How does reaction rate vary with substrate concentration

The single most important property of enzymes is the ability to increase the rates of reactions occurring in living organisms, a property known as catalytic activity. Because most enzymes are proteins, their activity is affected by factors that disrupt protein structure, as well as by factors that affect catalysts in general.

Factors that disrupt protein structure include temperature and pH; factors that affect catalysts in general include reactant or substrate concentration and catalyst or enzyme concentration. The activity of an enzyme can be measured by monitoring either the rate at which a substrate disappears or the rate at which a product forms. 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. The catalytic site of the enzyme is empty, waiting for substrate to bind, for much of the time, and the rate at which product can be formed is limited by the concentration of substrate which is available.

B As the concentration of substrate increases, the enzyme becomes saturated with substrate. As soon as the catalytic site is empty, more substrate is available to bind and undergo reaction. The rate of formation of product now depends on the activity of the enzyme itself, and adding more substrate will not affect the rate of the reaction to any significant effect.

The rate of reaction when the enzyme is saturated with substrate is the maximum rate of reaction, Vmax. The relationship between rate of reaction and concentration of substrate depends on the affinity of the enzyme for its substrate.

This is usually expressed as the Km Michaelis constant of the enzyme, an inverse measure of affinity. For practical purposes, Km is the concentration of substrate which permits the enzyme to achieve half Vmax. In the reaction process, there is no bond formed between the enzyme and the substrate, so the enzyme goes back to its original shape and can be used again. The enzyme binds to the substrate through the active site to form an enzyme-substrate complex. They are very specific in the reaction and also to the substrate they are binding with.

When the shape of the substrate matches the active site of the enzyme, the function of the enzyme is correct, and their functioning is dependent on its three-dimensional structure. They make the reaction easier to occur by reducing the activation energy and activating more molecules, thus carrying out the catalyst. The concentration of the enzyme is important in chemical reaction as it is needed to react with the substrate. Often a small amount of enzyme can consume a large amount of substrate.

However, with the increase of enzyme concentration, the effectiveness of the active sites also increases, so these active sites will convert the substrate molecules into products. This basically means that if the concentration of the enzyme is to be increased, there needs to be an excess of substrate, in other words, which means that the reaction must be independent of the concentration of the substrate.

A competitive inhibitor for example, cyanide competes with the substrate for the active site of the enzyme, reducing the rate of reaction at lower substrate concentrations. Given a high enough concentration of substrate the inhibitor can be overcome, so the same Vmax as the reaction without inhibition can be reached although the Km will be changed. Noncompetitive inhibitors such as penicillin do not use the active site of the enzyme, perhaps binding in another place and changing the conformational shape an allosteric inhibitor.

Increasing substrate concentration should still increase the reaction rate, but because enzymes can be inhibited regardless of how saturated their active sites are, both the Vmax and the Km will be changed.

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