2.1 Introduction to Metabolism and Enzyme Activity

Fundamentals of Cellular Metabolism and Physiology

• Metabolism refers to the exhaustive sum of all chemical reactions occurring within a cell.

• It is categorized as a core foundation of cellular functioning and physiology.

• The study of metabolism involves understanding specific vocabulary, the types of chemical reactions present in cells, the mechanics of enzymes, and the processes involved in the production of Adenosine Triphosphate (ATP).

Metabolic Pathways: Catabolism vs. Anabolism

• Metabolism is organized into pathways consisting of various types of reactions.

• Catabolic Pathways:

  • Function: These pathways involve the digestion and breakdown of larger molecules into smaller units or building blocks known as monomers.

  • Energy Profile: Catabolic reactions overall release energy which is then captured by the cell.

  • Utility: They generate energy and provide the necessary building blocks for the cell to construct functional molecules.

  • Examples and Nutrients: Cells utilize nutrient sources such as polysaccharides, monosaccharides, and proteins. Even autotrophs (cells that make their own food) utilize these pathways.

  • Metaphor: The term "catabolic" can be associated with the word "catastrophic" to visualize the process of molecules being blown apart or broken down.

  • Focus Instance: Cellular respiration is a primary catabolic process where glucose is broken down to generate energy.

• Anabolic Pathways:

  • Function: These pathways are responsible for "building up" molecules.

  • Energy Profile: Anabolism requires an investment of energy to construct complex molecules.

  • Outcomes: The cell uses the building blocks released by catabolism to reform macromolecules like proteins, nucleic acids, and carbohydrates necessary for cellular functions.

  • Example: Anabolic steroids, such as testosterone, are used to build up muscle mass, illustrating the "building" nature of these pathways.

• Shared Characteristics of Metabolic Pathways:

  • Both catabolic and anabolic pathways are essential for cell survival.

  • Regulation: Both are tightly regulated to ensure they only occur when the cell needs them, allowing the cell to conserve nutrients, building blocks, and energy.

  • Enzymatic Requirement: Both types of pathways require enzymes to proceed.

Enzyme Definition and Mechanics of Catalysis

• Definition of an Enzyme: An enzyme is a protein that acts as a catalyst for a specific chemical reaction.

• Catalysis: This is the acceleration of a reaction by making it more energetically favorable.

• Metaphor for Catalysis: Lighter fluid serves as a catalyst for fire; while fire can happen without it, lighter fluid makes the event much more likely to occur.

• Activation Energy ($E_a$): This is the energy barrier that must be overcome for a chemical reaction to occur.

  • The Riverbed Metaphor: Imagine a ball bouncing in waves in a dry riverbed. If the waves are not high enough, the ball cannot move over a hill. An enzyme "lowers the playing field" (effectively lowering the riverbed or increasing the relative energy of the waves), making it far more likely for the ball (the reaction) to move forward.

  • Energetic Efficiency: Enzymes lower the activation energy, meaning the reaction requires less energy investment and takes place much more quickly.

• Reaction Components:

  • Reactant: The starting material before the chemical reaction occurs.

  • Product: The substance formed after the reaction takes place.

  • Without enzymes: Reactions take more time and require higher energy investment.

  • With enzymes: Reactions occur faster and require lower energy investment.

Enzyme Structure, Binding, and Regulation

• Cofactors: These are metal ions, such as Zinc ($Zn^{2+}$), that serve to enhance or activate an enzyme’s activity. Without the presence of the required cofactor, certain enzymes remain inactive and cannot produce products.

• Substrate: The specific target molecule that an enzyme acts upon.

• Active Site: The specific region of the enzyme where the substrate binds.

• The Induced Fit Model: When a substrate binds to the active site, it triggers a conformational change or alteration in the enzyme’s structure. This "induced fit" brings the chemical groups of the active site into the perfect position to catalyze the reaction.

• Enzyme-Substrate Complex: The temporary structure formed when the enzyme and substrate are bound.

• Recycling: Enzymes are not permanently changed by the reactions they catalyze. Once the product is released, the active site becomes empty and the enzyme returns to its original form, ready to be reused.

• Specificity: Enzymes are highly selective; only specific substrates with the correct structure can fit into the active site of a particular enzyme.

Genetic and Environmental Factors Influencing Enzymes

• Genetic Control:

  • Decisions regarding which metabolic pathways a cell can perform are dictated by the cell's DNA.

  • The Central Dogma (DNA $→$ RNA $→$ Protein) applies here; because enzymes are proteins, their recipes are stored in the DNA.

  • Implications: A mutation in the DNA can lead to a faulty protein/enzyme. If the enzyme is defective, the associated metabolic reaction will not occur correctly, which can lead to disease in humans/animals or altered metabolism in microbes.

