#7 Drug design III: orthosteric drugs
Inhibition of Enzymes Overview
The lecture provides an in-depth exploration into the fascinating world of enzyme inhibition, with a particular emphasis on sophisticated mechanisms that extend beyond simple competitive binding. The core principle discussed is how inhibitors can achieve potent effects by interacting with enzymes at a molecular level to stabilize transition states or high-energy intermediates, rather than merely blocking the active site in a substrate-like fashion. This approach leverages the intrinsic catalytic mechanism of enzymes for targeted drug design.
Enzymes and Their Role
Definition of Enzymes: Enzymes are highly specialized, usually proteinaceous (though some RNA molecules, or ribozymes, also exist), biological catalysts. They dramatically increase the rate of specific biochemical reactions, which would otherwise proceed at infinitesimally slow rates under physiological conditions (moderate temperature, neutral pH). Life as we know it would not be possible without their remarkable efficiency.
Rate Enhancement by Enzymes: Enzymes achieve colossal rate enhancements, typically accelerating reaction rates by factors ranging from 10^6 to 10^{17} compared to their uncatalyzed counterparts. Crucially, enzymes do not alter the overall thermodynamics of a reaction; they do not change the standard free energy change (\Delta G) between reactants and products, nor do they affect the equilibrium constant (K_{eq}) of a reversible reaction. Instead, their profound impact is purely kinetic: they accelerate the attainment of equilibrium.
Mechanism of Action: Enzymes function by providing an alternative reaction pathway that requires significantly less activation energy (\Delta G^eq ). They do this primarily through:
Preferential Binding to the Transition State: The most fundamental principle is that enzymes bind to and stabilize the reaction's transition state much more tightly than they bind to the substrates or products. This differential binding energy is harnessed to lower the energy required to reach the transition state.
Optimal Orientation: Bringing substrates together in the correct spatial orientation to facilitate bond formation or breakage.
Strain Induction: Distorting or straining substrates to physically resemble the geometry of the transition state, making them more reactive.
Microenvironment Creation: Providing a localized environment within the active site that is ideally suited for the reaction, such as specific pH values, electrostatic interactions, or exclusion of water.
Common types of reactions catalyzed include hydrolysis (e.g., breaking peptide bonds), various group transfer reactions, and sophisticated bond formation processes.
Understanding Transition States
Transition State Concept: The transition state (often denoted as X^{eq} or ES^{eq} for enzyme-bound transition states) represents the highest energy point along the reaction pathway on a reaction coordinate diagram. It is a transient, unstable molecular configuration where existing bonds are partially broken, and new bonds are partially formed. It exists for an extremely short period (on the order of femtoseconds, 10^{-15} s).
Detailed Example (SN2 reaction revisited): In a nucleophilic substitution bimolecular (SN2 ) reaction, for instance, a nucleophile attacks a carbon atom while a leaving group simultaneously departs. The transition state features the central carbon atom simultaneously bonded to five groups (the original three unchanged bonds, the partially formed bond to the nucleophile, and the partially broken bond to the leaving group). This carbon is temporarily trigonal bipyramidal, with partial negative charges on both the attacking nucleophile and the departing leaving group. This highly strained, high-energy arrangement is intrinsically unstable and cannot be isolated. It rapidly rearranges to form products.
Characteristics of Transition States: They possess no significant lifetime and are theoretical constructs that cannot be directly observed or isolated. Their existence and properties are inferred from kinetic studies, isotope effects, and computational chemistry. Overcoming the energy barrier (activation energy) to reach this state is the rate-limiting step of a reaction. Enzymes dramatically lower this energy barrier, enabling reactions to occur rapidly and efficiently at physiological temperatures, which are far too low to provide the necessary kinetic energy for uncatalyzed reactions.
Energetics of Enzyme Reactions
Enzymatic Reaction Pathway: The general sequence of an enzymatic reaction involves several key stages (though variations exist):
Reactants (S): Substrates initially present.
Enzyme-Substrate Complex (ES): Substrates bind to the enzyme's active site.
Transition State (ES^{ eq}): The enzyme-bound complex enters the unstable, high-energy transition state, where catalysis truly occurs via preferential binding.
Intermediates: In multi-step reactions, stable (though often transient) intermediates may form after the transition state and before the final products.
