CIE AS Biology 3.2: Rate of Reaction

The rate of reaction is the measure of how long it takes for a reaction to occur. In biochemical reactions, it is a measure of how long it takes for $a reactant to be used up$ or for $a product to be formed$.

To measure the rate of reaction, we can measure:

1. the amount of product formed or
2. the rate of disappearance of a substrate using the complementary enzyme

The initial rate of reaction is the fastest. As the reaction proceeds, more substrates bind to active sites, forming enzyme-substrate complexes and producing products.

Eventually, all the active sites are occupied and the concentration of enzymes becomes the limiting factor - this is where the graph plateaus and Vmax is calculated.

At half of Vmax, the Michaelis-Menten constant (Km) is calculated.

• the Michaelis-Menten constant (Vmax) is a $measure of the affinity of an enzyme for a substrate$ during a reaction at half of its maximum capacity
• it is expressed as a concentration
• high Km → less affinity for its substrate; low Km → high affinity for its substrate
• affinity can vary depending on the substrate, temperature, pH, presence of particular ions, and presence of inhibitors

Factors that Affect the Rate of Reaction

• temperature   * as temperature increases, the rate of reaction increases     * due to collision theory, the substrates have more kinetic energy to collide with active sites with sufficient energy and with the right orientation   * temperature is too low → not enough collisions; temperature is too high → hydrogen bonds in the amino acid sequence are broken, enzyme denatures and loses shape   * optimal temperature is usually 30-40 degrees C

• pH   * as pH increases, the rate of reaction increases   * too acidic/basic → ionic bonds holding the tertiary structure are disrupted, enzyme denatures   * optimal pH is dependent on each enzyme     * stomach enzymes have an optimal pH of around 1-2     * intestinal enzymes have an optimal pH of around 9 to 11

• enzyme concentration   * as enzyme concentration increases, the rate of reaction increases   * slows down when substrate concentration becomes the limiting factor     * more enzymes than substrate → more empty active sites

• substrate concentration   * as substrate concentration increases, the rate of reaction increases   * slows down when enzyme concentration becomes the limiting factor     * more substrate than enzymes → all active sites are occupied

• inhibitors   * competitive inhibitor: binds reversibly to the active site   * non-competitive inhibitor: binds to another part of the enzyme, altering the enzyme’s shape   * reversible inhibitors: binds to an enzyme to temporarily stop its processes     * used in metabolic reactions to act as regulators for product inhibition