Enzymes: Structure, Function, and Kinetics (Chapter 6 Study Notes)

Enzymes: Structure, Function, and Kinetics

What are Enzymes?

Enzymes are biological catalysts that accelerate chemical reactions in living organisms. Most enzymes are proteins, though some RNA molecules (ribozymes) also possess catalytic activity.

• Enzyme Catalysis: Enzymes catalyze specific reactions, increasing reaction rates without being consumed.

• Composition: Usually proteins; some catalytic RNA molecules exist.

• Historical Context: First enzymes were extracted from yeast (used in bread and beer production).

• Example: Ribozymes catalyze RNA splicing reactions.

Enzyme Function and Classification

Enzymes act on a wide variety of substrates but catalyze a limited number of reaction types. The International Classification of Enzymes groups enzymes by the type of reaction they catalyze.

Class	Type of Reaction Catalyzed
Oxidoreductases	Transfer of electrons (redox reactions)
Transferases	Group transfer reactions (e.g., transfer of methyl or amino groups)
Hydrolases	Hydrolysis reactions (breaking bonds with water)
Lyases	Addition/removal of groups to double bonds
Isomerases	Transfer of groups within molecules to form isomers
Ligases	Formation of bonds coupled to ATP cleavage

Cofactors and Coenzymes

Many enzymes require non-protein molecules called cofactors to function. Cofactors can be small metal ions or large organic molecules (coenzymes).

• Apoenzyme: Enzyme without its cofactor; inactive.

• Holoenzyme: Enzyme with its cofactor; active.

• Prosthetic Group: Tightly bound coenzyme.

• Reversibly Bound Coenzyme: Loosely associated and can dissociate.

Mechanism of Enzyme Action

Enzymes convert substrates into products by binding them at the active site, where catalysis occurs.

• Substrate: The molecule upon which the enzyme acts.

• Active Site: Region of the enzyme where substrate binding and catalysis take place.

• Reaction: $E + S \rightleftharpoons ES \rightleftharpoons EP \rightleftharpoons E + P$

Enzyme Catalysis and Reaction Rates

Enzymes increase the rate of reaction by lowering the activation energy required to reach the transition state, but do not alter the reaction equilibrium.

• Transition State: High-energy state between reactants and products; not an intermediate.

• Activation Energy ($\Delta G^{\ddagger}$): Energy required to reach the transition state.

• Standard Free Energy Change ($\Delta G^\circ$): Energy difference between reactants and products at pH 7.0.

• Spontaneity: Reactions with $\Delta G^\circ < 0$ are spontaneous.

Energetics of Catalysis

Enzymes lower the activation energy ($\Delta G^{\ddagger}$) but do not affect the overall free energy change ($\Delta G^\circ$) of the reaction.

• Rate-Limiting Step: The slowest step in a multistep reaction, determined by the highest activation energy.

• Reaction Coordinate Diagram: Illustrates energy changes during a reaction, showing how enzymes lower activation energy.

Reaction Rates and Orders

The rate of a reaction depends on substrate concentration and can be described by rate equations.

• First Order Reaction: $V = k[S]$ (depends only on substrate; $k$ in s-1)

• Second Order Reaction: $V = k[S_1][S_2]$ (depends on two substrates; $k$ in M-1s-1)

• Relationship to Energy: $k = \frac{bT}{h} e^{-\Delta G^{\ddagger}/RT}$ (Boltzmann and Planck constants)

How Enzymes Lower Activation Energy

Enzymes use binding energy ($\Delta G_B$) and weak interactions to stabilize the transition state, effectively lowering the activation energy barrier.

• Binding Energy: Derived from entropy changes, desolvation, weak interactions, and induced fit.

• Transition State Stabilization: Enzymes are optimized to bind the transition state more tightly than the substrate or product.

Types of Catalysis

Enzymes employ different catalytic mechanisms to accelerate reactions.

• Acid-Base Catalysis: Proton transfers using water or amino acid side chains (e.g., Glu, Asp, His, Lys, Arg, Cys, Ser, Tyr).

• Covalent Catalysis: Formation of a temporary covalent bond between enzyme and substrate.

• Metal Ion Catalysis: Use of metal ions to facilitate redox reactions or stabilize charged intermediates.

Amino Acid	Proton Donor (Uncharged)	Proton Acceptor (Charged)
Glu, Asp	R-COOH	R-COO-
Lys, Arg	R-NH2	R-NH3+
Cys	R-SH	R-S-
His	R-NH	R-N-
Ser	R-OH	R-O-
Tyr	R-OH	R-O-

Enzyme Kinetics

Enzyme kinetics studies the rates of enzyme-catalyzed reactions and how they are affected by various factors.

• Initial Rate ($V_0$): Measured under conditions of excess substrate.

• Steady-State Approximation: Assumes [ES] remains constant during the reaction.

• Michaelis-Menten Equation: $V_0 = \frac{V_{max}[S]}{K_m + [S]}$

• Michaelis Constant ($K_m$): Substrate concentration at which $V_0 = \frac{1}{2}V_{max}$; reflects enzyme affinity for substrate.

• Maximum Velocity ($V_{max}$): Rate when enzyme is saturated with substrate.

• Turnover Number ($k_{cat}$): Number of substrate molecules converted per enzyme per unit time.

• Specificity Constant ($k_{cat}/K_m$): Measures catalytic efficiency.

Enzymes with Multiple Substrates

Some enzymes catalyze reactions involving more than one substrate, following different mechanisms.

• Ternary Complex Mechanism: Both substrates bind to the enzyme before catalysis.

• Ping-Pong Mechanism: One substrate binds and reacts, releasing a product before the second substrate binds.

Enzyme Inhibition

Enzyme activity can be reduced or stopped by inhibitors, which are important in regulation and drug design.

• Reversible Inhibition: Inhibitor can dissociate; includes competitive, uncompetitive, and noncompetitive mechanisms.

• Competitive Inhibition: Inhibitor competes with substrate for active site; increases $K_m$, does not affect $V_{max}$.

• Uncompetitive Inhibition: Inhibitor binds only to ES complex; decreases both $K_m$ and $V_{max}$.

• Noncompetitive (Mixed) Inhibition: Inhibitor binds to enzyme or ES complex; affects both $K_m$ and $V_{max}$.

Type	Binding Site	Effect on $K_m$	Effect on $V_{max}$
Competitive	Active site	Increases	No change
Uncompetitive	ES complex	Decreases	Decreases
Noncompetitive	Enzyme or ES	Variable	Decreases

• Irreversible Inhibition: Inhibitor covalently modifies the enzyme, permanently inactivating it.

Summary of Key Equations

• First Order Rate: $V = k[S]$

• Second Order Rate: $V = k[S_1][S_2]$

• Michaelis-Menten: $V_0 = \frac{V_{max}[S]}{K_m + [S]}$

• Lineweaver-Burk Plot: $\frac{1}{V_0} = \frac{K_m}{V_{max}} \frac{1}{[S]} + \frac{1}{V_{max}}$

• Relationship to Energy: $k = \frac{bT}{h} e^{-\Delta G^{\ddagger}/RT}$

Additional info:

• Some context and details were inferred from standard biochemistry knowledge to clarify mechanisms and terminology.