Enzyme-Product Complex (EP): Products are still bound to the enzyme.
Products (P): Products are released from the enzyme, regenerating the free enzyme.
Barrier Energy (\Delta G^{ eq}): The peak on the reaction coordinate diagram represents the free energy of the transition state. The difference in free energy between the reactants and the transition state is the activation energy (\Delta G^{eq}). A higher activation energy corresponds to a slower reaction rate. Enzymes do not primarily lower the energy of the substrates or products; instead, they specifically decrease the G^{e} of the transition state, effectively reducing the energy required for the reaction to proceed. This is achieved by creating an active site environment that is geometrically and electronically complementary to the transition state, resulting in a more stable ES^{e} complex than the uncatalyzed X^{e} .
Enzyme Design and Mechanism for Inhibitors
The objective in modern drug discovery often revolves around designing compounds that precisely mimic the geometric and electronic features of the transition state or high-energy intermediates of an enzyme-catalyzed reaction. These compounds are known as transition-state analogues (TSAs).
Principle of TSAs: Since enzymes have evolved to bind their reaction's transition state with extraordinarily high affinity (often 10^8 to 10^{15}-fold greater affinity than for the substrate), a stable molecule that closely resembles this transition state should also bind with exceptionally high affinity and act as a potent inhibitor.
Drug Design Challenges: Developing effective TSAs is challenging due to inherent properties of transition states:
Stability: Actual transition states are inherently unstable and fleeting. To be viable as drugs, TSAs must be thermodynamically stable enough to be synthesized, stored, and administered, while maintaining the critical structural features of the unstable transition state.
Chemical Structure Fidelity: Potential inhibitors must precisely mimic the transient geometry, bond lengths, angles, and crucial partial charges of the transition state. This is a delicate balance, as they must not be actual substrates, which would undergo catalysis and be rapidly turned over by the enzyme.
Drug-like Properties: Beyond potent enzyme inhibition, TSAs must possess favorable pharmacokinetic (absorption, distribution, metabolism, excretion) properties, including adequate bioavailability (e.g., oral bioavailability), membrane permeability (to reach intracellular targets), and metabolic stability to be effective therapeutic agents.
Application in Aspartic Protease Inhibition
Case Study - HIV Protease: Human Immunodeficiency Virus (HIV) protease is a critical enzyme in the HIV life cycle, belonging to the family of aspartic proteases. This viral enzyme is indispensable for the proteolytic processing of large, inactive viral precursor polyproteins into smaller, functional proteins (e.g., gag, pol, env proteins) required for the assembly of new, infectious virion particles. Inhibition of HIV protease effectively blocks viral maturation and therefore replication, making it a prime target for antiretroviral therapy.
Aspartic Protease Mechanism: Aspartic proteases utilize a highly conserved catalytic diad of two aspartate residues in their active site. The general mechanism involves:
General Acid-Base Catalysis: One aspartate residue acts as a general base, abstracting a proton from a water molecule, making it a more potent nucleophile (hydroxide-like).
Nucleophilic Attack: This activated water molecule performs a nucleophilic attack on the carbonyl carbon of the scissile (cleavable) peptide bond of the substrate.
Tetrahedral Intermediate Formation: This attack leads to the formation of a high-energy tetrahedral intermediate where the carbonyl carbon of the peptide bond briefly becomes sp\text{3} hybridized, and the carbonyl oxygen gains a negative charge, often stabilized by a proton from the second aspartate (which acts as a general acid).
Collapse and Product Release: The tetrahedral intermediate is highly unstable and rapidly collapses. The original N-terminal portion of the peptide (now bearing a free amino group) leaves, and a proton is transferred to the leaving amino group. The C-terminal portion of the peptide (now bearing a free carboxyl group) is also released. Both aspartates are regenerated, ready for another catalytic cycle.
Protease (Tetrahedral Intermediate of Amide Hydrolysis): The specific high-energy intermediate targeted for HIV protease inhibition is a gem-diol-like tetrahedral intermediate formed during the hydrolysis of the amide (peptide) bond. In this unstable state, the original carbonyl carbon is now bonded to two oxygen atoms (one from the original carbonyl, one from the attacking water) and the nitrogen of the amide bond, all in a tetrahedral geometry. The carbonyl oxygen is typically negatively charged or protonated (as in a gem-diol). Inhibitors are designed to be stable, non-cleavable mimics of this specific, highly ordered transition state. They typically incorporate a non-hydrolyzable group, such as a hydroxyl group (-OH) or a reduced amide, precisely where the scissile carbonyl would have been, thus preventing cleavage while maintaining the critical interactions for tight binding.
