6. Proteins as catalysts
6.1 Enzymes in metabolism
An organism uses the raw materials it has available to supply energy and the chemical building blocks it needs to grow. The chemistry it uses (its metabolism) is complex but we can group the reactions into two simple categories; those that break down large molecules into smaller units (catabolism) and those that build the small units back into different large molecules (anabolism). In general catabolic reactions release energy to a network of small molecules that then make it available for the anabolic reactions to use.
Organisms have enzyme catalysts to make sure that each reaction in its metabolism happens at the correct speed. Enzymes themselves are regulated so that metabolism as a whole is a co-ordinated web of reactions, happening in moderate conditions of temperature, pressure and pH.
Until recently scientists thought all biological catalysts were proteins, but they have discovered that a group of nucleic acid molecules, called ribozymes, act as catalysts in some single celled organisms. In this section, though, we will only look at protein catalysts.
6.2 How enzymes work
Catalysts change the speed of a chemical reaction without suffering any permanent chemical change themselves.
To understand how enzymes work as catalysts we first need a simple picture of what happens to the particles when two chemicals react.
The particles may be atoms, ions or molecules but we will refer just to molecules to keep it simple.
For a reaction to happen the two reacting molecules must:
Any change to the reaction conditions that alters any of these three will alter the speed of a reaction.