Enzymes: Biological Catalysts – Principles, Mechanisms, and Regulation

Enzymes: Biological Catalysts

What is a Catalyst?

Catalysts are substances that increase the rate of chemical reactions without being consumed or permanently altered in the process. In biological systems, enzymes serve as highly efficient and specific catalysts.

• Definition: A catalyst accelerates the approach to equilibrium for a reaction, but does not affect the thermodynamic favorability.

• Activation Energy: Catalysts lower the activation energy ($E_a$) required for the transition state, facilitating substrate conversion to product.

• Equilibrium: Catalysts do not change the position of equilibrium, only the rate at which it is achieved.

Major Classes of Enzymes

Enzymes are classified based on the type of reaction they catalyze. The six major classes are:

Class	Example (Reaction Type)	Reaction Catalyzed
Oxidoreductases	Alcohol dehydrogenase (oxidation with NAD+)	Ethanol → Acetaldehyde
Transferases	Hexokinase (phosphorylation)	Glucose → Glucose-6-phosphate
Hydrolases	Carboxypeptidase (peptide bond cleavage)	Polypeptide → Peptide fragments
Lyases	Pyruvate decarboxylase (decarboxylation)	Pyruvate → Acetaldehyde + CO2
Isomerases	Malate isomerase (cis-trans isomerization)	Malate → Fumarate
Ligases	Pyruvate carboxylase (carboxylation)	Pyruvate → Oxaloacetate

How Enzymes Act as Catalysts: Principles and Examples

Models for Substrate Binding and Catalysis

Enzyme-substrate interactions are explained by two primary models:

• Lock-and-Key Model: The substrate fits precisely into the enzyme's active site, like a key into a lock.

• Induced Fit Model: The enzyme undergoes a conformational change upon substrate binding, optimizing the interaction and catalytic efficiency.

• Example: Hexokinase exhibits an induced fit upon glucose binding, altering its conformation to facilitate catalysis.

Mechanisms for Achieving Rate Acceleration

Enzymes employ several strategies to accelerate reaction rates:

• General Acid/Base Catalysis (GABC): Enzyme side chains donate or accept protons to stabilize transition states.

• Covalent Catalysis: Formation of transient covalent bonds between enzyme and substrate.

• Electrostatic Stabilization: Stabilization of charged intermediates via ionic interactions.

• Proximity Effects: Bringing substrates into close proximity and correct orientation.

• Preferential Stabilization of the Transition State: Enzymes bind transition states more tightly than substrates or products.

• Protein Conformational Changes: Dynamic changes in enzyme structure facilitate catalysis.

Reaction Coordinate Diagram:

• Enzyme-catalyzed reactions lower the activation energy ($riangle G^{ eq}$) compared to noncatalyzed reactions.

• Equation for a simple enzyme-catalyzed reaction: $E + S ightleftharpoons ES ightleftharpoons EP ightleftharpoons E + P$

Coenzymes, Vitamins, and Essential Metals

Coenzyme or Cofactor Function in Catalysis

Many enzymes require non-protein helpers for catalytic activity:

• Coenzymes: Organic molecules, often derived from vitamins, that participate in catalysis but are not permanently altered.

• Cofactors: Inorganic ions (often metals) that assist in enzyme function.

Vitamin	Coenzyme	Reaction involving the coenzyme
Thiamine (B1)	Thiamine pyrophosphate	Activation and transfer of aldehydes
Riboflavin (B2)	Flavin mononucleotide; flavin adenine dinucleotide	Oxidation-reduction
Niacin (B3)	Nicotinamide adenine dinucleotide; nicotinamide adenine dinucleotide phosphate	Oxidation-reduction
Pantothenic acid (B5)	Coenzyme A	Acyl group activation and transfer
Pyridoxine (B6)	Pyridoxal phosphate	Various reactions involving amino acid activation
Biotin	Biotin	CO2 activation and transfer
Lipoic acid	Lipoamide	Acyl group activation; oxidation-reduction
Folic acid (B9)	Tetrahydrofolate	Activation and transfer of single-carbon functional groups
Vitamin B12	Adenosyl cobalamin, methyl cobalamin	Isomerization and methyl group transfers

Metal Ions in Enzymes

Metal ions serve as essential cofactors in many enzymes, facilitating catalysis through various mechanisms.

