Chapter 6 Reading Guide

Chapter 6: Enzyme Catalysis

6.1 Introduction to Enzymes

• What is a Catalyst?

  • A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It does not alter the equilibrium position of a reaction; instead, it helps reach equilibrium faster by lowering the activation energy ($E_a$).

  • Enzymes are biological catalysts, almost all of which are proteins.

• Ways to Increase Reaction Rate (and their practicality in cells):

  1. Increasing Temperature: Increases the kinetic energy of molecules, leading to more frequent collisions with sufficient energy to overcome the activation barrier. However, high temperatures can denature proteins and other biomolecules, making it impractical for cells.

  2. Increasing Reactant Concentration: Increases the frequency of collisions between reactant molecules. While effective, maintaining high concentrations of all reactants within the confined space of a cell can be metabolically expensive and challenging.

  3. Adding a Catalyst (Enzyme): Lowers the activation energy of the reaction. This is the most practical way for cells because:

     • Enzymes provide enormous rate accelerations (often $10^6$ to $10^{17}$-fold).

     • They operate under mild physiological conditions (neutral pH, moderate temperature, atmospheric pressure).

     • They exhibit high specificity, ensuring that only desired reactions occur.

     • Their activity can be regulated precisely, allowing cells to control metabolic pathways.

• Superiority of Enzymes to Other Chemical Catalysts:

  • Milder Reaction Conditions: Enzymes function optimally under physiological conditions (e.g., $37^{\circ}\text{C}$ in humans, pH 7.4), unlike many inorganic catalysts that require extreme temperatures, pressures, or pH.

  • Higher Reaction Rates: Achieve significantly greater rate enhancements than most synthetic catalysts, often by many orders of magnitude.

  • Greater Reaction Specificity: Enzymes are highly specific, catalyzing only one or a few very specific reactions with particular substrates, minimizing unwanted side reactions. Synthetic catalysts are often less specific.

  • Capacity for Regulation: Enzyme activity can be precisely regulated (turned on/off, increased/decreased) by various cellular mechanisms (e.g., allosteric control, covalent modification, proteolytic cleavage), allowing dynamic control over metabolic pathways. Non-biological catalysts generally lack this regulatory capability.

• Reaction Catalyzed by Chymotrypsin:

  • Chymotrypsin is a serine protease that catalyzes the hydrolysis of peptide bonds (amide bonds) in proteins.

  • It specifically cleaves peptide bonds on the C-terminal side of large aromatic or hydrophobic amino acid residues, such as Phenylalanine (Phe), Tryptophan (Trp), and Tyrosine (Tyr).

  • It can also catalyze the hydrolysis of ester bonds, which is exploited in experimental studies.

• Use of p-Nitrophenylacetate to Study Chymotrypsin:

  • Methodology: p-Nitrophenylacetate is an artificial chromogenic substrate used to study the kinetics of chymotrypsin. When chymotrypsin hydrolyzes p-nitrophenylacetate, p-nitrophenolate is released, which is yellow and can be easily detected and quantified using a spectrophotometer (absorbance at $400 ext{ nm}$).

  • Kinetic Insights: This allows researchers to observe the reaction in two distinct phases: an initial rapid burst phase (representing the fast acylation of the enzyme) followed by a slower steady-state phase (representing the rate-limiting deacylation step). This two-phase kinetics provides strong evidence for the formation of an acyl-enzyme intermediate.

  • Difference from Natural Substrate: p-Nitrophenylacetate is an ester, whereas natural substrates for chymotrypsin are peptides (amides). While it mimics the acyl-enzyme intermediate formation, it lacks the complex molecular recognition features (e.g., specific interactions within the S1, S2, P1', P2' pockets) that natural protein substrates engage with, which contribute to the enzyme's high specificity.

6.2 Enzyme Mechanisms

• Reaction Coordinate Diagram for a Spontaneous, but Slow One-Step Reaction:

  • A reaction coordinate diagram visually represents the free energy changes during a chemical reaction.

  • X-axis: Reaction Coordinate (progress of the reaction from reactants to products).

  • Y-axis: Free Energy (G).

  • Features:

    • The energy level of reactants (A) is higher than that of products (B) for a spontaneous reaction.

    • The difference in free energy between products and reactants is the standard Gibbs free energy change ($\Delta G$). For a spontaneous reaction, $\Delta G < 0$ (exergonic).

    • There is an energy maximum, representing the transition state ($S^\ddagger$), an unstable, high-energy intermediate state.