Darunavir: Darunavir is a potent and highly effective HIV protease inhibitor. It exemplifies the successful application of transition state analogue design. Its structure incorporates a hydroxyethylene isostere as a crucial component that mimics the tetrahedral intermediate of the natural substrate. This stable chemical moiety effectively replaces the scissile peptide bond, preventing Darunavir from being cleaved by the enzyme. The hydroxyl group within the hydroxyethylene unit precisely occupies the position of the attacking water molecule in the tetrahedral intermediate, allowing it to form essential hydrogen bonds and electrostatic interactions within the HIV protease active site, leading to exceptionally tight binding and potent enzyme inhibition. Darunavir's improved pharmacokinetic profile and enhanced binding affinity contribute to its efficacy against both wild-type and many drug-resistant HIV strains.
Example of Transition State Mimics: Purine Nucleoside Phosphorylases (PNPs)
A Classical Example: Another prominent example of transition state mimicry involves inhibitors of purine nucleoside phosphorylases (PNPs). PNPs are crucial enzymes in the purine salvage pathway, catalyzing the reversible phosphorolysis of purine nucleosides (like inosine and guanosine) into a free purine base and ribose-1-phosphate.
Importance as a Drug Target: Human PNP (hPNP) is particularly important because it is highly expressed in T-lymphocytes. Inhibiting hPNP leads to the accumulation of toxic deoxyguanosine nucleosides, specifically in T-cells, causing their selective depletion. This makes hPNP inhibitors attractive candidates for immunosuppression (e.g., in autoimmune diseases, organ transplant rejection) and for certain T-cell leukemias.
Human PNPs and Subtle Differences in Transition State: The transition state for PNP catalysis is characterized by a significant oxocarbenium ion-like character on the ribose moiety and a developing positive charge on the purine base as it departs. High-affinity inhibitors are designed to mimic this highly charged and electronically delocalized transition state. For instance, immucillin-H type compounds are designed with a positively charged iminoribitol ring system that precisely mimics the partial positive charge and distorted conformation of the ribose in the transition state. Furthermore, understanding the subtle structural and electronic differences in the transition states of hPNP versus, for example, bacterial or parasitic PNPs, allows for the rational design of highly selective inhibitors, minimizing off-target effects and maximizing therapeutic index. These differences can be as minute as slight variations in bond angles or the exact distribution of partial charges, which computational methods are critical in discerning.
Neuraminidase Inhibitors (Tamiflu) & Limitation
Neuraminidase (NA): Neuraminidase is a glycoprotein enzyme found on the surface of the influenza virus (both A and B strains). Its crucial role in the viral life cycle is to cleave terminal sialic acid residues from glycoconjugates (glycoproteins and glycolipids) on the surface of infected host cells and on newly formed virions. This cleavage is essential for:
Release of New Virions: Enabling newly formed viral particles to bud off from the host cell rather than aggregating on the cell surface.
Preventing Self-Aggregation: Preventing newly released virions from clumping together and ensures their efficient spread to uninfected cells.
Tamiflu (Oseltamivir): Oseltamivir (marketed as Tamiflu) is a widely used antiviral drug that acts as a potent and selective competitive inhibitor of influenza virus neuraminidase. It is a transition state analogue that mimics the positively charged oxocarbenium ion-like transition state that forms during the neuraminidase-catalyzed cleavage of sialic acid. Oseltamivir is administered orally as a prodrug (oseltamivir phosphate) and is converted in vivo to its active carboxylate form. Its structure features a carboxylate group that effectively substitutes for the carboxylate of sialic acid, and a cyclohexane ring that adopts a conformation mimicking the distorted half-chair conformation of the sialic acid moiety in the transition state. By binding tightly to the neuraminidase active site, Tamiflu prevents the enzyme from performing its essential function, thereby halting the spread of the infection.
Limitations: Despite its efficacy, Tamiflu has several limitations:
Timing of Administration: For maximal benefit and to significantly shorten illness duration, Tamiflu must be initiated within 48 hours of symptom onset. After this window, its clinical effectiveness is considerably reduced.
