Comprehensive Study Notes on Enzymes and Metabolism and Metabolism Notes: Enzymes as Biological Catalysts

Fundamental Principles of Enzymes and Metabolism

• Definition of Catalyst: A catalyst is a substance that increases the rate of a chemical reaction but remains unchanged by the reaction itself.

• Efficiency and Reusability: Because catalysts are not consumed or used up during the process, they can be utilized multiple times. Consequently, only small amounts of a catalyst are required relative to the volume of reactants.

• Enzymes as Biological Catalysts: Enzymes are specialized biological catalysts produced by living cells to accelerate biochemical reactions.

• Substrate Conversion: In enzymatic reactions, the enzyme facilitates the conversion of substrates into products ($\text{Substrate} \rightarrow \text{Product}$).

• Essentiality for Life: Without enzymes, most chemical reactions vital for life would occur too slowly at normal body temperatures to sustain life. Critical processes such as respiration, digestion, growth, and movement would be severely inhibited.

Metabolism and Metabolic Pathways

• Definition of Metabolism: Metabolism is defined as the complex network of interdependent and interacting chemical reactions within living organisms.

• Scope of Reactions: Living organisms contain thousands of metabolic reactions. While most occur inside cells (intracellular), some take place outside cells (extracellular), such as digestion in the intestine.

• Pathways: Metabolic reactions are organized into pathways where one molecule is transformed into another through a sequence of small, incremental steps.

  • Chains: Most pathways consist of linear reaction chains.

  • Cycles: Some pathways are organized in cyclical patterns.

• Metabolic Maps: The maps of these pathways are extremely complex. The Kyoto Encyclopedia of Genes and Genomes (KEGG) serves as a major online resource for documenting these pathways.

• Enzyme Specificity: A defining characteristic of enzymes is their specificity. Unlike non-biological catalysts (e.g., platinum, which can catalyze varied reactions), each enzyme typically catalyzes only one specific reaction or a specific group of reactions.

• Cellular Control: Specificity allows cells to exert precise control over metabolism. By producing specific enzymes, cells can trigger reactions that would otherwise not occur or would be extremely slow. Cells regulate reaction rates by:

  • Producing more enzymes ($\uparrow$ reaction rate).

  • Producing fewer enzymes ($\downarrow$ reaction rate).

  • Temporarily inhibiting enzymes when reactions are not required.

• Complexity across Species: Even simple prokaryotic cells produce hundreds of enzymes, while complex human cells (e.g., liver cells) produce thousands.

Anabolic and Catabolic Reactions

Metabolism is categorized into two primary divisions: anabolism and catabolism.

Anabolism (Building Up)

• Function: Anabolic reactions synthesize large, complex molecules from smaller constituent molecules.

• Energy Requirement: These reactions are endergonic and require energy, typically supplied by Adenosine Triphosphate (ATP).

• Process: Monomers are linked together to form macromolecules.

• Condensation Reactions: Many anabolic processes are condensation reactions, where water is produced as a byproduct.

• Examples:

  • Photosynthesis: Combining $CO_2$ and $H_2O$ using light energy to form larger organic molecules.

  • Protein Synthesis (Translation): Conducted by ribosomes.

  • DNA Synthesis (Replication): Building genetic material.

  • Carbohydrate Synthesis: Synthesizing starch, cellulose, or glycogen.

Catabolism (Breaking Down)

• Function: Catabolic reactions break down large, complex molecules into smaller components.

• Energy Release: These reactions are exergonic and release energy. This energy is often captured by coupling the reaction to the synthesis of ATP.

• Examples:

  • Digestion: Breaking down food in the mouth, stomach, and small intestine.

  • Cell Respiration: The oxidation of glucose or lipids into $CO_2$ and $H_2O$ in aerobic conditions.

  • Decomposition: The breakdown of dead organic matter by decomposers.

Molecular Structure and the Active Site

• Globular Proteins: Enzymes are globular proteins characterized by a precise 3D structure.

