What does lemon juice, snake venom and cyanide have in common?

enzymes are protein molecules that act as catalysts, speeding up chemical reactions without themselves getting used up. Each enzyme will only speed up a specific reaction, for example, catalase will speed up the decomposition of hydrogen peroxide into water and oxygen but it will not speed up the breakdown of starch into glucose. Enzymes (e.g. catalase) have active sites with specific shapes that bind to the substrate molecule (e.g. hydrogen peroxide) forming an enzyme-substrate complex. The enzyme-substrate complex then breaks down into the enzyme and product, allowing the enzyme to go on and react with another substrate molecule. Temperature and pH affect enzyme function because they can change the shape of the enzyme’s active site, preventing it from binding to the substrate, just as a broken lock will no longer fit the key. When the shape of an enzyme changes we call this denaturation. Any factor that increases the frequency of collisions between enzymes and substrates (increasing concentration, surface area or temperature) will increase the rate of reaction.

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