    • The energy difference between the reactants and the transition state is the activation energy ($E_a$ or $\Delta G^\ddagger$). This barrier must be overcome for the reaction to proceed.

    • A slow spontaneous reaction implies a relatively high activation energy, even if the overall free energy change is favorable.

  • [Imagine a graph: A starting high, dipping to a peak (transition state), then flowing downhill to B, which is lower than A. The peak is labeled Transition State. The height from A to the peak is $E_a$. The drop from A to B is $\Delta G$ (negative value).]

• Free Energy Release for 1000-Fold Rate Increase (Rule of Thumb):

  • The rule of thumb states that a 10-fold increase in reaction rate at $25^{\circ}\text{C}$ (or $298 ext{ K}$) corresponds to a decrease of $5.7 ext{ kJ/mol}$ (or $1.4 ext{ kcal/mol}$) in the activation free energy ($\Delta G^\ddagger$).

  • To achieve a 1000-fold ($10^3$) increase in reaction rate, the enzyme must lower the activation free energy by:
    $3 imes 5.7 ext{ kJ/mol} = 17.1 ext{ kJ/mol}$ (or $3 imes 1.4 ext{ kcal/mol} = 4.2 ext{ kcal/mol}$).

  • This reduction in activation energy is primarily achieved by the enzyme's preferential binding to and stabilization of the transition state.

• Definitions: Cofactor, Coenzyme, Cosubstrate, and Prosthetic Group:

  • Cofactor: A non-protein chemical component essential for the activity of an enzyme. Cofactors can be inorganic ions (e.g., $Mg^{2+}$, $Zn^{2+}$, $Fe^{2+}$) or complex organic molecules (coenzymes).

  • Coenzyme: An organic cofactor, often derived from vitamins (e.g., NADH from niacin, FAD from riboflavin, Coenzyme A from pantothenate). Coenzymes directly participate in catalysis, often acting as transient carriers of functional groups (e.g., electrons, atoms, or small molecules).

  • Cosubstrate: A type of coenzyme that binds transiently to the enzyme, participates in catalysis, and then dissociates from the enzyme, behaving much like a second substrate. It is chemically modified during the reaction and subsequently regenerated in a separate reaction (e.g., ATP, NAD+, FAD).

  • Prosthetic Group: A coenzyme or metal ion that is very tightly or even covalently bound to the enzyme throughout enzymatic catalysis. It remains associated with the enzyme during its catalytic cycle, often playing a permanent structural or catalytic role (e.g., heme in cytochrome c, retinal in rhodopsin, FAD in succinate dehydrogenase).

• Three Kinds of Catalytic Mechanisms Used by Enzymes:

  1. General Acid-Base Catalysis: Involves the transfer of protons to or from the substrate or an intermediate by groups within the active site (e.g., His, Asp, Glu, Lys, Cys, Tyr). A general acid donates a proton to stabilize a developing negative charge, while a general base accepts a proton to facilitate nucleophilic attack or stabilize a developing positive charge. This mechanism avoids the need for extreme pH changes that would be destructive to the cell.

  2. Covalent Catalysis: Involves the formation of a transient, reversible covalent bond between the enzyme and the substrate during the catalytic process. This often involves a nucleophilic residue in the active site (e.g., Ser, Cys, His, Lys) attacking an electrophilic center on the substrate, forming an intermediate that is subsequently resolved. This provides a different reaction pathway with a lower activation energy.

  3. Transition State Stabilization (Preferential Binding to the Transition State): This is the fundamental principle by which enzymes achieve high rate enhancements. Enzymes are designed to bind the transition state of a reaction with much greater affinity than they bind the ground state (substrates or products). This tight binding lowers the energy of the transition state, thereby reducing the activation energy ($\Delta G^ ext{\ddagger}$) and vastly increasing the reaction rate.

  • Other important mechanisms include: Metal Ion Catalysis, Proximity and Orientation Effects (discussed later).

• DIPF to Elucidate Chymotrypsin Mechanism:

  • DIPF (diisopropylphosphofluoridate) is a chemical reagent that was used as an irreversible inhibitor of chymotrypsin.

  • Mechanism of Action: DIPF reacts covalently with a specific hydroxyl group of a serine residue in the active site of chymotrypsin (Ser195 in particular). This reaction forms an irreversible diisopropylphosphoryl-enzyme adduct, permanently inactivating the enzyme.

