enzymes

• Types of reactions catalyzed by enzymes

  • Enzymes can either catalyse reactions that build complex molecules from simple units, or breakdown complex substances into simple units
  • Reactions that build up new molecules are known as anabolic reactions
  • Reactions that breakdown molecules are known as catabolic reactions
• Enzymes

  2/29/2024
• Enzyme active site

  • Enzymes are specific
  • Only a small portion of the enzyme (around 2–4 amino acids) is directly involved in catalysis, called the catalytic site
• Enzymes mode of action

  1. Active site

  2. Enzyme-substrate complex

  3. Enzyme specificity: Lock and key hypothesis, Induced fit hypothesis

  4. Lowering of activation energy
• The substrate interacts/binds to the active site of the enzyme
• The catalytic site and binding site together compose the enzyme's active site
• The substrate is converted to products
• The lock and key hypothesis: 'The active site has a specific shape in which only one substrate type can fit exactly'
• An enzyme's name is often derived from its substrate or the chemical reaction it catalyzes
• Enzymes

  • Globular proteins that act as catalysts (= bio-catalysts)
  • Almost all metabolic processes in the cell need enzyme catalysis in order to occur at rates fast enough to sustain life
  • Enzymes increase the rate of the chemical reaction
  • There are generally two types of enzymes, depending on the site of action: Intracellular enzymes catalyze reactions inside the cell, Extracellular enzymes are secreted and catalyze reactions outside the cell
• Enzymes act on substrates
• This momentarily forms the enzyme-substrate complex
• The enzyme remains unchanged at the end of the reaction
• The catalytic site is located next to one or more regions where the substrate binds
• The enzyme and the substrate in the enzyme-substrate complex are held together by bonds between the R groups in the protein’s binding site and the substrate
• The induced fit model is generally accepted as the best current model of enzyme action
• Induced fit hypothesis

  • The active site has a distinctive shape, but it is not rigid
  • Binding of the substrate modifies the shape to fit and form the active complex
  • Proposed by Daniel Koshland in 1958
  • It is a modification to the lock and key model
  • States that the active site is continuously reshaped by interactions with the substrate
  • The active site continues to change until the substrate is completely bound, at which point the final shape and charge distribution is determined
  • Induced fit may enhance the fidelity of molecular recognition in the presence of competition
• Enzymes speed up reactions by lowering the activation energy needed for the reaction to start
• Enzymes increase the rate of reactions that are otherwise possible
• At the beginning of the reaction, products are produced very swiftly. The graph of the amount of product produced vs. time is therefore steepest at the beginning of the reaction. As the reaction progresses, the amount of product produced reduces until it eventually stops completely. The graph gradually flattens
• Lock and key hypothesis

  • The active site has a specific shape in which only one substrate type can fit exactly
  • Proposed by Emil Fischer in 1894
  • He proposed that the enzyme has a particular shape into which the substrate exactly fits
• Enzyme's active site

  The enzyme and the substrate in the enzyme-substrate complex are held together by bonds between the R groups in the protein’s binding site and the substrate
• It is possible to carry out an investigation into the rate at which substrates are converted into products during an enzyme-controlled reaction
• Enzymes do not alter the position of the chemical equilibrium of the reaction but only the speed at which it is reached
• Activation energy is the minimum quantity of energy which the reacting species must possess in order to undergo a specified reaction
• The induced fit model explains enzyme specificity but fails to explain the stabilization of the transition state that enzymes achieve
• Other molecules, despite being similar to the specific substrate, cannot exactly fit. Therefore, their reactions cannot be catalysed
• The rate of an enzyme-catalyzed reaction is always fastest at the beginning of the reaction
• Factors that affect the rate of enzyme activity

  • Temperature
  • pH
  • Substrate concentration
  • Enzyme concentration
  • Enzyme inhibition
• Importance of considering the initial rate of an enzyme when investigating factors that affect enzyme reaction
• Reaction progress

  1. At the beginning of the reaction, products are produced very swiftly

  2. As the reaction progresses, the amount of product produced reduces until it eventually stops completely
• The pH at which the enzyme catalyses the reaction at the maximum rate is called the optimum pH of the enzyme
• pH is a measure of the concentration of hydrogen ions [H+] in a solution
• As the temperature increases, the rate of the reaction also increases
• Graph of amount of product produced Vs time

  • Is steepest at the beginning of the reaction
  • Gradually flattens as the reaction progresses
• Initial rate of the reaction

  The rate of the reaction at the very beginning
• At low temperature, the reaction is slow because molecules are moving relatively slow
• Calculating the initial rate of the reaction

  Calculate the slope/gradient of the curve at a time as close to zero as possible
• Most human enzymes have an optimum temperature of about 40°C
• Changes in pH affect the formation of hydrogen bonds, ionic bonds, disulfide bonds, etc.