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In biochemical reactions, the substrate is the substance on which an enzyme acts. Such a reaction generally changes the molecules of the substrate into some other substance. The term "substrate concentration" is used to describe the number of substrate molecules in a solution. It is one of the factors that affects the rate of reactions, and can be a limiting factor for them.
Enzymes are highly specific, which means that they usually only act on one substrate. During an enzyme-controlled reaction, the enzyme combines with the substrate at the active site. To do so, the enzyme has a very specialized shape that fits exactly with the substrate. An enzyme-substrate complex is formed when the enzyme attaches to the substrate. When the reaction is finished and the products are created, they are released from the enzyme, which can now catalyze another reaction.
Whether a reaction will occur depends on whether the substrate molecule will collide with and combine with the required enzyme. Substrate concentration, enzyme concentration, temperature and pH are all factors that will affect the rate of enzyme-controlled reactions. The factor that is at the lowest level becomes the limiting factor for the reaction.
Substrate concentration is the number of substrate molecules found in a particular solution, while enzyme concentration is the number of enzymes. One enzyme can only act on one substrate molecule at a time, so an increase in enzymes means that more substrate molecules can be converted into the products of the reaction. Also, with more molecules present, there is a greater chance of the substrate and enzymes colliding. As the number of enzymes is increased, the substrate concentration now becomes the limiting factor.
If there are more enzymes in solution than substrate molecules, adding more substrate, or increasing the substrate concentration, will increase the rate of the reaction initially. As the reaction progresses, the substrate molecules are used up as they are converted into the products of the reaction. This means that the substrate concentration decreases over time and once again becomes the limiting factor on the rate of the reaction. Substrate concentration can be increased until the point where all the enzymes are being used, which will be the maximum rate of the reaction for that concentration of enzymes.
Changes in temperature and pH can also affect the rate of the reaction even if the maximal substrate concentration and enzyme concentration is used. Increased temperature increases the rate of the reaction while decreased temperature decreases the rate. This is due to the fact that the molecules have more energy and are more likely to combine. Temperature can only be increased to a certain point or it will cause the enzymes to break down, as will changes in pH. Enzymes have an optimum temperature and pH at which they will work and will provide the highest rate of reaction.