You know when you look at a word too much and it starts to look weird? That will definitely happen with the word ‘enzyme’ as we move through this topic. Today we’re going to take a look at the basics of what enzymes are and how they act.
What are enzymes?
Perhaps the first phrase which pops into your head when you hear that question is ‘biological catalysts‘. And that is absolutely what enzymes are – they speed up metabolic reactions without being used up themselves. Another answer to that question could be ‘enzymes are globular proteins‘. Remember that globular proteins are round and soluble. The tertiary protein structure is extremely important because the active site of an enzyme must be very specific to bind its substrate. The active site is complementary to the substrate, so there is normally only one substrate per enzyme. However, inhibitors can sometimes fit the active site too – more on this in a future article.
Enzymes can be intracellular (found within cells) or extracellular (found outside of cells). You will come across lots of examples of these if you are studying A-Level biology:
- Intracellular enzymes include DNA polymerase, RNA polymerase, catalase (breaks down toxic hydrogen peroxide to oxygen and water), ATP synthase, DNA helicase.
- Extracellular enzymes include amylase, trypsin, lipase (i.e. digestive enzymes – see article on digestion).
Lowering activation energy
Sometimes to get something done you have to put in a bit of extra effort to get started on something (exam revision for example). Activation energy works in a similar way. For a chemical reaction to occur, a minimum amount of energy needs to be provided so that the chemical bonds can break. Normally that energy is provided as heat from the surroundings. We won’t go into detail about the chemistry, but that is the general idea.
An enzyme can lower the activation energy in a couple of ways. Firstly, it could hold reactants close together so they are able to form new bonds more easily without repelling each other. Secondly, it could contort and put strain on a substrate that needs breaking up to help the chemical bonds to break more easily.
Models of enzyme action
The classic model of enzyme action is the ‘lock and key‘ model, where the shape of the active site and the shape of the substrate are a perfect fit. However, since that theory was put forward further evidence has come along to change what is now accepted. The more up-to-date ‘induced fit‘ model better describes how enzymes work.
The induced fit model suggests that even though the active site and substrate are similar in shape, the enzyme actually has to change structure slightly in order to bind the substrate. When the substrate is bound to the enzyme, the enzyme-substrate complex is formed. This is the transition state when the activation energy is lowered and bonds are broken or formed. Then the products are released and the enzyme returns to its original structure.
Don’t forget that the polarity of the active site (whereabouts there are areas of positive or negative charge) also affects binding of the substrate which is why a change in pH can affect enzyme activity. We will look at this in more detail in a future article. Hydrophilic and hydrophobic interactions also have a role in substrate specificity.
Summary
- Enzymes are globular proteins which act as biological catalysts, and can be intracellular or extracellular.
- Enzymes speed up metabolic reactions by lowering the activation energy.
- The lock and key model of enzyme action has been replaced by the induced fit model, which suggests that the enzyme changes structure slightly to bind the substrate.
