Premed · Premed · Biochemistry
Lecture 9: Enzyme Mechanisms and Catalysis
Biochemistry
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the major catalytic strategies used by enzymes
- Explain the roles of cofactors, coenzymes, and prosthetic groups
- Describe the catalytic mechanism of serine proteases (chymotrypsin)
- Explain how lysozyme catalyzes glycosidic bond cleavage
- Describe the concept of transition state stabilization and its therapeutic applications
- Explain the role of metal ions in enzyme catalysis
Lecture Content
I. Catalytic Strategies
Enzymes employ several fundamental strategies to accelerate reactions, often using them in combination.
Proximity and Orientation Effects
Enzymes bring substrates together in the correct orientation within the active site, creating an effective concentration of reactants far higher than in bulk solution. This reduces the entropic cost of the reaction by minimizing the loss of translational and rotational freedom, and it can enhance reaction rates by factors of 10^2 to 10^5.
Transition State Stabilization
The enzyme active site is complementary to the transition state rather than the substrate itself. While the substrate binds well, the transition state binds even better, a principle first articulated by Linus Pauling. The binding energy released from transition state interactions is used to lower the activation energy. Transition state analogs are stable molecules that mimic the transition state and bind more tightly than the substrate (often by 10^2 to 10^6 fold), making them potent enzyme inhibitors. HIV protease inhibitors such as saquinavir and ritonavir, for example, mimic the tetrahedral transition state of peptide bond hydrolysis.
Acid-Base Catalysis
In general acid catalysis, a donor other than water donates a proton to the substrate, stabilizing a developing negative charge. In general base catalysis, an acceptor other than water abstracts a proton from the substrate, generating a nucleophile. Many enzymes use both simultaneously in concerted acid-base catalysis. Active site residues commonly involved include histidine (imidazole, pKa approximately 6, which can act as both acid and base at physiological pH), glutamate and aspartate (as general acids), lysine (as a general base), and cysteine (as a general acid or base).
Covalent Catalysis
In covalent catalysis, a transient covalent bond forms between the enzyme and the substrate, creating a new reaction pathway with a lower activation energy. This requires a nucleophilic residue in the active site, typically serine (hydroxyl), cysteine (thiol), histidine (imidazole), or lysine (amino group). The covalent intermediate must be broken down in a subsequent step to regenerate the enzyme. Serine proteases, cysteine proteases, and pyridoxal phosphate-dependent enzymes all use this strategy.
Metal Ion Catalysis
Approximately one-third of all enzymes require metal ions. Metals serve multiple roles: as Lewis acid catalysts, they accept electron pairs and stabilize negative charges on intermediates. In redox catalysis, metals such as iron, copper, and manganese participate in electron transfer. Metals also play structural roles, stabilizing active site conformations (as zinc does in zinc finger proteins). Additionally, metal-coordinated water has a lower pKa, creating a metal-hydroxide nucleophile for water activation. Carbonic anhydrase uses Zn2+ to activate water for hydroxide formation and attack on CO2. Carboxypeptidase A uses Zn2+ to polarize the substrate carbonyl. Cytochrome oxidase employs iron and copper in electron transfer.
II. Cofactors and Coenzymes
Many enzymes require non-protein molecules for catalytic activity. A cofactor is the general term for any non-protein component required for enzyme activity. Coenzymes are organic cofactors, often derived from vitamins. A loosely bound coenzyme that dissociates after the reaction is called a cosubstrate (examples include NAD+ and CoA), while a tightly bound coenzyme that remains attached is a prosthetic group (examples include FAD, heme, biotin, and PLP). Metal ion cofactors can be tightly or loosely bound. The complete, catalytically active enzyme is called the holoenzyme, consisting of the apoenzyme (protein alone) plus its cofactor(s).
Important Coenzymes and Their Vitamin Precursors
NAD+/NADH (from niacin, vitamin B3) serves as a hydride carrier in oxidation-reduction reactions. FAD/FADH2 (from riboflavin, vitamin B2) is an electron carrier in redox reactions. Coenzyme A (from pantothenic acid, vitamin B5) carries acyl groups. Thiamine pyrophosphate or TPP (from thiamine, vitamin B1) participates in decarboxylation reactions. Pyridoxal phosphate or PLP (from pyridoxine, vitamin B6) is essential for amino acid metabolism, including transamination and decarboxylation. Biotin (vitamin B7) carries CO2 in carboxylation reactions. Tetrahydrofolate or THF (from folate, vitamin B9) mediates one-carbon transfers. Cobalamin (vitamin B12) participates in rearrangement reactions and methyl transfer. Lipoic acid facilitates acyl group transfer as part of the pyruvate dehydrogenase complex.
