What is an enzyme?

Enzymes are proteins that are folded into specific shapes to allow for certain 'substrates', the other molecule involved, to bind. This area of binding is known as the 'active site', as it's where all the activity occurs!

This binding forms an 'enzyme-substrate complex', in which the substrate fits into the enzyme to bind due to their compementary shapes. I often imagine this binding like the game tetris - the substrate will only bind if it fits into the specific shape of the active site of the enzyme. If there are slight differences, the pieces will not fall into place - the enzyme and substrate will not bind.

This binding allows for the enzyme to be activated, meaning that it is able to perform its function. The purpose of an enzyme is to provide a different pathway for the reaction to occur along that involves a lower activation energy. This speeds up ('catalyses') a reaction, or simply allows it to occur in conditons where it would normaly not. To get my head around this intially, I imagined that I was walking home across a muddy field near my house. If i take the path alongside it, it would take me ten minutes to get home. However, if i wore my wellington boots to allow me to cut across the muddy grass, then I could get home in five minutes. The wellington boots in this analogy are enzymes: they allow me to take a different route to speed up the reaction (or, in this case, my walk home).

Unless you live in a parellel universe, I'm going to assume that when you get into your house, your shoes do not magically disappear.. The same is true of enzymes - as they are not directly involved in the reaction (in chemical terms, they are neither reactants nor products), therefore are not used up. This is why they are so useful!

Enzymes can go wrong, however.  A change in pH or temperature can cause the enzyme to change shape, meaning that the substrate is no longer complementary and therefore cannot bind to the active site - the enzyme is said to have been 'denatured' (i.e. it has lost it's shape and therefore it's 'nature'). This means that the reaction will either occur far more slowly, or won't occur at all. This is why your body fights to maintain your body temperature at 37.5 degrees - this is your enzymes 'optiumum temperature', or the temperature that they like the best. If your body temperature rises abov 40 degrees, your enzymes begin to denature (spoiler alert: you die!).

To conclude, an enzyme is a protein that catalyses a chemical reaction without being used up in the process. Learn to love them, because they keep you alive!