Metal	Example of Enzymes	Role of Metal
Fe	Cytochrome oxidase	Oxidation-reduction
Cu	Ascorbic acid oxidase	Oxidation-reduction
Zn	Alcohol dehydrogenase	Helps bind NAD+
Mn	Histidine ammonia lyase	Aids in catalysis by electron withdrawal
Co	Glutamate mutase	Co is part of cobalamin coenzyme
Ni	Urease	Catalytic site
Mo	Xanthine oxidase	Oxidation-reduction
V	Nitrate reductase	Oxidation-reduction
Se	Glutathione peroxidase	Replaces S in one cysteine in active site
Mg	Many kinases	Helps bind ATP

Enzyme Inhibition

Drugs, Toxins, and Enzymatic Activity

Enzyme inhibition is a key mechanism by which drugs and toxins exert their effects. Inhibitors can be classified as reversible or irreversible.

• Reversible Inhibitors: Bind noncovalently and can dissociate from the enzyme.

• Irreversible Inhibitors: Bind covalently, permanently inactivating the enzyme.

Types of Reversible Inhibition

• Competitive Inhibition: Inhibitor competes with substrate for the active site. Equation: $[E]_T = [E] + [ES] + [EI]$ Only substrate is processed; inhibitor blocks substrate access.

• Uncompetitive Inhibition: Inhibitor binds only to the enzyme-substrate (ES) complex, not to the free enzyme, reducing catalytic activity.

• Mixed (Noncompetitive) Inhibition: Inhibitor can bind to both the free enzyme and the ES complex, affecting both substrate binding and catalysis.

Irreversible Inhibition

• Mechanism: Inhibitor forms a covalent bond with the enzyme, often at the active site, leading to permanent inactivation.

• Example: Diisopropyl fluorophosphate (DFP) binds to the active site serine of acetylcholinesterase, causing paralysis.

The Regulation of Enzyme Activity

Controlling Enzyme Functions in the Cellular Context

Cells regulate enzyme activity through multiple mechanisms to maintain metabolic balance and respond to changing conditions.

• Substrate Level Control: Reaction rate increases with substrate concentration ($[S]$), but large changes are needed for significant effects.

• Regulation at Committed Steps: Feedback inhibition or activation at key control points, often mediated by allosteric enzymes, is an efficient means to maintain homeostasis.

• Covalent Modification: Enzyme activity can be reversibly or irreversibly modified (e.g., phosphorylation, zymogen activation).

Feedback Inhibition

• End product of a pathway inhibits an earlier step, preventing overproduction and maintaining balance.

Allostery

• Allosteric Enzymes: Usually multisubunit proteins that change conformation upon binding substrates or effectors.

• Homoallostery: Cooperativity in substrate binding.

• Heteroallostery: Regulation by nonsubstrate effector molecules.

Covalent Modifications Used to Regulate Enzyme Activity

Reversible and Irreversible Modifications

• Phosphorylation: Addition of phosphate groups by protein kinases; reversible by protein phosphatases.

• Zymogen Activation: Irreversible activation of enzymes by proteolytic cleavage.

• Functional Switches: Covalent modifications often serve as on/off switches for enzyme activity.

Nonprotein Biocatalysts: Catalytic Nucleic Acids

Ribonucleic Acids as Enzymes

Some RNA molecules, known as ribozymes, can catalyze chemical reactions. This discovery supports the hypothesis that early life may have relied on RNA-based catalysis before the evolution of protein enzymes.

• Ribozymes: Catalytic RNA molecules involved in self-replication and other reactions.

• RNA World Hypothesis: Suggests ancient cells used RNA for both genetic information and catalysis.

Additional info: Ribozymes play roles in RNA splicing, gene regulation, and viral replication.