Your life depends upon billions of chemical reactions every second. Your breath, the beating of your heart and your thoughts would all stop in the absence of chemical reactions. But most of those reactions would not occur fast enough without help from enzymes -- protein molecules that bring substrate molecules together and combine them into new molecules. Without any substrate molecules, enzymes don't do anything, but they start working when substrate molecules are around.
Enzymes in Your Body
A molecule called adenosine triphosphate, ATP, provides energy for every muscle in your body. But when it gives up its energy it becomes adenosine diphosphate, ADP. For your body to keep working, you need to convert the ADP back into ATP -- as many as 10 million times a second for just one muscle cell. That's an example of a reaction your body needs to survive -- and one that could not happen quickly enough without enzymes. Every chemical reaction needs some energy to get started. That's called the activation energy. Enzymes work by lowering the activation energy required to promote a chemical reaction. The molecules an enzyme works on are called the substrates, and the end results of the chemical reaction are called the products.
Enzymes and Substrates
You can think of a chemical reaction as something like trying to push a foam ball into a circular hole in a wooden block. The foam ball is just a tiny bit smaller than the hole, so it needs to be squished in the hole. If you stirred a bucket full of foam balls and wooden blocks, some balls would stick in the holes, but not too many. Let's say you built a little machine that holds a wooden block and squeezes the foam ball. If you threw a few of those machines in the bucket you would end up with a lot more finished reactions, where the ball was stuffed into the hole in the block. Substrates are like the balls and blocks, and enzymes are like the machines.
Temperature, pH and salt concentration can all affect reaction rates, but the two components that must be present to have any chance of an enzyme-mediated reaction are the enzymes and the substrates. If you have plenty of enzyme, but no substrate, then you'll have no reactions. If the same as having a bucket full of machines to put blocks and balls together, but no blocks and balls. When you start adding balls and blocks, the reaction will start. If you add twice as many balls and blocks, you'll get twice as many reactions. The more balls and blocks you add to the bucket, the more reactions you'll get.
If you keep adding substrate to an area with enzyme, you'll continue to generate more product -- until you put in so much substrate that every enzyme is already working. Then your new substrate will have to wait for a reaction to finish before it can connect with an enzyme. If you add balls and blocks to a bucket where every machine is already busy, the new balls and blocks will have to wait until one of the machines is done. That is called the saturation point. With no substrate, there's no reaction. Then as you add substrate the reaction rate increases linearly for a while. As you put in more and more substrate the increase in reaction rate slows down until you reach the saturation point. Then adding more substrate doesn't increase the reaction rate at all.. The curve of enzyme activity versus substrate concentration is called the Michaelis-Menten plot, after the two biochemists who discovered it.
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