Metabolism Basics and Energy Currency

Metabolic Overview

• Cells continuously run two complementary sets of chemical reactions:
  • Anabolism (biosynthesis)
  • Builds larger, more complex molecules from smaller sub-units.
  • Requires an external input of energy (endergonic).
  • Typical mechanism: dehydration (condensation) synthesis → removal of \mathrm{H_2O} forms a new covalent bond.
  • Catabolism (degradation)
  • Breaks large molecules into smaller units.
  • Releases energy stored in chemical bonds (exergonic).
  • The two pathways are metabolically intertwined—energy liberated by catabolism powers anabolism.

Anabolism via Dehydration Synthesis

• General reaction schema:
  \text{Monomer}1\;{+}\;\text{Monomer}2 \;\xrightarrow[\text{energy}]{\text{dehydration}}\;\text{Dimer} + \mathrm{H_2O}
• Specific macromolecular examples:
  • Carbohydrates
  • Two monosaccharides (e.g., glucose + fructose)
    → hydroxyl (OH) removed from one sugar + hydrogen (H) removed from the other → water released → new glycosidic bond → disaccharide produced.
  • Lipids
  • Glycerol + 3 fatty acids
    → each fatty acid loses an OH, glycerol loses three H atoms → 3 \mathrm{H_2O} molecules expelled → ester bonds yield a triglyceride.
  • Proteins (peptide bonds)
  • Two amino acids
    • Each has a carboxyl group (\mathrm{COOH}) and an amino group (\mathrm{NH_2}).
    • OH removed from the carboxyl of one amino acid + H removed from the amino group of the next → water formed.
    • Resulting peptide bond (–CO–NH–) links residues; energy stored in newly formed bond.
• Significance: Dehydration synthesis underlies formation of all major biomolecules (polysaccharides, triglycerides, polypeptides, nucleic acids).

Catabolism

• Reverse of anabolism; employs hydrolysis (addition of \mathrm{H_2O}) to cleave bonds.
• Key example: breakdown of polysaccharides → glucose units.
  • Reaction releases free energy that can be captured as \mathrm{ATP}.
• Complete aerobic catabolism of glucose (cellular respiration):
  \mathrm{C6H{12}O6 + 6\,O2 \;\rightarrow 6\,CO2 + 6\,H2O + \text{energy (\sim 30{-}32 ATP)}}
• Catabolic and anabolic pathways are inseparable; energy yield from one directly funds the other.

Integration: Krebs (Citric Acid) Cycle as a Hub

• Glycolysis converts glucose → 2 pyruvate molecules → decarboxylation → acetyl-CoA.
• Acetyl-CoA enters Krebs cycle (still catabolic), generating \mathrm{CO2}, NADH, FADH$2$ and GTP/ATP.
• Intermediates serve dual purposes (amphibolic):
  • Citrate → fatty acid & sterol synthesis (anabolic diversion).
  • α-Ketoglutarate → precursor for amino acids & nucleotides.
  • Any step’s intermediate may be siphoned off for biosynthetic needs—evidence that catabolism directly feeds anabolism.

ATP: Cellular Energy Currency

• Molecular structure:
  • Adenine (nitrogenous base)
  • Ribose (five-carbon sugar)
  • Three phosphate groups (α, β, γ) linked by high-energy phosphoanhydride bonds.
• Nomenclature:
  \mathrm{ATP = Adenosine\;Tri\,Phosphate}
  \mathrm{ADP = Adenosine\;Di\,Phosphate}
• Hydrolysis reaction that releases usable energy: \mathrm{ATP + H2O \;\rightarrow ADP + Pi + \text{energy}}
  • \Delta G of hydrolysis ≈ -30.5\;\text{kJ·mol}^{-1} (physiological standard)
• Energy captured drives:
  • Bond formation in macromolecules (peptide bond creation, nucleic acid polymerization).
  • Mechanical work (muscle contraction, cytoskeletal movement).
  • Active transport across membranes.

ATP–ADP Recycling (ATP Cycle)

• Step 1: Energy release
  • Cell cleaves γ-phosphate bond of ATP.
  • Generates ADP + inorganic phosphate (P_i) + free energy.
• Step 2: Energy investment
  • Catabolic pathways (e.g., oxidative phosphorylation) add P_i back to ADP.
  • Requires energy input; reforming high-energy bond recreates ATP.
• Continuous loop ensures a rapid turnover; an average human recycles entire ATP pool every 1–2 minutes.

Key Take-Home Connections

• Anabolism ≈ building; catabolism ≈ breaking—but they run concurrently and interdependently.
• Dehydration synthesis is the common anabolic mechanism for carbohydrates, lipids, proteins.
• Hydrolysis/respiration pathways funnel energy into ATP, which is the immediate power source for endergonic biosynthetic reactions.
• The Krebs cycle exemplifies the amphibolic nature of metabolism: catabolic degradation produces intermediates that are also anabolic precursors.
• Efficient energy management (via ATP cycling) is central to cell viability, growth, and replication.