What Are Enzymes?
Enzyme: A biological catalyst , a globular protein that speeds up chemical reactions in living organisms without being consumed or permanently altered in the process.
Enzymes are absolutely central to life. Without them, most biochemical reactions would occur far too slowly to sustain metabolism. Here's what makes enzymes special:
- They are proteins , their function depends entirely on their three-dimensional shape
- They are catalysts , they speed up reactions by lowering the activation energy required
- They are specific , each enzyme acts on a particular substrate or type of reaction
- They are reusable , after catalysing a reaction, they return to their original form
Activation Energy: The minimum amount of energy required to initiate a chemical reaction. Enzymes lower this barrier, allowing reactions to proceed rapidly at the mild temperatures found in living cells.
Think of activation energy as a hill that reactants must climb before they can roll down to become products. Enzymes act like a tunnel through the hill , providing a lower-energy route to the same destination. The overall energy difference between reactants and products stays the same; enzymes just make it easier to get there.
Enzymes do not alter the overall energy change (ΔG) of a reaction , they only lower the activation energy. A reaction that is thermodynamically unfavourable cannot be made favourable by an enzyme.
The Active Site: Where Catalysis Happens
Active Site: A small, precisely shaped three-dimensional pocket or cleft on the enzyme surface, formed by a specific arrangement of amino acids. This is where the substrate binds and catalysis takes place.
Although enzymes can be enormous molecules with hundreds of amino acids, only a handful of those amino acids actually form the active site. These critical amino acids may be far apart in the linear sequence of the protein, but folding brings them together into the right configuration.
Key features of the active site:
- Shape complementarity , the active site's 3D shape is precisely matched to the substrate
- Chemical compatibility , amino acids in the active site have specific charges and polarity that interact with the substrate through weak bonds (hydrogen bonds and ionic interactions)
- Small size relative to the enzyme , only a tiny region of the enzyme is the active site
The specificity of an enzyme comes from its active site. If the shape or chemistry of the active site is disrupted , by heat, pH extremes, or chemicals , the enzyme loses its ability to function. This is called denaturation (the irreversible or severe alteration of the enzyme's 3D shape, covered in detail in a later section).
Enzyme-Substrate Complex (ESC): The temporary intermediate formed when a substrate binds to the active site of an enzyme. The ESC is a distinct stage in the catalytic cycle and is the point at which the conformational change of induced fit occurs.
