Enzyme Activity and Interactions

Enzymes Overview

• Enzymes facilitate chemical reactions by interacting with substrates at their active sites.
• Models of Enzyme-Substrate Interaction:
• Lock and Key Model: Suggests a rigid correspondence between substrate and enzyme active site.
• Induced Fit Model: Proposes a flexible interaction where the enzyme structure adjusts to better fit the substrate upon binding.

Chemical Interactions at Active Sites

• Substrates are held at the active sites through several types of interactions:
• Hydrophobic Interactions
• Hydrogen Bonds
• Salt Bridges
• These interactions are typically weak and non-covalent because:
• The aim is for substrates to bind temporarily, undergo transformation, and then release products quickly.
• Strong covalent bonds would keep substrates bound for longer than desired, impeding enzyme function.

Pyruvate Kinase Function

• Function: Transfers a phosphate unit from phosphoenolpyruvate (P) to adenosine diphosphate (ADP), producing adenosine triphosphate (ATP) and pyruvate (pyr).
• Chemical Reaction:
  \text{P} + \text{ADP} \rightarrow \text{ATP} + \text{pyruvate}
• The phosphate group:
• Composed of a phosphorus atom single bonded to three oxygens, exhibiting a tri-negative charge.

Enzyme Classification

• Pyruvate kinase is classified as a transferase because it transfers phosphate groups.

Active Site Visualization

• The structure of the active site includes specific amino acids that interact with substrates:
• Phosphoenolpyruvate is highlighted in pink.
• ADP is shown in orange.
• Pyruvate Kinase (PK) is illustrated in blue.
• Active site consists of:
• A crevice formed by folding of the enzyme which allows substrates to bind.

Interactions in the Active Site

1. Hydrophobic Pocket:

• Nonpolar amino acids cluster around the nonpolar parts of P to create a hydrophobic environment.

1. First Salt Bridge:

• Basic side chains with positive charges interact with the negative phosphate groups of ADP, stabilizing its position.

1. Second Salt Bridge:

• Acidic side chains hold a magnesium co-factor (Mg²⁺) in place, necessary for catalysis.

1. Metal Ion Salt Bridge:

• Further stabilizes the binding of P via interaction with the magnesium co-factor.

1. Potassium Salt Bridge:

• Potassium ions (K⁺) create additional interactions that help align ADP and P for the phosphate transfer.

Enzyme Activation Energy Reduction

• Proper alignment of substrates reduces the activation energy needed for the reaction, thus increasing the reaction rate.

Visualization Techniques

• X-ray crystallography can be employed to see active sites in detail, revealing how substrates interact with enzymes.

Importance of Co-factors

• Co-factors like magnesium and potassium are vital for proper enzyme function and substrate binding.
• These nutrients are also essential in a balanced diet for maintaining enzyme activity.