7. Enzyme

What is enzyme?

Enzymes are biological catalysts that speed up reactions without consuming themselves in the process. They are mostly composed of proteins, but a few of them consist of RNA molecules. Here, we will discuss the mechanism of the function of enzymes.

Function of enzyme

To achieve a chemical reaction, regardless of whether it is exergonic or endergonic, we have to input some amount of energy. This minimal amount of energy needed to initiate a chemical reaction is called activation energy. Each chemical reaction has a specific amount of activation energy. The higher the activation energy is, the slower the reaction is, because the system needs lots of energy to initiate.

Catalysts can create alternative pathways of chemical reactions that have lower activation energies than the original reaction. In this way, catalysts can lower the overall activation energy of chemical reactions and make them faster.

Let’s say there is a substrate A, which will become B after a chemical reaction. The original reaction is indicated as A → B, whose activation energy is high and the reaction speed is slow. When we add a catalyst C to this reaction, this will create another pathway of reaction, which is A + C → AC → B + C. Here, each step of the reaction has low activation energy, so the reaction speed is fast. You may notice that the catalyst C is not consumed throughout the whole reaction. This is also an important characteristic of catalysts. You may have such a question about what the difference between A + C and AC is. One thing is that A and C are separated and independent substrates in A + C, while two are bonded in AC. Another thing is that the structures of A and C in these two states are different. Through the binding of A and C, the enzyme C changes its structure to perform its catalytic activity. This theory about the mechanism of enzymes is called the “induced-fit model.”

Structure of enzyme

Enzymes are mostly composed of proteins, and the rest of them are comprised of RNA, as we discuss. Regardless of that, an enzyme has two parts: the apoenzyme and the cofactor.

An apoenzyme is the protein part of an enzyme that, by itself, is inactive. It requires another component, called a cofactor, to become a fully functional enzyme, called a holoenzyme.

Cofactors can be inorganic molecules, like metal ions (e.g., magnesium, zinc), or organic molecules, like vitamins. They bind to the apoenzyme at a specific site and help it carry out its catalytic activity.