As seen from the post before enzymes are highly specific molecules, meaning that they do not bind to any kind of substrate molecule. From this two theories are born, each contrasting the other.
The Lock and Key Hypothesis
This theory was developed by Emil Ficher(1890), and he stated that due to the enzymes being highly specific the substrate must be an exact fit to the active site of the enzyme for the substrate to catalyze into products. In simple terms, it means that it must fit like a key inside a lock. Once combined, the resulting structure is referred to as a enzyme-substrate complex (ES). Therefore no other key can work in the lock if it does not compliment the exact shape of the active site. The products have a different form than the original substrate, therefore once formed they they leave the active site and the enzymes moves on to another substrate molecule and repeats.
Figure 1 shows the lock and key method for both breaking and reforming reactions.
Induced Fit Theory
In 1959, a scientist by the name of Daniel Koshland, proposed that enzymes change the conformation to suit that of the substrate. He proposed that some enzymes can in fact change their shapes to be able to turn more substrates into products. This change in enzyme conformation occurres when the substrate is bind onto the substrate, the enzymes active site slightly changes to accomodate the substrate. The resulting active site will be conformed to a precise shape for the reaction to take place.
Figure 2: showing a substrate joining and enzyme changes its shape to accommodate the substrate.
Information gathered from http://www.saburchill.com/IBbiology/chapters01/033.html accessed on April 6th, 2013.