  • Significance: The identification of Ser195 as the modified residue definitively indicated that this specific serine residue is absolutely crucial for chymotrypsin's catalytic activity, identifying it as the nucleophile that forms the acyl-enzyme intermediate during catalysis. This was a key piece of evidence in understanding the catalytic mechanism of serine proteases.

• Chymotrypsin's Catalytic Triad: Residues and Roles:

  • The catalytic triad of chymotrypsin consists of three amino acid residues: Serine 195 (Ser195), Histidine 57 (His57), and Aspartate 102 (Asp102). These residues are precisely positioned in the active site to work cooperatively.

  • Ser195 (Nucleophile): The hydroxyl group of Ser195 acts as the primary nucleophile. It is activated by His57, which abstracts its proton, making it a highly reactive alkoxide ion. This alkoxide then attacks the carbonyl carbon of the scissile peptide bond, forming a transient covalent acyl-enzyme intermediate.

  • His57 (General Acid-Base Catalyst): His57 acts as a general base, accepting a proton from Ser195 to enhance its nucleophilicity. Later, it acts as a general acid, donating a proton to the departing amine group. In the deacylation step, it acts as a general base to activate a water molecule and then as a general acid to donate a proton back to Ser195.

  • Asp102 (Stabilizer/Orientator): Asp102 is buried within the active site and forms a strong hydrogen bond with His57. Its role is to precisely orientate His57 and to stabilize the positively charged imidazolium ion formed on His57 during catalysis, effectively increasing His57's basicity and its ability to deprotonate Ser195. This enhances the overall nucleophilicity of Ser195 and prevents the charge from dissipating into the solvent.

• Chymotrypsin's Catalytic Mechanism (Steps):
  The mechanism proceeds in two main phases: acylation (formation of acyl-enzyme intermediate and release of the C-terminal product) and deacylation (hydrolysis of acyl-enzyme and release of the N-terminal product), regenerating the free enzyme.

  1. Substrate Binding: The polypeptide substrate binds to the active site, positioning the scissile peptide bond (the bond to be cleaved) near S195. The side chain of the residue N-terminal to the scissile bond (P1 residue) parks into the S1 specificity pocket.

  2. Nucleophilic Attack (Acylation, Step 1): The activated Ser195 (as a potent alkoxide, due to proton abstraction by His57 stabilized by Asp102) performs a nucleophilic attack on the electrophilic carbonyl carbon of the scissile peptide bond.

  3. First Tetrahedral Intermediate Formation (Acylation, Step 2): This attack leads to the formation of a tetrahedral intermediate. The carbonyl oxygen of the substrate becomes negatively charged (an oxyanion) and is stabilized by hydrogen bonds from the backbone amides of Gly193 and Ser195 in the oxyanion hole.

  4. Amine Departure & Acyl-Enzyme Formation (Acylation, Step 3): The tetrahedral intermediate collapses. The C-N bond of the peptide is cleaved. His57, now acting as a general acid, donates its proton to the amino group of the original leaving group (the C-terminal fragment of the cleaving peptide), which then departs from the active site. The N-terminal fragment of the substrate remains covalently attached to Ser195, forming an acyl-enzyme intermediate.

  5. Water Entry (Deacylation, Step 1): A water molecule enters the active site.

  6. Second Nucleophilic Attack (Deacylation, Step 2): His57 (now acting as a general base) abstracts a proton from the incoming water molecule, activating it into a nucleophilic hydroxide ion. This hydroxide ion then attacks the carbonyl carbon of the acyl-enzyme intermediate.

  7. Second Tetrahedral Intermediate Formation (Deacylation, Step 3): A second tetrahedral intermediate is formed. Again, the carbonyl oxygen becomes an oxyanion and is stabilized in the oxyanion hole by hydrogen bonds from Gly193 and Ser195.

  8. Carboxylate Departure & Enzyme Regeneration (Deacylation, Step 4): The second tetrahedral intermediate collapses. The covalent bond between Ser195 and the acyl group is broken. His57 (now acting as a general acid) donates a proton back to Ser195's oxygen, regenerating the active serine hydroxyl group. The carboxyl group of the N-terminal fragment (now the second product) departs, and the enzyme is fully regenerated and ready for a new catalytic cycle.

6.3 Enzyme-Substrate Interactions and Evolution

• Lock-and-Key Model vs. Induced Fit Model:

  • Lock-and-Key Model (Emil Fischer, 1894):

    • Concept: Proposed that the enzyme active site has a rigid, pre-formed structure that is perfectly complementary to the substrate, much like a specific key fits into a specific lock. The binding is static.

