Please help! .. Explain why an enzyme is specific in the reaction it catalyses?

Could you tell us what you know about enzymes, so that we know how much we need to tell you?

Where you cam from

Well I don't no much only started the topic few days ago..they are 3d and made of protein that about it really

Okay, so you know they are made of proteins.

Now those proteins, they have been made from different amino acids, controlled by genes in cells. The 3D shape that the enzyme will take, the different amino acids the enzyme contains will determine what type of reaction the enzyme can catalyse.

This is because the different substrates/products have different structures. Each enzyme has one particular structure for one particular substrate/product.

If there was another enzyme present, the reaction would most probably not be catalysed at all, because the latter enzyme has a different structure which doesn't 'fit' the substrate/product and hence cannot perform the process.

You might want to read about the Lock and Key mechanism of enzymes.

In this analogy, the lock is the enzyme and the key is the substrate. Only one particular key can fit in the lock, a different one won't work.