Enzyme Mechanisms: Acid-Base, Covalent, and Metal-Ion Catalysis

Why study enzyme mechanisms?

To understand biochemical reactions at the molecular level, predict and design reactions, interpret experimental data, and connect chemistry with biology.

How do enzyme mechanisms connect chemistry and biology?

They apply chemical principles such as acid-base reactions and nucleophilic attack to biological systems.

What are the six major enzymatic catalytic mechanisms?

Acid-base catalysis; covalent catalysis; metal-ion catalysis; electrostatic interactions; proximity and orientation effects; transition-state binding.

What is acid-base catalysis?

Catalysis involving proton transfer that increases reaction rate.

What is a general acid?

A proton donor; it donates H+.

What is a general base?

A proton acceptor; it abstracts H+.

How does a Brønsted base increase reaction rate?

By partially abstracting a proton from a reactant.

How does a Brønsted acid increase reaction rate?

By partially donating a proton to a reactant.

What is mutarotation?

The interconversion of glucose anomers through the linear form, accompanied by a change in optical activity.

How can glucose mutarotation be monitored?

By observing changes in optical activity.

What happens during acid-base-catalyzed glucose mutarotation?

An acid protonates the ring oxygen while a base removes a proton from the OH on carbon 1, allowing formation of linear glucose.

How does glucose return to its cyclic form during mutarotation?

The carbonyl is attacked from the other side, regenerating the cyclic form.

What happens to observed mutarotation rate when acid and base concentrations increase?

The observed rate increases with increasing acid and base concentrations.

What does Kobs represent in the glucose mutarotation experiment?

The observed apparent first-order rate constant.

What is an aprotic solvent?

A solvent without acid or base groups, such as dimethyl sulfoxide (DMSO) or dimethylformamide (DMF).

Does glucose undergo mutarotation in aprotic solvents without catalysts?

No; the lecture notes that mutarotation does not occur in these solvents without added catalysts.

Which catalysts promote glucose mutarotation in aprotic solvents?

Phenol, a weak acid, and pyridine, a weak base.

What is the significance of alpha-pyridone in glucose mutarotation?

Alpha-pyridone can catalyze the reaction much more efficiently than phenol and pyridine together.

How much more efficient is alpha-pyridone in the lecture's example?

The reported catalytic efficiency is approximately 7,000-fold greater.

Which biochemical reactions commonly require acid-base catalysis?

Peptide hydrolysis, reactions involving phosphate groups, tautomerizations, and additions to carboxyl groups.

Which six amino acids participate in general acid-base catalysis?

Aspartate (Asp), glutamate (Glu), histidine (His), cysteine (Cys), tyrosine (Tyr), and lysine (Lys).

Why can Asp, Glu, His, Cys, Tyr, and Lys participate in acid-base catalysis?

Their side chains have pKa values near physiological pH.

How can enzymes promote concerted acid-base catalysis?

They arrange several catalytic groups around the substrate so multiple catalytic events can occur together.

What is RNase?

An enzyme that catalyzes hydrolysis of RNA phosphate linkages.

Which amino acid residues perform acid-base catalysis in the RNase example?

Histidine 12 (His12) and histidine 119 (His119).

What role does His12 perform in the first RNase catalytic step?

His12 acts as a general base, abstracting a proton from the RNA 2'-OH.

What role does His119 perform in the first RNase catalytic step?

His119 acts as a general acid, donating a proton to the phosphate group.

What happens during the second RNase catalytic step?

His12 reprotonates the 2'-OH while His119 abstracts a proton from water, generating OH- that attacks the phosphate.

What is the overall result of RNase catalysis?

Hydrolysis of the RNA phosphate linkage.

What is covalent catalysis?

Catalysis involving formation of a transient covalent bond between the enzyme and substrate.

What does covalent catalysis usually require?

A nucleophile.

What is a nucleophile?

An electron-rich species that donates an electron pair to form a covalent bond.

How does nucleophilicity differ from basicity in the lecture?

A base abstracts a proton, whereas a nucleophile attacks and forms a covalent bond.

What are the three general stages of covalent catalysis?

Nucleophilic attack forms a covalent bond; electron withdrawal makes the catalyst electrophilic; elimination releases the catalyst.

What happens during the first stage of covalent catalysis?

A nucleophilic reaction forms a temporary covalent bond with the substrate.

What happens during the second stage of covalent catalysis?

Electron withdrawal by the catalyst creates an electrophilic intermediate.

What happens during the third stage of covalent catalysis?

