Enzymes are proteins that act as catalysts to help speed up reactions in a living organism. They are specific to the reaction they catalyze so if you lose or damage a specific enzyme a different enzyme can not take its place. The specificity is related to the shape of the enzyme which is determined by the way the protein folds, which is determined by its initial amino acid sequence. The specificity between an enzyme and its substrate is compared to a 'lock and key' mechanism.
Here is a YouTube video giving some of the basics of enzymes.
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Enzyme Basics - a nice animated tutorial about enzymes, enzyme inhibitors and negative feedback inhibition.
More Enzyme Basics - a nice site showing simple animations to talk about specificity and denaturing.
Enzymes and Energy - a tutorial explaining the energy of reactions and how enzymes work to lower the activation energy necessary for a reaction to occur.