Enzymes can increase the rate of reaction

As with many chemical reactions, the rate of an enzyme-catalysed reaction increases as the temperature increases. However, at high temperatures the rate decreases again because the enzyme becomes denatured and can no longer function. This is shown in the graph below. As the temperature increases so does the rate of enzyme activity.

An optimum activity is reached at the enzyme's optimum temperature. A continued increase in temperature results in a sharp decrease in activity as the enzyme's active site changes shape. At this point, so much substrate is present that essentially all of the enzyme active sites have substrate bound to them.

In other words, the enzyme molecules are saturated with substrate. The excess substrate molecules cannot react until the substrate already bound to the enzymes has reacted and been released or been released without reacting.

Ten taxis enzyme molecules are waiting at a taxi stand to take people substrate on a minute trip to a concert hall, one passenger at a time. If only 5 people are present at the stand, the rate of their arrival at the concert hall is 5 people in 10 minutes. If the number of people at the stand is increased to 10, the rate increases to 10 arrivals in 10 minutes.

With 20 people at the stand, the rate would still be 10 arrivals in 10 minutes. The rate would simply be higher 20 or 30 people in 10 minutes before it leveled off. This is true for any catalyst; the reaction rate increases as the concentration of the catalyst is increased.

To some extent, this rule holds for all enzymatic reactions. This fact has several practical applications. We sterilize objects by placing them in boiling water, which denatures the enzymes of any bacteria that may be in or on them. We preserve our food by refrigerating or freezing it, which slows enzyme activity.

Because most enzymes are proteins, they are sensitive to changes in the hydrogen ion concentration or pH. Ionizable side groups located in the active site must have a certain charge for the enzyme to bind its substrate.

An enzyme exhibits maximum activity over the narrow pH range in which a molecule exists in its properly charged form. In biology, chemical reactions are often aided by enzymes , biological molecules made of proteins which can be thought of as facilitators or catalysts. Enzymes speed the reaction, or allow it to occur at lower energy levels and, once the reaction is complete, they are again available.

In other words, they are not used up by the reaction and can be re-used. Enzymes are designed to work most effectively at a specific temperature and pH.

Outside of this zone, they are less effective. At very high temperatures, enzymes, because they are made of protein, can be denatured or destroyed. The material on which the enzyme will act is called the substrate. The enzyme attaches to the substrate molecule at a specific location called the active site. When the enzyme has attached to the substrate, the molecule is called the enzyme-substrate complex. For example, the sugar found in milk is called lactose.

With the aid of the enzyme, lactase , the substrate, lactose, is broken down into two products, glucose and galactose. Enzyme action can be blocked by molecules that obstruct the enzyme's active site.

Herbicides and pesticides often work in this way. The active site of an enzyme has a very specific 3-dimensional shape. Therefore, enzymes are specific to particular substrates, and will not work on others with different configurations. There are several factors that can increase the rate of a reaction. Raising the temperature can speed a reaction because the molecules have more energy and therefore bump into each other more frequently.

The same effect can be obtained by physically stirring the ingredients. A reaction can also be speeded by increasing the concentration of reactants, the chemicals that are necessary for the reaction to proceed; this is called the Law of Mass Action , or by decreasing the concentration of products, the chemicals that result from the reaction. Some reactions can even run in both directions depending on the concentration of molecules. For example, carbonic anhydrase can catalyse the conversion of bicarbonate, a blood pH buffer, into water and carbon dioxide, or can catalyse the reaction in the opposite direction when water and carbon dioxide are more abundant.

The graph below shows that the rate or velocity V of a reaction depends on substrate K concentration up to a limit.