, containing less potential energy.

Redox Reactions

Oxidation and Reduction

• OIL RIG: Oxidation Is Losing electrons; Reduction Is Gaining electrons.

• LEO goes GER: Lost Electrons Oxidized; Gained Electrons Reduced.

• Redox reactions may transfer electrons alone or with protons (H+).

Electron Carriers: FAD and NAD

• NAD+ and FAD are important electron carriers, cycling between oxidized and reduced forms (NADH, FADH2).

• Reducing FAD and NAD is endergonic (requires energy input) and must be coupled to an exergonic reaction.

Coupling Reactions

Energy Coupling in Cells

• An exergonic reaction (energy-releasing) can drive an endergonic reaction (energy-consuming) if the two are coupled.

• This coupling is essential for cellular metabolism, allowing unfavorable reactions to proceed by linking them to favorable ones.

ATP: The Energy Currency of the Cell

Structure and Function

• ATP (Adenosine Triphosphate): Contains three phosphate groups with high potential energy due to clustered negative charges.

• Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate ($\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{Energy}$).

• ATP hydrolysis is a catabolic reaction, not a redox reaction.

The ATP Cycle

• ATP is continuously recycled in cells, coupling exergonic and endergonic reactions.

• Exergonic reactions (e.g., cellular respiration) regenerate ATP from ADP; endergonic reactions (e.g., active transport, biosynthesis) use ATP hydrolysis for energy.

Clarification on Bond Energy

• Breaking bonds requires energy; forming bonds releases energy.

• In biology, the term "high-energy bond" refers to the net energy released when an unstable bond is broken and replaced by a more stable bond.

Enzymes

Role in Biochemical Reactions

• Enzymes are biological catalysts that speed up biochemical reactions by lowering activation energy.

• They contain binding sites for substrates and can facilitate coupled reactions (exergonic and endergonic).

Enzyme Kinetics

• Enzymes increase reaction rates, but there is a maximum rate determined by substrate concentration and enzyme availability.

• Maximum rates can range from 1 to 40 million molecules per second for different enzymes.

Types of Biochemical Reactions

Catabolic	Anabolic
Reactions that break down molecules	Reactions that build molecules

Regulation of Enzyme Activity

• Competitive Inhibition: A molecule similar to the substrate binds to the enzyme's active site, blocking substrate binding.

• Allosteric Regulation: Binding of a regulatory molecule at a site other than the active site changes the enzyme's shape, affecting activity.

• Allosteric activation can occur via phosphorylation, which changes the enzyme's 3D shape to reveal or hide the active site.

Cofactors

• Cofactors are nonprotein molecules or inorganic ions required for enzyme function. They include coenzymes (organic cofactors) and metal ions.

Cofactor	Role in Catalyzed Reaction
Iron (Fe2+, Fe3+)	Electron acceptor in electron transport
ATP	Provides energy for many cellular reactions
NADH/NAD+	Electron carrier in redox reactions
FADH2/FAD	Electron carrier in redox reactions

Putting It All Together: Metabolic Pathways

Integration of Concepts

• Metabolic pathways involve a series of enzyme-catalyzed reactions, often coupled to ATP hydrolysis or synthesis and redox reactions.

• Example: Glycolysis involves the transfer of phosphate groups, redox reactions (NAD+ to NADH), and ATP synthesis/hydrolysis.

Example: The conversion of phosphoenolpyruvate to pyruvate by pyruvate kinase is highly exergonic and coupled to ATP synthesis.

Additional info: Understanding these principles is foundational for later topics such as cellular respiration, photosynthesis, and metabolic regulation.