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Lehninger Principles of Biochemistry Overview

• Authors: David L. Nelson, Michael M. Cox

Chapters Overview

• Chapter 13: Bioenergetics and Biochemical Reaction Types
• Chapter 14: Glycolysis, Gluconeogenesis, and the Pentose Phosphate Pathway
• Chapter 15: Principles of Metabolic Regulation
• Chapter 16: The Citric Acid Cycle
• Chapter 17: Fatty Acid Catabolism
• Chapter 18: Amino Acid Oxidation and the Production of Urea
• Chapter 19: Oxidative Phosphorylation

Carbon Cycle

• Carbon, oxygen, and water are cycled between heterotrophic and autotrophic worlds
  • Solar energy serves as the driving force for this extensive process (Refer to Fig. 1).

Nitrogen Requirement

• All living organisms require a source of nitrogen.
  • Nitrogen is essential for the synthesis of:
  • Amino acids
  • Nucleotides
  • Other nitrogen-containing compounds (Refer to Fig. 2).

Energy Relationships in Metabolism

• Metabolic pathways are classified into two categories:
  • Catabolism: Pathways that yield energy.
  • Key energy carriers:
    • ATP (Adenosine triphosphate)
    • NADH (Nicotinamide adenine dinucleotide)
    • NADPH (Nicotinamide adenine dinucleotide phosphate)
    • FADH2 (Flavin adenine dinucleotide)
  • Anabolism: Pathways that consume energy to synthesize macromolecules from small precursor molecules.
  • Precursor molecules:
    • Amino acids
    • Sugars
    • Fatty acids
    • Nitrogenous bases
  • Energy-depleted end products include:
  • $CO_2$
  • $H_2O$
  • $NH_3$
  • Figure 3 demonstrates how catabolic pathways supply chemical energy via ATP, NADH, NADPH, and FADH2, which is utilized in anabolic pathways.

Bioenergetics and Reaction Types

Standard Free-Energy Change

• The standard free-energy change ($riangle G'^{ ext{o}}$) is directly related to the equilibrium constant (See Tables 13-2 and 13-3).

Relationships Among $K_{eq}$, $riangle G'^{ ext{o}}$, and Reaction Directions

Standard Free-Energy Changes of Chemical Reactions (Table 13-4)

Oxidation States of Carbon in Biomolecules

• The oxidation states of carbon change during various biochemical reactions.
• Carbon dioxide is considered the most oxidized form of carbon in living systems.
  • Reactions that decrease the number of hydrogen atoms are known as dehydrogenations.
  • Enzymes that facilitate these reactions are called dehydrogenases.

Example of an Oxidation-Reduction Reaction

• The oxidation of lactate to pyruvate is an example of such a reaction:
  • Lactate
    ightarrow Pyruvate
  • Catalyzed by lactate dehydrogenase
  • Electrons transferred to the cofactor NAD.
  • Reaction is reversible:
  • Pyruvate
    ightarrow Lactate (reduction process).

ATP and Phosphoryl Group Transfers

ATP as a Major Energy Carrier

• ATP is recognized as the primary carrier of chemical energy in all cells (Refer to Fig. 13-11).

Importance of Other Phosphorylated Compounds

• Other biologically important phosphorylated compounds and thioesters exhibit significant free energies of hydrolysis (See Table 13-6, Fig. 13-13, and Fig. 13-16).

Coenzymes and Electron Carriers

Types of Coenzymes

• Various types of coenzymes and proteins serve as universal electron carriers:
  • NAD+ (Nicotinamide Adenine Dinucleotide)
  • NADP+ (Nicotinamide Adenine Dinucleotide Phosphate)
  • FMN (Flavin Mononucleotide)
  • FAD (Flavin Adenine Dinucleotide)
  • Quinones
  • Iron-sulfur proteins
  • Cytochromes

NAD+ and NADP+

• Both NAD+ and NADP+ act as freely diffusable coenzymes for many dehydrogenases.
  • They accept two electrons and one proton.
  • Reactions:
  • NAD^+ + 2e^- + 2H^+
    ightarrow NADH + H^+
  • NADP^+ + 2e^- + 2H^+
    ightarrow NADPH + H^+
  • Over 200 enzymes utilize NAD+ or NADP+.
  • Enzymes are classified as oxidoreductases or dehydrogenases.
  • Example:
  • Alcohol Dehydrogenase catalyzes:
    • ext{CH}_3 ext{CH}_2 ext{OH} + NAD^+
      ightarrow ext{CH}_3 ext{CHO} + NADH + H^+
    • Reactants: Ethanol and NAD+
    • Products: Acetaldehyde, NADH, and proton (H+)

FMN and FAD as Prosthetic Groups

• FAD and FMN serve as tightly bound prosthetic groups of flavoproteins, capable of accepting either:
  • One or two electrons
  • One or two protons
• Reactions:
  • FMN + 2e^- + 2H^+
    ightarrow FMNH_2
  • FAD + 2e^- + 2H^+
    ightarrow FADH_2