The catalyst is eliminated, approximately reversing the initial bond-forming step.

Can covalent catalysis be described as nucleophilic or electrophilic?

Yes. Its classification depends on which stage is rate-limiting.

Why is decarboxylation by primary amines described as electrophilic in the lecture?

The nucleophilic Schiff-base-forming step is very fast, so a later electrophilic stage limits the reaction.

Which amino acid commonly forms Schiff bases?

Lysine.

What is a Schiff base?

An imine-containing intermediate formed when an amine reacts with a carbonyl group.

Which other functional groups can participate in covalent catalysis?

Thiol groups, imidazole groups, acidic groups, and hydroxyl groups.

Which coenzymes can participate in covalent catalysis?

Thiamine pyrophosphate (TPP) and pyridoxal phosphate (PLP).

Approximately what fraction of known enzymes requires metal ions?

Approximately one-third.

What is a metalloenzyme?

An enzyme containing a tightly bound metal ion.

Which metal ions are commonly found in metalloenzymes?

Fe2+, Fe3+, Cu2+, Zn2+, Mn2+, and Co2+.

What is a metal-activated enzyme?

An enzyme that loosely binds metal ions.

Which ions commonly activate enzymes through loose binding?

Na+, K+, Mg2+, and Ca2+.

What are three major roles of metal ions in enzyme catalysis?

Substrate orientation, redox reactions, and electrostatic stabilization or negative-charge shielding.

How can metal ions participate in redox reactions?

By gaining or losing electrons.

Why are metal ions effective catalysts at neutral pH?

They can be present at higher concentrations than protons and have charges greater than 1.

How do metal ions affect bound water?

They make bound water more acidic than free water, promoting formation of hydroxide ions.

Why is metal-bound hydroxide useful in enzyme catalysis?

OH- is a potent nucleophile.

What is an example of an enzyme that uses zinc in metal-ion catalysis?

Carbonic anhydrase.

What is electrostatic stabilization?

Stabilization of charged groups through electrostatic interactions.

What is charge shielding by metal ions?

Metal ions stabilize negative charges by shielding them through their positive charge.

What is the proximity effect?

Bringing reacting molecules close together to increase the likelihood of interaction.

Why does proximity alone usually produce a relatively small rate enhancement?

Reactants already encounter neighboring molecules in solution, and reactions occur only when molecules make contact.

What rate enhancement is given for proximity effects in the lecture's example?

Approximately 4.6-fold.

What does the lecture's imidazole and phenyl acetate example demonstrate?

Proximity can increase reaction rates, but the enhancement is relatively modest.

What is the orientation effect?

Holding reacting molecules in the correct alignment for a reaction.

How much can proper orientation increase reaction rates?

Up to approximately 100-fold.

Why does proper orientation increase reaction rates?

It aligns reactive groups correctly and reduces the entropic cost of arranging molecules for reaction.

What is the transition state?

The high-energy state between reactants and products through which a reaction proceeds.

What is preferential transition-state binding?

An enzyme binds the transition state more tightly than the substrate or product.

What is the rack mechanism?

An enzyme strains or distorts a substrate toward the transition-state geometry, promoting faster reaction.

Why does substrate strain promote faster reactions?

The strained substrate more closely resembles the transition state.

What determines the degree of catalytic rate enhancement from transition-state binding?

The enzyme's relative affinity for the transition state compared with the substrate.

What are KT and KR in the lecture's transition-state binding explanation?

KT represents binding affinity for the transition state; KR represents binding affinity for the substrate.

What happens when an enzyme binds its transition state much more tightly than its substrate?

Transition-state stabilization increases, producing a greater catalytic rate enhancement.

What affinity increase corresponds to a 10^6-fold rate enhancement?

Approximately 10^6-fold greater affinity for the transition state relative to the substrate.

What energy difference corresponds to a 10^6-fold rate enhancement in the lecture?

Approximately 34.2 kJ/mol.

How can two additional hydrogen bonds enhance catalysis?

Two hydrogen bonds formed in ES‡ but not in ES can provide approximately 10^6-fold rate enhancement.

What is ES?

The enzyme-substrate complex, also called the Michaelis complex.

What is ES‡?

The enzyme-transition-state complex.

What are transition-state analogues?

Molecules that resemble the transition state of an enzyme-catalyzed reaction.

Why do transition-state analogues bind strongly to enzymes?

Enzymes preferentially bind structures resembling their transition states.

What type of inhibitors are transition-state analogues according to the lecture?

Competitive inhibitors.