• Active Site Composition: The active site is a specific region on the enzyme where catalysis occurs. Its size varies depending on the substrate.

• Chemical Environment: Only a few amino acids within the active site are directly essential; they create several chemical conditions required to convert substrates to products.

• Folding Significance: Amino acids forming the active site are often not adjacent in the primary polypeptide sequence. They are brought together by the complex folding of the polypeptide chain into its tertiary or quaternary structure.

• Sensitivity to Alteration: If the enzyme's structure is altered, the active site may change shape, rendering the enzyme ineffective. This occurs regardless of whether the change happens at the active site or in another part of the protein scaffold.

Interactions Between Substrate and Active Site

• Induced-Fit Binding: The historical "Lock and Key" model is considered inappropriate because it implies a rigid structure. In reality, interactions between the substrate and the active site cause both to change shape upon binding. This includes alterations in bond angles and bond lengths to achieve an "induced fit."

• Stabilizing the Transition State: By binding to substrates, enzymes stabilize the transition state. This makes it more energetically feasible for bonds to break or form.

• Catalytic Cycle:

  1. Substrate approaches the active site via random molecular motion (collisions).

  2. Chemical properties on the enzyme surface attract the substrate when it is in close proximity.

  3. Binding occurs, causing conformational changes (induced fit).

  4. Substrate is converted to products.

  5. Products detach from the active site.

  6. The enzyme returns to its original state, ready for another cycle.

• Reaction Equation: $E + S \rightarrow E-S \rightarrow E-P \rightarrow E + P$

Collision Theory and Factors Affecting Catalyst Rate

• Successful Collisions: For a reaction to occur, a substrate must successfully collide with the enzyme's active site with the correct alignment.

• Molecular Motion: Binding can only occur if the substrate moves very close to the enzyme due to thermal motion.

• Factors Increasing Collision Frequency:

  • Temperature: Increases kinetic energy and molecular speed.

  • Substrate Concentration: Increases the density of particles.

  • Enzyme Concentration: Increases the availability of active sites.

• Orientation: Some enzymes use chemical properties to attract substrates or adjust their orientation, though these forces only work over very short distances.

Activation Energy ($E_a$)

• Transition States: Chemical reactions require an input of energy to reach a high-energy transition state before becoming products.

• Energy Change in Exothermic Reactions: In an exothermic reaction, the energy released during bond formation exceeds the activation energy required.

• Enzyme Impact on energy: Enzymes significantly decrease the activation energy needed to reach the transition state. They weaken bonds within the substrate, allowing the reaction to proceed faster (often by a factor of $1.0 \times 10^6$ or more).

• Net Energy: While enzymes lower the $E_a$, the net energy released or absorbed in the reaction remains constant.

Measuring Enzyme-Catalyzed Reactions

• Rate of Reaction: Calculated as $1 / \text{time taken (s)}$ or as the change in the amount of reactant/product per unit of time (e.g., $mmol\,s^{-1}$).

• Variables:

  • Independent: The factor deliberately varied (e.g., pH, Temperature, $[S]$).

  • Dependent: The quantity measured to calculate the rate (Enzyme Activity).

  • Control: Factors kept constant to ensure a fair test.

• Experimental Approaches:

  • Measure the amount of substrate used or product formed in a fixed, short time interval.

  • Measure the time taken for a reaction to reach completion (total conversion).

• Measurement Methods:

  • Mass: Electronic balance ($g$ or $mg$).

  • Volume: Pipettes, syringes, or cylinders ($dm^3$ or $cm^3$).

  • Acidity: pH meter or universal indicator.

  • Light Absorbance: Colorimeter (percentage absorbance).

Enzyme Kinetics and Inhibition

Basics of Kinetics

• $V_{\max}$: The maximum rate of reaction reached when all enzyme active sites are occupied by substrate.

• Substrate Concentration: Increasing $[S]$ increases the rate until $V_{\max}$ is achieved.

