Metabolism: Enzymes, Catabolism, and Anabolism

Enzyme-Substrate Dynamics

• Scenario: High Enzyme Concentration, Low Substrate Availability
  • When there are abundant enzymes but limited or no substrate, the reaction rate will be relatively low.
  • Products will be released, and new substrate will eventually replace it, but the overall rate remains low.
  • This is because not all active sites on the enzymes are occupied due to the insufficient amount of substrate.
• Graphical Representation of Reaction Rate
  • If enzyme concentration increases (assuming other factors are constant), the reaction rate will initially increase.
  • However, in a scenario with limited substrate, the reaction rate will reach a maximum and then level off.
  • It will not continue to increase indefinitely because there isn't enough substrate to occupy all active sites and sustain a higher rate.

Metabolism: An Overview

• Definition: Metabolism is the sum of all chemical reactions that occur within an organism.
• Purpose: Organisms constantly engage in making and breaking molecular bonds as they live and grow.
• Components: Metabolism comprises two main types of processes:
  • Catabolism
  • Anabolism

Catabolism: Breaking Down Molecules

• Definition: Catabolism includes processes that break down complex molecules into simpler molecules.
• Function: These processes are designed to harvest energy from complex molecules and store it, typically in the form of $ext{ATP}$.
• Energy Input: Catabolic pathways may require initial inputs of energy to proceed.
• Example: The breakdown of starch.
  • Starch is a large, complex molecule composed of many glucose units in a multi-branched structure.
  • Breaking down starch into simpler sugars uses enzymes like amylase.

Anabolism: Building Up Molecules

• Definition: Anabolism includes processes that build more complex molecules from simpler precursor molecules.
• Energy Source: Anabolic processes are driven by the energy (in the form of $ext{ATP}$) acquired and stored through catabolic reactions.
• Example: The construction of starch.
  • Simple molecules, like glucose one phosphate, can be built into a complex molecule like starch.
  • An enzyme involved in this process is phosphorylase.

Energy Efficiency and Loss

• Efficiency: Neither catabolic nor anabolic processes are completely efficient.
• Energy Loss: In each step of these metabolic pathways, some of the available energy is inevitably lost into the environment as heat.

The Role of $ext{ATP}$ (Adenosine Triphosphate)

• $ext{ATP}$ serves as the primary energy currency of the cell.
• Catabolism harvests energy, often to synthesize $ext{ATP}$.
• Anabolism utilizes the energy stored in $ext{ATP}$ to drive the synthesis of complex molecules.
• Catabolic pathways are essentially the opposite of anabolic pathways in terms of energy flow and molecular complexity.

Enzyme Naming Conventions

• General Rule: Almost all enzymes have names that end with the suffix "$ext{-ase}$ ".
  • Examples: Maltase, Catecholase, Amylase, Phosphorylase.
• Exceptions: There are some enzymes that do not follow this naming convention.
  • Example: Pepsin.

Examples in Digestion

• Initial Starch Digestion: The very first reaction in the digestion of food containing starch begins in the mouth.
  • Salivary amylase (an enzyme present in saliva) starts breaking down starch molecules as soon as food is consumed.