III. The Serine Protease Mechanism (Chymotrypsin)
Serine proteases are one of the best-studied enzyme families, including chymotrypsin, trypsin, elastase, thrombin, and subtilisin. Their active site features a catalytic triad of Ser195, His57, and Asp102 (using chymotrypsin numbering). The mechanism proceeds in two phases.
Acylation Phase: The substrate polypeptide binds in the active site with the scissile peptide bond positioned near the catalytic triad. His57, acting as a general base, abstracts a proton from the Ser195 hydroxyl, with Asp102 stabilizing the positive charge on His57 and enhancing its basicity. The activated Ser195 oxygen then acts as a nucleophile and attacks the carbonyl carbon of the peptide bond. A tetrahedral intermediate forms, with the developing negative charge (oxyanion) stabilized by the oxyanion hole -- backbone NH groups of Gly193 and Ser195 that donate hydrogen bonds to the oxyanion. The tetrahedral intermediate collapses as His57, now acting as a general acid, donates a proton to the leaving amine group. The first product (the amine portion) departs, leaving an acyl-enzyme intermediate in which Ser is covalently bonded to the acyl portion of the substrate.
Deacylation Phase: Water enters the active site, and His57 acts again as a general base, abstracting a proton from water to generate a hydroxide ion. This hydroxide attacks the acyl-enzyme intermediate, forming a second tetrahedral intermediate that is again stabilized by the oxyanion hole. The intermediate collapses, releasing Ser195 and the second product (the carboxylate portion), thereby regenerating the free enzyme.
<image>A detailed step-by-step mechanism of chymotrypsin catalysis. Panel A: The catalytic triad (Asp102-His57-Ser195) shown with hydrogen bonding network and the substrate peptide bond positioned in the active site. Panel B: The acylation phase in 3 steps — nucleophilic attack by Ser195, formation of the tetrahedral intermediate with the oxyanion hole stabilizing the negative charge, collapse of the intermediate and departure of the amine product to form the acyl-enzyme intermediate. Panel C: The deacylation phase in 3 steps — water activation by His57, nucleophilic attack on the acyl-enzyme, second tetrahedral intermediate, and release of the carboxylate product with regeneration of the free enzyme. Curved arrows indicate electron movement throughout.</image>
IV. Lysozyme Mechanism
Lysozyme cleaves the glycosidic bond between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) in bacterial cell walls. The active site residues Glu35 (protonated, acting as a general acid) and Asp52 (deprotonated, stabilizing positive charge) drive the catalytic mechanism. The substrate binds in the active site cleft, which has six sugar binding subsites (A through F). The sugar in subsite D is distorted into a half-chair conformation, representing transition state stabilization. Glu35 donates a proton to the glycosidic oxygen (general acid catalysis), cleaving the bond. An oxocarbenium ion intermediate forms on the sugar in subsite D, and Asp52 stabilizes this positively charged intermediate through electrostatic stabilization. Water then attacks the oxocarbenium ion to complete hydrolysis. Glu35 is reprotonated and the products are released. Lysozyme holds special historical significance as the first enzyme whose structure was determined by X-ray crystallography, by David Phillips in 1965.
V. Transition State Analogs as Drugs
Since enzymes bind the transition state most tightly, designing stable molecules that mimic the transition state produces potent inhibitors with broad therapeutic applications. HIV protease inhibitors (saquinavir, ritonavir, indinavir) contain a non-hydrolyzable group that mimics the tetrahedral transition state of peptide hydrolysis. Statins (atorvastatin, rosuvastatin) mimic the transition state of HMG-CoA reductase. Enalaprilat, an ACE inhibitor, mimics the transition state of angiotensin-converting enzyme. Oseltamivir (Tamiflu) is a transition state analog inhibitor of influenza neuraminidase. The general drug design strategy involves determining the transition state structure and designing stable analogs that replicate its geometry and interactions.
<image>A figure showing transition state analog drug design. Panel A: The normal substrate and transition state of HIV protease, showing the tetrahedral intermediate at the scissile peptide bond with a hydroxyl group. Panel B: The structure of the HIV protease inhibitor saquinavir, highlighting the hydroxyethylamine moiety that mimics the tetrahedral transition state. Panel C: A superposition of the transition state and the drug in the active site of HIV protease, showing how the drug occupies the same space and makes the same interactions as the transition state. Panel D: A comparison table of Ki values for substrate versus transition state analogs, demonstrating the much tighter binding of transition state analogs.</image>
VI. Catalytic Antibodies (Abzymes)
Antibodies raised against transition state analogs can catalyze the corresponding reaction, providing compelling evidence that transition state stabilization is sufficient for catalysis. These catalytic antibodies are generally less efficient than natural enzymes because they lack the optimized active site dynamics, but they serve as a powerful proof of concept for the transition state theory of enzyme catalysis.
