Factors affecting enzyme activity (temperature, pH, concentration) Knowledge-Based Study

Study notes

1. Enzymes and Reaction Rates

   Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy required for a reaction to occur. An enzyme's function depends directly on the three-dimensional conformation of its active site, which binds specifically to complementary substrate molecules.

   The rate at which an enzyme catalyzes a reaction is not constant. Instead, it fluctuates depending on the physical and chemical conditions of the cellular environment. The primary factors that influence enzyme activity include:

   • Temperature
   • pH (acidity or alkalinity)
   • Substrate concentration
   • Enzyme concentration

   Each enzyme has evolved to function best under specific environmental conditions known as its optimum conditions.

   Temperature and Kinetic Energy

   Temperature affects enzyme activity by altering the kinetic energy of both enzyme and substrate molecules:

   • Low Temperatures: At low temperatures, molecules move slowly due to low kinetic energy. As a result, collisions between the active site and substrate molecules are infrequent, leading to a low reaction rate. Enzymes remain intact but are temporarily inactive.
   • Rising Temperature toward Optimum: As temperature increases, kinetic energy rises. Molecules move faster and collide more frequently and with greater energy, increasing the rate of successful enzyme-substrate complex formation.
   • Optimum Temperature: This is the temperature at which the rate of reaction reaches its maximum. For most human enzymes, the optimum temperature is approximately 37°C.
   • High Temperatures (Denaturation): Above the optimum temperature, excessive thermal agitation disrupts the weak hydrogen and ionic bonds stabilizing the enzyme's tertiary structure. The active site permanently loses its shape, preventing substrate binding. This irreversible loss of structure and catalytic function is known as denaturation.

   pH and Enzyme Conformation

   The pH of a solution measures the concentration of hydrogen ions (H+). Enzymes are sensitive to pH changes because ion concentrations directly affect the charges of acidic and basic amino acid side chains within the protein.

   • Optimum pH: Every enzyme exhibits a characteristic optimum pH where its tertiary structure and the electrical charge of its active site are ideal for binding substrate.
   • Deviations from Optimum: If the pH shifts significantly above or below the optimum level, the alteration in H+ or OH- ion concentrations disrupts the ionic bonds and hydrogen bonds that hold the enzyme in its active conformation.
   • Denaturation by Extreme pH: Severe deviations from the optimum pH lead to irreversible denaturation, causing the reaction rate to fall to zero.

   Different enzymes operate at vastly different optimum pH values depending on their native physiological environments:

   • Pepsin (a stomach protease): functions optimally at approximately pH 1.5–2.0.
   • Salivary Amylase (mouth): functions optimally at near-neutral pH 6.8–7.0.
   • Trypsin (small intestine): functions optimally in an alkaline environment around pH 8.0.

   Substrate and Enzyme Concentration

   The availability of interacting molecules plays a critical role in determining reaction velocity when environmental conditions (temperature and pH) remain constant.

   Substrate Concentration

   • At low substrate concentrations, active sites are freely available, and the substrate acts as the limiting factor. Increasing substrate concentration increases collision frequency, leading to a proportional increase in reaction rate.
   • As substrate concentration continues to rise, active sites become occupied. Eventually, a point is reached where all active sites are continuously engaged in catalysis. This is known as the point of saturation.
   • Beyond saturation, any further increase in substrate concentration will not increase the reaction rate; the maximum velocity (Vmax) has been attained.

   Enzyme Concentration

   • Assuming substrate is present in excess, the reaction rate is directly proportional to enzyme concentration. Adding more enzyme introduces more available active sites, facilitating a higher number of successful collisions per unit time.
   • If substrate becomes limited, the reaction rate plateaus unless additional substrate is added.

   Check Your Understanding

   Review the key concepts below to evaluate your understanding of the factors affecting enzyme action:

   1. Why does cooling an enzyme-catalyzed reaction on ice slow the reaction down without destroying the enzyme?
      Answer: Low temperatures reduce kinetic energy and collision frequency between enzyme and substrate molecules, but they do not break the covalent or non-covalent bonds that maintain the active site's structure.
   2. What happens to the structure of pepsin if it passes into the alkaline environment of the duodenum?
      Answer: The drastic increase in pH alters the ionization of amino acid side chains, disrupting ionic and hydrogen bonds, leading to denaturation and inactivation of pepsin.
   3. Why does increasing substrate concentration fail to increase the reaction rate indefinitely?
      Answer: When all available enzyme active sites are fully saturated with substrate molecules, the rate of catalysis is limited strictly by enzyme turnover, reaching a plateau.