3.1: Enzyme Structure

So far, you've learned about all the different types of molecules and structures that make up a living organism, but how are those things formed? How do they interact? Topic 3.1 answers both of those questions: cells use enzymes.

Vocab List

• Enzyme
  • Catalyst
  • Activation energy
  • Active site
  • Cytoplasm/Cytosol
• Substrate
• Enzyzme-substrate complex

Written Explanation

Enzymes:

All actions taken by cells are defined by a set of three possible steps. Cells may move materials, form a bond between atoms, or split a bond between atoms. Any larger and more complex action is a combination and repetition of those steps. While the cell uses active and passive transport for movement, it uses enzymes for the other two steps. Enzymes are proteins (or groups of proteins) that catalyze chemical reactions. They do so by connecting to a substrate and lowering the activation energy of a reaction. And now for some definitions:

Chemical reaction:

A chemical reaction is a process that results in a chemical or structural change to a substance. For example, when salt (NaCl) is added to water, it dissolves into Na+ and Cl-, thus undergoing a chemical change. In a combustion chemical reaction, hydrocarbons and oxygen molecules react (with heat) to produce water and carbon dioxide molecules. This occurs through a rearrangement of the atoms in the original molecules (the reactants) to form new compounds (the products).

Catalysts:

In everyday speech, a catalyst is something (an action or object) that causes/boosts some other event. Similarly, a biological catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change ^{(Oxford Languages)}. Essentially, a catalyst turns a set of (1+) molecules into a different set of (1+) molecules. It's important to remember that although enzymes exist to cause a desired chemical change, most chemical reactions actually go both ways. All an enzyme does is speed up the rate of that reaction, swapping between the initial and final substrate.

Activation energy:

As you would expect, causing a chemical reaction requires energy. For some reactions (like combustion), the required amount of energy is obtainable from the environment (heat). However, many chemical reactions in cells require much more energy to occur. Activation energy is the minimum amount of energy needed to induce a reaction. The way enzymes serve as catalysts (speeding up a reaction) is by reducing this activation energy.

Substrates:

So an enzyme is the biological catalyst that causes molecules to join together and break, but what does it act on? Well, substrates are the materials that enzymes act on.

Active site:

And where does this chemical reaction occur? Such reactions occur in the active site of an enzyme. Active sites are specially "designed" for each enzyme to perfectly fit the substrate that it acts upon. This includes having a compatible charge and shape. As you would imagine, it's quite hard to connect two positive sides of magnets, or fit a square in a circular hole. Therefore, an enzyme works on only very specific substrates, and has a very limited action.

Lastly, when a substrate is bound to an enzyme at its active site, the whole compound is called the enzyme-substrate complex.