Biochemistry: Enzyme Function and Properties

Active Site Formation

• Enzymes play an important role in catalysis by bringing substrates together in a specific orientation and promoting bond formation.
• Enzymes can stretch the bonds in substrate molecules, making them unstable.
• Enzymes can temporarily add chemical groups to substrates.
• Enzymes lower the activation energy of a reaction by bringing substrates together in a way that facilitates the reaction.

Catalyzed Reactions

• The rate of a catalyzed reaction depends on substrate concentration.
• At saturation, all enzymes are bound to substrates, leading to a maximum rate.
• The rate can be used to calculate the enzyme efficiency: the number of substrate molecules converted to product per unit time (turnover).

Properties of Enzymes

• Most enzymes are proteins; however, ribozymes are RNA molecules that act as enzymes.
• Most enzyme names end in "-ase."
• Enzymes lower the activation energy of a reaction but do not change the equilibrium constant (ΔG).
• Enzymes are very specific for the reactions they catalyze and have a specific active site that binds to the substrate.
• Enzymes can be regulated by different factors, including temperature, pH, cofactors, and inhibitors.

Enzyme-Catalyzed Reactions

• Enzymes are not consumed during a reaction and can act repeatedly.
• Reactants acted upon by enzymes are called substrates.
• The enzyme-substrate complex is held together by various interactions: hydrogen bonds, electrostatic interactions, hydrophobic interactions, or even covalent bonds.

Activation Energy

• Activation energy is the initial energy required for a reaction to take place, even if the overall reaction is energetically favorable (ΔG < 0).
• Enzymes lower the activation energy barrier by bringing reactants together, increasing reaction rates.
• Enzymes do not change the ΔG of a reaction; they only accelerate the rate at which equilibrium is reached.

Enzyme Shape and Active Site

• The active site is the catalytic center of the enzyme.
• The shape of the active site allows specific substrates to bind to the enzyme, often described as a "lock and key" model.
• Often, enzymes undergo a conformational change upon substrate binding, called induced fit.
• The active site stabilizes the transition state, further lowering the activation energy.

Enzyme Inhibitors

• Enzyme inhibitors are substances that block the action of an enzyme.
• Two important types of inhibitors:
  • Competitive inhibitors: resemble the substrate and bind to the active site of the enzyme.
  • Non-competitive inhibitors: bind to a different site on the enzyme, affecting its catalytic activity.
• Inhibitors can be reversible or irreversible.
• Irreversible inhibition occurs when the inhibitor covalently binds to the active site, permanently deactivating the enzyme.

Regulation of Enzyme Activity - Temperature

• Each enzyme has an optimal temperature for activity.
• At high temperatures, non-covalent bonds in the enzyme can break, leading to denaturation.
• Denaturation is the loss of the enzyme's three-dimensional structure, resulting in the loss of function.

Regulation of Enzyme Activity - pH

• Each enzyme has an optimal pH for maximum activity.
• pH influences the ionization state of functional groups, affecting the enzyme's structure and catalytic activity.

Regulation of Enzyme Activity - Activators

• Enzyme activators are molecules that accelerate the activity of an enzyme.

Metabolic Pathways

• Metabolic reactions are organized into pathways, which are orderly series of enzymatic reactions.
• The first reaction in a pathway is often the commitment step, and subsequent reactions follow in sequence.
• These pathways are controlled by enzymes, ensuring the efficient flow of energy and molecules within a cell.