Catalysts, Enzymes & Inhibitors – Reaction Kinetics (Ch. 22 Pt 3)

Factor 5 Affecting Reaction Rate – Catalysts

• A catalyst is a chemical substance that:
• Is not a reactant or product (neither created nor destroyed overall).
• Does participate in, and stabilise, the activated complex.
• Provides an alternative reaction pathway with lower activation energy $\big(E_a\big)$.
• Can repeatedly assist successive molecules once released from the activated complex.

• Core textbook definition to memorise – “A catalyst lowers the activation energy of a reaction and therefore speeds the reaction rate.”

• Because the catalyst offers a new pathway but does not change the energies of reactants or products:
• $\Delta H$, $\Delta E$ and overall thermodynamics remain unchanged.
• Only $E_a$ and the shape/height of the energy diagram hump (transition‐state peak) change.

• Orientation effect – Catalysts often hold reactants so functional groups meet at the correct angle & distance, ensuring more successful collisions.

Energy-Profile Sketch (Conceptual)

• Uncatalysed: high peak ⇒ slow rate.
• Catalysed: lower peak ⇒ fast rate.
• Reactant & product baselines identical for both curves ⇒ $\Delta H<em>{catalysed}=\Delta H</em>{uncatalysed}$.

Two Major Catalyst Classes

1. Homogeneous Catalysts (same phase as reactants)
   • Actually form bonds with reactants → temporary part of the activated complex.
   • Excellent for orienting large, complicated molecules (e.g. proteins, macromolecules).
   • Common biological example: enzymes (proteins functioning at mild pH ≈7.4 and body temperature ≈310 K).

2. Heterogeneous Catalysts (different phase)
   • Usually a solid providing a surface on which gaseous or liquid reactants adsorb.
   • Reaction occurs on/near the surface and products desorb, freeing sites.
   • Key surface mechanism: adsorption (see next section).

Adsorption vs Absorption

• $\textbf{absorption}$ – species penetrate into bulk of a material (paper towel absorbs water).
• $\textbf{adsorption}$ – species adhere only to surface (chewing-gum adsorbs to shoe sole). Heterogeneous catalysis relies on adsorption.

Case Study – Catalytic Converter (Environmental Relevance)

• Device inserted in automobile exhaust; interior coated with platinum (Pt) or other noble metals.
• Gaseous pollutants (e.g. $NO$, $CO$, unburned hydrocarbons) adsorb to Pt.
• Example surface reaction:
$2\,NO<em>{(g)} \xrightarrow[\text{Pt surface}]{ } N</em>2<em>{(g)} + O</em>2_{(g)}$
• Outcome: fewer toxic gases reach the atmosphere → lowered acid-rain potential, reduced photochemical smog.

Biological Catalysis – Enzymes

• Speed enhancement ranges millions to trillions-fold compared with uncatalysed pathway.
• Enable life-critical pathways (e.g. citric acid cycle, metabolism) under gentle conditions (physiological pH, 37 °C, 1 atm).

• Structural models:
• "Lock & key" (historical, oversimplified).
• Induced-fit (currently accepted): enzyme slightly changes conformation to cradle substrate.

• Illustrative enzyme: carbonic anhydrase
• Reaction: $CO<em>2 + H</em>2O \leftrightarrow H<em>2CO</em>3$.
• Rate ↑ $\approx 10^7$–$10^8$‐fold when catalysed; active site features a coordinated $Zn^{2+}$ that polarises water, eases proton removal.

• Why catalysts matter physiologically:
• Raising temperature or altering pH enough to match enzyme rate would kill cells.

Recap of Key Equations & Concepts

• Activation-energy reduction: Ea^{\text{catalysed}} < Ea^{\text{uncatalysed}}.
• Enthalpy change unaffected: $\Delta H = H<em>{products} - H</em>{reactants}$ (same for both pathways).

Inhibitors (Reaction Quenching)

• $\textbf{Definition}$ – A chemical that stops (not merely slows) a reaction; often termed a “quench” in organic chemistry.

• Mechanistic categories:

1. Competitive inhibition
   • Inhibitor resembles substrate / signal molecule.
   • Binds the active site (or receptor) → blocks substrate access.
   • Examples/benefits:
   • Drug design against tuberculosis: inhibitor occupies bacterial enzyme site → stops cell-wall synthesis.
   • Natural isothiocyanates in broccoli & Brussels sprouts compete for MRSA bacterial enzymes → reduces infection.

2. Non-competitive (allosteric) inhibition
   • Inhibitor binds elsewhere; changes protein shape so active site stops functioning.

• Energetic depiction of a quench:
• Alternative pathway with lower $E_a$ and also lower final energy (product more stable) → reaction “prefers” inhibitor route and reactants never reach original product.
• Conceptual diagram: original curve vs. inhibitor curve starting at same reactant energy but dropping to a deeper product well.

• Cell-signalling angle:
• Virus or native ligand may dock on receptor → downstream gene expression (protein synthesis).
• Introduce competing inhibitor → receptor filled, signalling cascade halted, harmful protein never produced.

Practical / Ethical / Real-World Connections

• Automotive catalysis – legislation for catalytic converters arose from 1980s acid-rain crisis; illustrates socio-environmental drive for chemical innovation.
• Pharmaceutical design – understanding enzyme catalysis & inhibition underpins antibiotics, antivirals, chemotherapy, and metabolic-disease drugs.
• Diet & public health – phytochemicals acting as mild inhibitors (e.g., cruciferous-vegetable isothiocyanates) show link between nutrition and molecular medicine.

Quick Study Checklist

• Memorise catalyst definition & its three main impacts: alternative pathway, lower $E<em>a$, unchanged $\Delta H$. • Differentiate homogeneous vs. heterogeneous catalysts and recall an example of each (enzyme vs. Pt converter).
• State adsorption vs. absorption.
• Interpret energy diagram: locate reactants, products, transition state, $E</em>a$ (with & without catalyst).
• Describe induced-fit enzyme model.
• Define inhibitor; distinguish competitive & non-competitive; give at least one biomedical example.
• Write & explain the pollution-control reaction $2\,NO \rightarrow N<em>2 + O</em>2$.
• Remember catalysts enable biological rates impossible by mere temperature or pH adjustments.

These notes capture every major/minor point from the lecture, include all examples, clarify concepts such as adsorption, and integrate broader environmental and biomedical contexts for comprehensive exam preparation.