Antiviral Resistance: The influenza virus can develop resistance to Tamiflu through mutations in the neuraminidase gene. These mutations can alter the active site, reducing the binding affinity of the drug while retaining enzyme function, leading to reduced clinical efficacy.
Modest Clinical Benefits: In many patient populations, particularly otherwise healthy adults, the clinical benefits of Tamiflu can be modest, often only shortening the duration of illness by approximately one day. Its primary value is in preventing severe complications or reducing hospitalizations in high-risk groups.
Prodrug Activation: As a prodrug, it requires esterase-mediated hydrolysis in the liver to become active, which may lead to varying systemic exposure depending on individual metabolic capacity.
Case Study: Renin Inhibitors - Aliskiren
Renin's Role: Renin is an aspartic protease secreted by the juxtaglomerular cells of the kidney. It plays the initial and rate-limiting step in the Renin-Angiotensin-Aldosterone System (RAAS), a crucial hormonal cascade that regulates blood pressure, fluid volume, and electrolyte balance. Renin cleaves angiotensinogen (a large plasma glycoprotein) to produce the decapeptide angiotensin I. Angiotensin I is then converted to angiotensin II (a potent vasoconstrictor) by angiotensin-converting enzyme (ACE).
Early Inhibitor Challenges: Initial attempts to develop renin inhibitors involved peptide-based compounds that mimicked a segment of angiotensinogen. While these showed in vitro activity, they suffered from extremely poor oral bioavailability, rapid in vivo degradation, and short half-lives due to their peptide nature, rendering them clinically impractical.
Aliskiren Design Evolution: Novartis successfully developed Aliskiren, the first orally available, non-peptide direct renin inhibitor, after extensive medicinal chemistry efforts. The design strategy ingeniously retained the key structural features necessary for mimicking the tetrahedral intermediate formed during renin's cleavage of angiotensinogen, while drastically reducing molecular size and improving drug-like properties. Aliskiren incorporates a hydroxyethylene moiety (specifically, a non-hydrolyzable alkyl-hydroxy-alkane chain where the hydroxyl group mimics the activated water molecule in the tetrahedral intermediate). This stable chemical structure effectively occupies the S1/S1' subsites of the renin active site and makes crucial hydrogen bonds with the catalytic aspartates, acting as a potent transition-state analogue. It precisely mimics the sp\text{3} hybridized carbon and the critical hydroxyl group of the tetrahedral intermediate, allowing it to bind with very high affinity and specificity to the renin active site (IC_{50} in the nanomolar range).
Outcome: The successful design modifications transformed large, metabolically labile peptide inhibitors into a small, stable, orally bioavailable molecule. By directly inhibiting renin at the very beginning of the RAAS cascade, Aliskiren effectively reduces the formation of angiotensin I and subsequently angiotensin II, leading to potent and sustained blood pressure reduction, offering a novel therapeutic option for hypertension.
Conclusion and Future Directions
The in-depth understanding of enzyme transition states and their specific energetics provides an incredibly powerful and often superior pathway for innovative drug design and discovery. Structurally stable mimics of these fleeting, high-energy catalytic intermediates have consistently proven to be highly successful in developing potent, selective, and clinically effective enzyme inhibitors.
Bioinformatics and advanced computational techniques, including molecular docking, quantum mechanics/molecular mechanics (QM/MM) simulations, and extensive molecular dynamics simulations, are continually evolving. These tools significantly enhance our ability to predict, design, and optimize stable inhibitors that precisely mimic high-energy states. They allow for the fine-tuning of molecular interactions, improving selectivity for target enzymes over homologous enzymes and minimizing unwanted off-target effects. This integration of computational and experimental approaches is crucial for accelerating the drug discovery pipeline.
Continued exploration, particularly focusing on the intricate details of various enzymes' catalytic mechanisms, their transition states, and the subtle differences between isoforms or species-specific enzymes, remains paramount for future therapeutic developments across a wide and complex range of human diseases.
Questions and Discussion
The floor is open for questions, encouraging further discussion on specific case studies (e.g., detailed kinetics of a particular inhibitor, structural analysis of an enzyme-inhibitor complex) or prompting clarification on any complex topics related to enzyme catalysis, transition state theory, or drug design strategies covered during the lecture.