    • Strengths: Explains the high specificity of enzymes.

    • Limitations: Does not adequately explain how enzymes lower the activation energy by stabilizing the transition state, nor does it account for the dynamic nature of enzyme-substrate interactions.

  • Induced Fit Model (Daniel Koshland Jr., 1958):

    • Concept: Proposes that the enzyme's active site is flexible and undergoes a conformational change upon substrate binding. This change is induced by the substrate and optimizes the fit not for the substrate itself, but for the transition state of the reaction, essentially molding the active site around the transition state.

    • Strengths: Better explains how enzymes achieve high specificity, lower activation energy by transition state stabilization, and can release products more easily (as the complex becomes less complementary after the reaction).

    • Current View: The induced fit model is more widely accepted, acknowledging the dynamic nature of enzymes and their role in stabilizing the transition state.

• Role of the Oxyanion Hole in Chymotrypsin:

  • The oxyanion hole is a critical structural feature within the active site of chymotrypsin (and other serine proteases) that plays a crucial role in stabilizing the transient, negatively charged tetrahedral intermediates formed during peptide bond hydrolysis.

  • During both the acylation and deacylation phases, the carbonyl oxygen of the amide or ester bond, upon nucleophilic attack, becomes an oxyanion (carrying a full negative charge). The oxyanion hole is a region formed by the backbone amide NH groups of Gly193 and Ser195 (among others), which are positioned to form strong hydrogen bonds with this developing negatively charged oxygen.

  • Stabilization Mechanism: These hydrogen bonds effectively delocalize and stabilize the negative charge on the oxyanion, dramatically lowering the energy of the transition state. This reduction in the activation energy for both the acylation and deacylation steps is a major contributor to the enzyme's extraordinary catalytic efficiency.

  • Significance: Without the specific stabilization provided by the oxyanion hole, the tetrahedral intermediates would be much higher in energy, significantly slowing down the reaction rate.

• Role of Proximity and Orientation Effects in Catalysis:

  • Proximity Effect (Effective Molarity): Enzymes bring reacting functional groups into close proximity within the active site, effectively increasing their local concentration. This confinement dramatically reduces the number of translational and rotational degrees of freedom for the reacting molecules, thereby increasing the frequency of productive collisions (collisions with correct alignment and sufficient energy) compared to reactions occurring freely in solution.

  • Exclusion of Water from the Active Site:

    • Mechanism: Many enzyme active sites are designed to exclude water molecules from the immediate vicinity of the reacting groups. This creates a microenvironment with a lower dielectric constant compared to bulk solvent.

    • Promotion of Catalysis: In an apolar or less polar environment, electrostatic interactions (such as those involved in transition state stabilization, e.g., hydrogen bonds, charge-charge interactions) are significantly strengthened. Water molecules can compete with active site residues for H-bonding partners or screen charges, thereby weakening crucial catalytic interactions. By excluding water, the enzyme ensures that its specific array of residues can exert their full catalytic potential, particularly in stabilizing charged transition states or intermediates, which would otherwise be solvated and stabilized by water in bulk solution. This reduction in the dielectric constant effectively lowers the free energy barrier for the reaction.

• Distinguishing between Divergent Evolution and Convergent Evolution:

  • Divergent Evolution:

    • Concept: Occurs when two or more species, or two or more proteins/enzymes, share a common ancestor but have evolved to become different over time, often adapting to different environments or functions. They share structural similarities (homology) reflecting their common origin, but their functions may vary.

    • Example in Enzymes: The serine protease family (e.g., chymotrypsin, trypsin, elastase) are excellent examples. They all share a common evolutionary ancestor and similar overall three-dimensional structures, including the catalytic triad, but have evolved different substrate specificities due to changes in their specificity pockets.

  • Convergent Evolution:

    • Concept: Occurs when unrelated species or proteins/enzymes independently evolve similar traits or functions due to similar selective pressures or functional requirements, without sharing a recent common ancestor. Their structures might be quite different, but they arrive at similar solutions.

    • Example in Enzymes: Subtilisin (a bacterial protease) and the mammalian serine proteases (like chymotrypsin) are an example. While both perform peptide bond hydrolysis using a catalytic triad (Ser, His, Asp), their overall three-dimensional structures are completely different, indicating they evolved independently to achieve the same catalytic mechanism.

