Enzymes: The Catalysts of Life

Definitions and Key Terms

• Enzyme: A biological catalyst, usually a protein (sometimes RNA), that accelerates chemical reactions in cells without being consumed.

• Substrate: The specific molecule upon which an enzyme acts.

• Active Site: The region of the enzyme where substrate binding and catalysis occur; its shape and chemical properties confer specificity.

• Allosteric Site: A site on the enzyme (distinct from the active site) where regulatory molecules can bind, affecting enzyme activity.

• Turnover Number (kcat): The number of substrate molecules converted to product per enzyme molecule per unit time when the enzyme is fully saturated.

• KM (Michaelis constant): The substrate concentration at which the reaction rate is half of Vmax; reflects enzyme affinity for substrate.

• Vmax: The maximum rate of an enzyme-catalyzed reaction when the enzyme is saturated with substrate.

• EA (Activation Energy): The energy barrier that must be overcome for a reaction to proceed.

Importance and Role of Enzymes in Cells

Enzymes are essential for life because they enable cellular reactions to occur rapidly and efficiently under physiological conditions. They lower the activation energy (EA) required for reactions, allowing cells to maintain a "metastable state" where reactions are possible but do not occur spontaneously without catalysis.

• Relation to ΔG: Enzymes do not alter the free energy change (ΔG) of a reaction; they only affect the rate by lowering EA.

• Metastable State: Cellular reactions are often in a metastable state, meaning they are thermodynamically possible (ΔG negative) but kinetically slow without enzymes.

Properties of Enzymes

• Protein Nature: Most enzymes are proteins, so their activity is sensitive to physical factors such as pH, temperature, and ionic strength. Some enzymes are ribozymes (RNA-based).

• Specificity: Enzymes display high specificity for their substrates due to the precise shape and chemical environment of the active site.

• Transient Interaction: Enzymes interact temporarily with substrates, forming an enzyme-substrate complex.

• Reaction Rate Enhancement: Enzymes increase reaction rates by lowering EA, allowing reactions to proceed at useful rates toward equilibrium.

• Direction of Equilibrium: Enzymes facilitate reactions in the direction dictated by the sign and value of ΔG.

Enzyme Kinetics: Substrate Concentration, Saturation, KM, and Vmax

The rate of enzyme-catalyzed reactions depends on substrate concentration. As substrate increases, the rate rises until the enzyme becomes saturated, reaching Vmax. KM is a measure of substrate affinity.

• Saturation: At high substrate concentrations, all enzyme active sites are occupied, and the reaction rate plateaus at Vmax.

• Michaelis-Menten Equation:

• KM: Lower KM indicates higher affinity for substrate.

• Turnover Number (kcat): Indicates catalytic efficiency.

Enzyme Inhibition

Enzyme activity can be inhibited by environmental molecules. Inhibitors are classified as irreversible or reversible, with reversible inhibitors further divided into competitive and noncompetitive types.

• Irreversible Inhibitors: Bind permanently to the enzyme, often at the active site, rendering it inactive.

• Reversible Inhibitors: Bind non-permanently and can be removed.

• Competitive Inhibitors: Compete with substrate for the active site; can be overcome by increasing substrate concentration.

• Noncompetitive Inhibitors: Bind to a site other than the active site (often allosteric), reducing enzyme activity regardless of substrate concentration.

Regulation of Enzyme Activity

Cells regulate enzyme activity at multiple levels to control metabolic pathways.

• Gene Expression: The first level of regulation is whether the cell synthesizes the enzyme (transcription and translation).

• Post-Synthesis Regulation: After synthesis, enzymes can be regulated by:

  • Allosteric Regulation: Binding of activators or inhibitors at allosteric sites alters enzyme activity.

  • Chemical Modification: Covalent modifications such as phosphorylation can activate or inhibit enzymes.

  • Proteolysis: Enzymes can be activated or inactivated by proteolytic cleavage.

  • Sequestration: Enzymes may be compartmentalized or sequestered to regulate activity.

• Multiple Mechanisms: Individual enzymes may be regulated by several mechanisms simultaneously.

Major Classes of Enzymes (Table 6-1)

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

Additional info: Other enzyme classes include lyases (cleavage without water), ligases (joining molecules), and synthetases.

Example: Enzyme Regulation in Glycolysis

Phosphofructokinase is a key regulatory enzyme in glycolysis, subject to allosteric regulation by ATP (inhibitor) and AMP (activator), demonstrating how cells fine-tune metabolic pathways.