Enzymes: Introduction and Biological Role

Definition and Function

Enzymes are biological catalysts, typically proteins (and some RNA molecules), that accelerate chemical reactions in living organisms. They are essential for mediating nearly all biochemical reactions in the body and are not consumed or permanently altered during the process.

• Biological Catalysts: Enzymes increase the rate of reactions by lowering the activation energy required.

• Specificity: Enzymes selectively guide substrates into useful metabolic pathways, ensuring proper cellular function.

• Metabolic Control: Enzymes direct all metabolic events, allowing for regulation and compartmentalization of cellular processes.

• Example: The conversion of substrate to product via the enzyme's active site, as illustrated in the provided diagram.

Enzyme Nomenclature and Classification

Naming Conventions

Enzymes are assigned two names: a short, recommended name and a systematic name that describes their catalytic action unambiguously.

• Recommended Name: Most enzyme names end with the suffix "-ase" (e.g., alcohol dehydrogenase).

• Systematic Name: Describes the chemical reaction catalyzed, including all substrates and products.

Major Classes of Enzymes

Enzymes are classified into six major groups based on the type of reaction they catalyze:

Enzyme Properties

Catalytic Efficiency and Specificity

Enzymes are highly efficient and specific, often increasing reaction rates by factors of 103 to 108 compared to uncatalyzed reactions.

• Turnover Number (kcat): The number of substrate molecules converted to product per enzyme molecule per second. Typical values range from 0.5 to 600,000.

• Specificity: Enzymes usually interact with one or a few substrates and catalyze only one type of chemical reaction.

Enzyme Structure: Cofactors and Coenzymes

Many enzymes require nonprotein components for activity.

• Holoenzyme: The active enzyme with its nonprotein component.

• Apoenzyme: The protein portion without its nonprotein moiety; inactive.

• Cofactor: A nonprotein moiety that is a metal ion (e.g., Zn2+, Fe2+).

• Coenzyme: An organic molecule required for enzyme activity.

• Prosthetic Group: A coenzyme or cofactor permanently associated with the enzyme.

Example: The activation of an apoenzyme by binding a cofactor or coenzyme, forming a holoenzyme capable of catalysis.

Mechanism of Enzyme Action

Active Site and Substrate Binding

The active site is a specialized region of the enzyme where substrate binding and catalysis occur.

• Induced Fit Model: Substrate binding induces a conformational change in the enzyme, optimizing the active site for catalysis.

• Enzyme-Substrate Complex (ES): The substrate binds noncovalently, forming the ES complex, which is converted to the enzyme-product (EP) complex.

Energy of Activation and Transition State

Enzymes accelerate reactions by lowering the activation energy (Ea), the energy required to reach the transition state.

• Activation Energy (Ea): The energy difference between reactants and the high-energy transition state (T*).

• Enzyme Function: Enzymes provide an alternative reaction pathway with a lower Ea, increasing the rate without altering the free energies of reactants or products.

Equation:

Enzyme Kinetics

Michaelis-Menten Kinetics

Most enzymes follow Michaelis-Menten kinetics, describing the relationship between reaction velocity and substrate concentration.

• Initial Velocity (v0): The rate of product formation when the reaction begins.

• Vmax: The maximal velocity achieved at saturating substrate concentration.

• Michaelis Constant (Km): The substrate concentration at which the reaction velocity is half of Vmax; reflects enzyme affinity for substrate.

Michaelis-Menten Equation:

• At low [S] (<< Km), velocity is proportional to [S].

• At high [S] (>> Km), velocity approaches Vmax (enzyme saturation).

Lineweaver-Burk Plot

The Lineweaver-Burk plot is a double reciprocal graph used to determine Vmax and Km and analyze enzyme inhibition.

Equation:

• The y-intercept is 1/Vmax; the x-intercept is -1/Km.

• Useful for distinguishing types of enzyme inhibition.

Enzyme Inhibition

Types of Inhibition

Enzyme inhibitors decrease the velocity of enzyme-catalyzed reactions and can be reversible or irreversible.

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

• Reversible Inhibitors: Bind noncovalently and can dissociate, restoring enzyme activity.

Competitive Inhibition

• Inhibitor binds reversibly to the active site, competing with the substrate.

• Can be overcome by increasing substrate concentration.

• Vmax remains unchanged; apparent Km increases.

• Lineweaver-Burk plot: Inhibited and uninhibited reactions intersect at the y-axis.

Noncompetitive Inhibition

• Inhibitor binds at a site distinct from the substrate-binding site.

• Cannot be overcome by increasing substrate concentration.

• Vmax decreases; Km remains unchanged.

• Lineweaver-Burk plot: Slope increases, but x-intercept (Km) is unchanged.

Clinical Application Example

• Statins: Competitive inhibitors of HMG-CoA reductase, used to lower cholesterol levels by inhibiting de novo cholesterol synthesis.

Regulation of Enzyme Activity

Allosteric Regulation

Allosteric enzymes are regulated by effectors that bind noncovalently at sites other than the active site.

• Positive Effectors: Increase enzyme activity.

• Negative Effectors: Decrease enzyme activity.

• Effectors can alter substrate affinity (K0.5), maximal activity (Vmax), or both.

• Homotropic Effectors: Substrate itself acts as an effector, showing cooperativity.

• Heterotropic Effectors: Different molecules act as effectors, often as feedback inhibitors.

Covalent Modification

• Enzyme activity can be regulated by addition or removal of phosphate groups (phosphorylation/dephosphorylation) on serine, threonine, or tyrosine residues.

• Protein kinases catalyze phosphorylation; protein phosphatases catalyze dephosphorylation.

Enzyme Synthesis and Degradation

• Enzyme levels can be regulated by altering the rate of synthesis or degradation.

• Induction or repression of enzyme synthesis is typically slow and occurs in response to developmental or physiological changes.

Enzymes in Clinical Diagnosis

Diagnostic Use of Enzymes

Enzyme levels in plasma can be used to diagnose tissue damage or disease.

• Secreted Enzymes: Actively released into the blood.

• Intracellular Enzymes: Released during cell turnover or damage.

• Elevated levels of tissue-specific enzymes (e.g., alanine aminotransferase in liver disease) indicate damage to the corresponding tissue.

Isoenzymes and Heart Disease

• Isoenzymes are different forms of an enzyme that catalyze the same reaction but differ in structure and properties.

• Electrophoresis can distinguish isoenzymes based on charge differences.

• Creatine kinase (CK) isoenzyme levels are used in the diagnosis of myocardial infarction (heart attack).

Summary

• Enzymes are protein catalysts that increase reaction velocity by stabilizing the transition state and lowering activation energy.

• They are highly specific, efficient, and regulated by various mechanisms, including allosteric effectors, covalent modification, and synthesis/degradation.

• Enzyme kinetics (Michaelis-Menten, Lineweaver-Burk) provide quantitative understanding of enzyme activity and inhibition.

• Enzymes have significant diagnostic and therapeutic value in medicine.