• Factors Affecting Reaction Rate:

  • Enzyme Concentration: Higher concentrations of enzymes increase the rate of reaction.

  • Substrate Concentration: Having an overwhelming amount of substrate compared to enzymes can limit the reaction rate.

  • Temperature: Enzymes have an optimal temperature range.

  • High Heat: Boiling temperatures can cause denaturing, where the protein unfolds and the active site is destroyed.

  • Cold: Lowering the temperature (e.g., refrigeration) slows down metabolic processes because enzymes do not function as well outside their optimal range.

  • pH: Enzymes have an optimal pH range; altering the pH typically decreases enzymatic activity.

Enzyme Nomenclature and Classification

• Naming Convention: Most enzymes end with the suffix "-ase."

• Common Functional Names:

  • Proteases: Enzymes that break down proteins.

  • Lipases: Enzymes that break down lipids.

  • Amylase: Enzymes that break down starches.

  • Catalase: An enzyme that breaks down hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and oxygen ($O_2$).

• Lysozyme Case Study:

  • Found in: Saliva, mucus, and tears.

  • Function: It acts as an antimicrobial by cleaving polysaccharide chains in bacterial cell walls, causing the bacteria to rupture.

• The Six Classes of Enzymes:

  • Hydrolases: Catalyze hydrolysis (breaking chemical bonds with the addition of $H_2O$). $AB + H_2O → A + B$.

  • Isomerases: Catalyze the rearrangement of bonds within a single molecule to produce an isomer. $AB → BA$.

  • Ligases: Catalyze the formation of covalent bonds to join molecules together. $A + B → AB$.

  • Lyases: Break chemical bonds by means other than hydrolysis or oxidation. $AB → A + B$.

  • Oxidoreductases (Redox Enzymes): Transfer electrons from a reductant (donor) to an oxidant (acceptor). These are critical for energy generation.

  • Transferases: Transfer functional groups (e.g., phosphate, acetyl, or methyl groups) from one molecule to another.

  • Kinases: A specific type of transferase that phosphorylates a molecule by adding a phosphate group.

Oxidation-Reduction (Redox) Reactions and Energy Carriers

• The Role of Electrons: Electrons carry energy. When an electron is transferred between molecules, energy is transferred with it.

• Redox Principles:

  • Oxidation: The loss of electrons from a molecule. Oxygen is a frequent electron acceptor in these reactions.

  • Reduction: The gain of electrons by a molecule. This reduces the overall charge of the molecule (e.g., from 0 to $-1$ or $-2$) because electrons are negatively charged.

• Mnemonics for Redox:

  • OIL: "Oxidation Involves Loss."

  • RIG: "Reduction Involves Gain."

• Activated Carriers (Energy Carriers):

  • $NAD^+$ (Nicotinamide Adenine Dinucleotide): The oxidized form, which lacks high-energy electrons and is ready to accept them.

  • NADH: The reduced form, representing high energy. It has gained electrons and a hydrogen atom, allowing it to carry energy to various parts of the cell for ATP production.

  • This cycle is circular: $NAD^+$ gains electrons (reduction) to become NADH, and NADH loses electrons (oxidation) to return to $NAD^+$ while releasing energy for metabolic pathways such as cellular respiration.

Enzymes

Most metabolic functions in cells rely on enzymes to facilitate the biochemical reactions. An enzyme is a protein, or group of proteins, that catalyze (speed up) chemical reactions. The enzyme is not consumed during the reaction and can be used repeatedly by the cell. The specificity and function of some enzymes can also be regulated by a cofactor — a small chemical component, usually metal ions, that assist enzymes during the catalysis reactions. Thus, cofactors serve as regulators of chemical reactions: In the absence of the proper cofactor, enzymes are inactive, while in its presence enzymes are active.

In order to produce sufficient levels of energy, microorganisms must break down complex nutrients into smaller, manageable (and useful) subunits. Enzymes must break down proteins, lipids, and polysaccharides into their smaller building-block molecules: proteins into amino acids, fats into glycerol/fatty acids, and polysaccharides into monosaccharides. In turn, cells must also be able to assemble or build the cellular components required for survival. Most metabolic processes can be classified as either catabolism or anabolism. Catabolism is the process of breaking down larger molecules into useful energy sources. Anabolism is the building up or biosynthesis of macromolecules (see Module 1) from smaller molecular units into larger complexes. For instance, the anabolic process is often used during growth and repair phases of the cell. The above processes require a great deal of energy in order to carry out the required reactions for life. Without enzymes and their ability to increase the rate of a specific chemical reaction, these reactions would take too long and exhaust too much energy. If a cell runs out of energy before the necessary reactions conclude, it dies. Simply put: Cells need energy