6.4 Proteolytic Enzymes and Regulation

• Role of the Specificity Pocket in Proteolytic Enzymes:

  • Description: The specificity pocket (often referred to as the S1 pocket for serine proteases like chymotrypsin) is a distinct binding pocket within the active site of proteolytic enzymes. It is specifically designed to accommodate the side chain of the amino acid residue (the P1 residue) immediately N-terminal to the peptide bond that the enzyme is programmed to cleave (the scissile bond).

  • Function: The chemical nature (size, charge, hydrophobicity) and dimensions of this pocket are crucial determinants of the enzyme's substrate specificity.

    • Chymotrypsin: Has a deep, hydrophobic S1 pocket, which allows it to preferentially bind large, hydrophobic, or aromatic amino acid side chains (Phe, Trp, Tyr) at the P1 position, directing the peptide bond of interest into the active site for hydrolysis.

    • Trypsin: Has a negatively charged aspartate residue at the bottom of its S1 pocket, making it specific for positively charged amino acid residues (Lys, Arg).

    • Elastase: Has smaller, bulkier residues at the entrance of its S1 pocket, restricting access to small, nonpolar residues (Ala, Gly, Val).

  • Significance: The specificity pocket is key to the enzyme's ability to selectively cleave particular peptide bonds within a protein, avoiding indiscriminate degradation and ensuring precise control over protein function.

• Why are most Proteases Synthesized as Zymogens?

  • Concept: Many proteolytic enzymes (proteases) are synthesized as inactive precursors called zymogens (also known as proenzymes).

  • Mechanism of Activation: Zymogens are typically activated by specific proteolytic cleavage events, often by another protease, which removes a small peptide segment or rearranges the protein structure, leading to a conformational change that unmasks or forms the active site.

  • Biological Rationale:

    1. Prevention of Autodigestion: Synthesizing proteases in an inactive form prevents them from digesting the very cells and tissues that produce them. This is particularly critical for digestive enzymes (e.g., trypsinogen, chymotrypsinogen) or blood clotting factors.

    2. Regulation and Timing: Zymogen activation allows for precise control over when and where proteolytic activity occurs. This is vital in processes such as blood coagulation, immune response, and programmed cell death (apoptosis), where uncontrolled activity would be disastrous.

    3. Localizing Activity: Activation can be localized to specific compartments or tissues, ensuring the enzyme acts only where needed. For instance, digestive enzymes are activated only in the lumen of the digestive tract.

6.5 Medical Applications/Examples

• Role of Thrombin in Coagulation and Enzyme Type:

  • Role in Coagulation: Thrombin is a key serine protease in the blood coagulation cascade. Its primary role is to convert soluble fibrinogen (a plasma protein) into insoluble fibrin monomers. These fibrin monomers then spontaneously polymerize to form a soft clot. Thrombin also activates Factor XIII, which cross-links the fibrin strands to form a stable, hard clot. Additionally, thrombin activates several other clotting factors (e.g., Factors V, VIII, XI, and XIII) and platelets, amplifying the coagulation response.

  • Enzyme Type: Thrombin is a serine protease, belonging to the same family of enzymes as chymotrypsin, trypsin, and elastase. It utilizes a catalytic triad (Ser, His, Asp) and an oxyanion hole in its active site to perform peptide bond hydrolysis. Its specificity pocket is designed to recognize and cleave specific Arg-Gly bonds in its substrates.

• How does Heparin work as an Anticoagulant?

  • Mechanism of Action: Heparin is a highly sulfated glycosaminoglycan, commonly used as an injectable anticoagulant. It does not directly inhibit thrombin or other clotting factors. Instead, heparin acts indirectly by binding to and markedly enhancing the activity of a plasma protein called antithrombin III (ATIII).

  • Antithrombin III Enhancement: ATIII is a natural inhibitor of thrombin and Factor Xa (and other coagulation enzymes). When heparin binds to ATIII, it induces a conformational change in ATIII, increasing its affinity for thrombin and Factor Xa by up to 1000-fold. This greatly accelerates the rate at which ATIII inhibits these crucial coagulation enzymes.

  • Result: By essentially acting as an allosteric activator for ATIII, heparin effectively neutralizes thrombin and Factor Xa, thereby preventing the formation of fibrin and inhibiting blood clot formation. Heparin is released from the ATIII-enzyme complex and can then bind to another ATIII molecule, acting catalytically.