Lecture 2.4 - Metabolism and Enzymes

Learning Objectives

• Explain what metabolism is and describe the two types of metabolic pathways.
• Discuss how living systems "pay" the energy cost of building more complex molecules.
• Describe how enzymes work.
• Explain why biological organisms don’t spontaneously combust.

What is Metabolism?

• Definition: Metabolism is the sum of all chemical reactions within a cell or organism.
  • This occurs simultaneously in cells.
  • Metabolism is an emergent property, meaning it arises from the interaction of simpler elements.

Metabolic Pathways

• Cells utilize metabolic pathways:
  • Definition: A series of chemical reactions where each step is catalyzed by a specific enzyme.
  • Products of one pathway often serve as substrates for others.
  • Pathways can be:
  • Linear: One reaction follows another in a straight line.
  • Reversible: Reactions can occur in both directions.
  • Circular: The end of the pathway regenerates the beginning.
  • Branched: Pathways diverge, leading to different products.

Two Types of Metabolic Pathways

1. Anabolic (Biosynthetic):

   • Function: Synthesis of complex molecules from simpler ones.
   • Example: Formation of sugars from CO2 during photosynthesis or proteins from amino acids.

2. Catabolic (Degradative):

   • Function: Breakdown of complex molecules into simpler ones.
   • Example: Hydrolysis of proteins to amino acids or oxidation of sugars to CO2 during respiration.

Energy in Metabolism

• Types of Energy:
  • Kinetic Energy: Energy of moving objects; thermal energy is a form associated with molecular motion.
  • Potential Energy: Energy stored in an object due to its position or structure; includes chemical energy available for reactions.
• Energy can be interconverted.

Gibbs Free Energy

• Definition: Indicates the amount of energy available to do work within a system.
  • Measured in kcal/mol or kJ/mol.
• ΔG (Change in Free Energy):
  • Used to predict spontaneity of reactions:
  • If ΔG < 0: Reaction is spontaneous (exergonic).
  • If ΔG > 0: Reaction requires energy (endergonic).

Free Energy and Stability

• Increased free energy indicates less stability; decreases in free energy lead to more stable products.
• Spontaneous changes (ΔG < 0) harness free energy to perform work, where instability leads to releases and transformations.

Work in Cells

• Cells perform three types of work:
  1. Chemical Work: Endergonic reactions requiring energy.
  2. Transport Work: Pumping molecules across membranes.
  3. Mechanical Work: Muscle contraction and chromosome movement during cell division.
• Energy Currency: Cells utilize ATP to pay energy costs for work.

ATP: The Energy Currency

• Structure:
  • Composed of adenine (nucleotide base), ribose (sugar), and three linked phosphate groups.
• Hydrolysis Reaction (ATP → ADP + Pi): Releases energy (ΔG = -7.3 kcal/mol).
• Energy Harvesting:
  • ATP hydrolysis fuels endergonic reactions by providing necessary energy (e.g., converting glutamic acid to glutamine).

The ATP Cycle

• ATP is synthesized from ADP and inorganic phosphate (requires energy) and is generated through exergonic reactions (catabolism).
• Renewable source of energy for cells during cellular work.

Enzymes and Their Function

• Definition: Enzymes are proteins that act as biological catalysts, speeding up chemical reactions by lowering activation energy.
  • Most enzymes are proteins, some are RNA molecules (ribozymes).
• Enzyme Specificity: Each enzyme is substrate-specific, often denoted with the suffix "-ase" (e.g., lipase, protease).

Mechanism of Enzymatic Action

1. Substrates enter the enzyme's active site, inducing a change in the enzyme's shape (induced fit).
2. Substrates are converted to products in the active site through weak interactions (hydrogen, ionic bonds).
3. Products are released, and the enzyme reverts to its original shape.

Regulation of Enzymes

• Enzymes are regulated to prevent chaos in metabolic pathways:
  • Natural processes (methylation, phosphorylation) can turn enzymes on or off.
  • Feedback inhibition occurs when the end products act as inhibitors to the enzymes.

Key Takeaways

• Metabolism encompasses all reactions in a living organism, involving anabolic and catabolic pathways.
• ATP is critical for providing energy to drive cellular processes.
• Enzymes increase the rate of chemical reactions and are subject to regulation to maintain homeostasis in biological systems.

Readings

• Campbell’s Biology, 12th Edition: Chapter 8 - An Introduction to Metabolism (Concepts 8.1 – 8.4)