Competitive Inhibition

• Mechanism: The inhibitor is chemically similar to the substrate and binds directly to the active site, competing for entry.

• Effect on Kinetics: At very high substrate concentrations, the substrate effectively out-competes the inhibitor. Therefore, $V_{\max}$ can still be reached, but it requires a higher substrate concentration.

• Example: Statins (e.g., lovastatin) are competitive inhibitors of the enzyme HMG-CoA reductase, which is the rate-limiting step in the liver's synthesis of cholesterol.

Non-Competitive Inhibition

• Mechanism: The inhibitor binds to an allosteric site (a site other than the active site). This binding changes the shape of the enzyme, rendering the active site non-functional.

• Effect on Kinetics: Because the inhibitor prevents the enzyme from functioning regardless of substrate levels, the effective number of enzymes is reduced. $V_{\max}$ is lowered and cannot be restored by adding more substrate.

Feedback Inhibition (End-Product Inhibition)

• Function: A regulatory mechanism where the end product of a metabolic pathway inhibits the first enzyme in that pathway.

• Mechanism: The end product binds to an allosteric site on the first enzyme (non-competitive inhibition).

• Nature: This is a form of negative feedback; it prevents the wasteful accumulation of intermediate products.

• Example: The pathway converting threonine to isoleucine. Isoleucine acts as a non-competitive inhibitor for the first enzyme, threonine deaminase.

Mechanism-Based (Irreversible) Inhibition

• Definition: Occurs when an inhibitor binds irreversibly to an enzyme, often through the formation of permanent covalent bonds with the active site.

• Heavy Metals: Mercury ($Hg$) and Lead ($Pb$) are non-specific irreversible inhibitors that bind to $-SH$ groups in cysteine residues.

• Penicillin: A specific mechanism-based inhibitor. It targets the enzyme transpeptidase, which is essential for cross-linking peptidoglycan in bacterial cell walls.

  • Penicillin forms a permanent covalent bond in the active site.

  • Cell wall links are not reformed, but breakdown continues, leading to a weakened wall and cell lysis (bursting) due to osmotic pressure.

• Chemical Weapons: Many toxins, such as Sarin, are mechanism-based inhibitors. Sarin inhibits acetylcholinesterase, causing over-stimulation of the nervous system and respiratory shutdown.

Data and Quantitative Relationships

• Catalytic Effectiveness (Table 1 Data):

  • Carbonic Anhydrase: Rate without enzyme = $1.3 \times 10^{-1}$, Rate with enzyme = $1.0 \times 10^6$, Ratio = $7.7 \times 10^6$.

  • OMP Decarboxylase: Rate without enzyme = $2.8 \times 10^{-16}$, Rate with enzyme = $3.9 \times 10^8$. This enzyme is the most effective catalyst among the data provided due to the highest ratio.

• Adenylate Kinase Case Study: Consists of a polypeptide of $214$ amino acids. It catalyzes the reaction: $ATP + AMP \rightleftharpoons 2ADP$. Mutants (e.g., valine to glycine substitutions like V135G) show significantly reduced thermal stability compared to the Wild Type (WT).

• Glycogen Synthesis: Gerty and Carl Cori isolated two enzymes for glycogen production. One forms 1,4-bonds, and another forms 1,6-bonds (side-branches). Multiple enzymes are needed because enzymes are specific to limited bond types.

Questions & Discussion

• Ethical Positions in Science: The transcript highlights the dual-use nature of scientific discovery.

  • Gerhard Schrader: Discovered Sarin while developing insecticides. Is he the "father of nerve agents"?

  • Fritz Haber: Developed industrial ammonia production (Nobel Prize 1918), which increased food yields via fertilizer, but also pioneered chlorine gas as a chemical weapon.

  • Haber's Quote: "During peacetime a scientist belongs to the World, but during war time he belongs to his country."

• Discussion Point: Should scientists be recognized for contributions that improve life if they also contribute to weapons of mass destruction? How should the scientific community weigh the responsibility of the creator versus the application of the discovery?