Enzymes and Protein Structure

Protein Structure

  • Key Definitions:

    • Protein Monomer: Amino acid.

    • Protein Polymer: Peptide chain composed of amino acids, also known as a peptide or protein.

Amino Acids

  • Role in Proteins:

    • The specific order of amino acid monomers in a polypeptide is crucial as it interacts with the environment to determine the overall shape of the protein.

Protein Shape

  • Characteristics:

    • A functional protein consists of one or more polypeptides that are twisted, folded, and coiled into a unique three-dimensional shape.

Enzymes

  • Definition: Specialized proteins that accelerate chemical reactions.

  • Energy Dynamics:

    • Energy Released: The energy released during the reaction when it occurs.

    • Energy Supplied: The energy needed to start the reaction.

    • Enzyme Action: Enzymes lower activation energy, the energy needed to start a reaction

      • Lower activation energy = faster reaction.

      • Enzymes do not change the reactants or products.

      • Enzymes do not change the total energy released in a reaction.

    • Without Enzyme: Higher activation energy required.

    • With Enzyme: Lower activation energy, leading to increased rate of reaction.

Enzyme Components

  • Definitions:

    • Enzyme: A type of protein that catalyzes reactions.

    • Substrate: The starting material that the enzyme acts upon.

    • Active Site: The specific region on the enzyme where the substrate binds and catalysis occurs.

Enzyme-Substrate Complex

  • Definition: The intermediate formed when a substrate molecule binds to the active site of an enzyme.

    • The enzyme and substrate join to form this complex, facilitating the reaction.

Active Site

  • Description:

    • A specialized region of an enzyme that specifically binds to the substrate.

    • It is a restricted area of the enzyme structure where the substrate fits perfectly.

Models of Enzyme Action

  1. Lock and Key Model:

    • Simplistic representation where the substrate fits directly into the three-dimensional structure of the enzyme’s active site.

    • Involves the formation of hydrogen bonds between the substrate and the enzyme.

  2. Induced Fit Model:

    • A more accurate representation suggesting that the substrate binding triggers a conformational change in the enzyme, enhancing the fit and catalysis.

Properties of Enzymes

  • Reaction Specificity:

    • Each enzyme is tailored to work specifically with a certain substrate due to a chemical fit between them.

  • Not Consumed in Reactions:

    • An individual enzyme molecule can catalyze thousands of reactions per second and remains unchanged after the reaction.

  • Sensitivity to Cellular Conditions:

    • Enzymes are affected by external conditions, as they are proteins, including:

    • Temperature

    • pH

    • Salinity

Naming Conventions for Enzymes

  • Enzymes are often named based on the reactions they catalyze:

    • Sucrase: Breaks down sucrose.

    • Proteases: Break down proteins.

    • Lipases: Break down lipids.

    • DNA Polymerase: Builds DNA by adding nucleotides to DNA strands.

    • Pepsin: Breaks down proteins (polypeptides).

Steps of Enzyme Action

  1. Step 1: Substrates enter the active site; the enzyme alters its shape to accommodate the substrates.

  2. Step 2: The enzyme (acting as a catalyst) reduces the energy needed to initiate the reaction, expediting the transformation of substrates into products.

  3. Step 3: Substrates are converted into new products that are then released from the enzyme.

  4. Step 4: The active site of the enzyme becomes free for two new substrate molecules, allowing the catalytic process to repeat.

Mechanisms of Enzyme Action

  • Methods to Lower Activation Energy:

    • Synthesis/Holding: The active site orients substrates in the optimal position for reaction and brings them closer together.

    • Digestion: The active site binds the substrate and stresses the bonds requiring separation, facilitating easier molecule dissociation.

Factors Affecting Enzyme Function

  • Enzyme Concentration:

    • Increasing enzyme levels typically leads to increased reaction rates due to more frequent collisions with substrate. However, the reaction rate plateaus when the substrate concentration becomes limiting.

  • Substrate Concentration:

    • Increased substrate concentration results in higher reaction rates until all active sites on enzymes are occupied, leading to saturation and maximum reaction rate.

  • Temperature:

    • Optimum Temperature: There exists an optimal temperature range (35°C to 40°C for most human enzymes) where reaction rates peak; body temperature is approximately 37°C.

    • Effect of Heat: Excess heat can disrupt molecular bonds, resulting in denaturation (loss of the 3D shape).

    • Effect of Cold: Low temperatures slow molecular movement, reducing collision rates between enzymes and substrates.

  • pH Levels:

    • Each enzyme has an optimal pH range for activity:

    • Most human enzymes work optimally between pH 6 and 8.

    • Specific enzymes like Pepsin function well at pH 2 to 3, while Trypsin functions best at pH 8.

    • Alterations in pH (by adding/removing H+) can disrupt chemical bonds and the enzyme’s three-dimensional structure, potentially leading to denaturation.