An enzyme makes a reaction proceed faster, but is not consumed in the reaction. This means that the more substrate there is, the more enzyme activity can be observed. However, the effect of substrate on enzyme activity is not simply to increase it. Substrate concentration has many different effects on enzyme activity, depending on the context of the reaction and the nature of the enzyme. Substrates increase enzyme activity by out-competing competitive inhibitors that block enzyme activity, protecting the enzyme against harsh pH environments, or activating a multi-part enzyme by positive cooperativity -- which is binding one part to turn on other parts. However, there is such a thing as negative cooperativity, so substrate concentration can also lower enzyme activity.
Let The Games Begin
An enzyme has a special part called an active site. The active site binds the substrate and catalyzes the change in the substrate. Active sites by themselves may not be active in that they do not take on the proper three-dimensional shape required for activity. The binding of a substrate at the active site induces a shape changed -- called a conformational change -- that turns on the active site. Since the enzyme is not consumed in the reaction, but remains intact and able to catalyze more reactions, adding more substrate will result in an increase in enzyme activity. However, at some point enzyme activity will saturate, meaning it stops increasing, even if more substrate is added.
Some enzymes have multiple parts, each with its own active site. The binding of a substrate to one active site causes the entire enzyme to change shape, resulting in the activation, or opening, of the other active sites. This chain reaction effect is called cooperativity. Human blood carries oxygen through hemoglobin, which is a four-part protein. The binding of an oxygen molecule to one of the four parts causes the other three to be more receptive to binding more oxygen. Hemoglobin is an example of positive cooperativity. However, not all cooperativity is positive. Negative cooperativity occurs when a substrate binds to one active site and causes the other active sites to be less receptive to the substrate. Thus, substrate concentration can increase or decrease enzyme activity, depending on the nature of cooperativity between a multi-subunit enzyme and its substrate.
A population of an enzyme has a certain rate of activity, depending on how much substrate is present. Graphing enzyme activity versus substrate concentration reveals what is called the kinetics of the enzyme. Enzymes and substrates that do not exhibit cooperativity, as discussed above, have a hyperbolic curve, which looks like the side of a plateau. Hyperbolic curves rise quickly and then flatten out, telling us that enzyme activity is gradually turned and increases until it saturates -- meaning it cannot become any faster. But enzymes that exhibit cooperativity exhibit what is called sigmoidal kinetics, meaning the curve is shaped like an S. Sigmoidal curves differ from hyperbolic curves in that the line rises to the right and flattens out much more quickly. Sigmoidal curves indicate that the substrates and enzymes are interacting with each other to change the speed of each enzyme, such that the rise in activity is much faster than in hyperbolic curves.
Beating The Competition
Enzyme activity can be stopped by a competitive inhibitor, which is a molecule that binds to the active site of an enzyme and prevents it from binding a substrate molecule. Certain types of competitive inhibitors only temporarily bind to active sites and are called reversible inhibitors. Since reversible inhibitors can bind to and fall out of the active site, they can be out-competed by the substrate if there is a lot more substrate than there is inhibitor. In this game of musical chairs, an abundance of substrate will prevent the competitive inhibitor from blocking enzyme activity. Thus, increasing substrate concentration can increase enzyme activity in the presence of reversible competitive inhibitors.
- Elmhurst College, Virtual Chembook: Mechanism of Enzyme Action
- Royal Society of Chemistry: Education: Factors affecting catalytic activity of enzymes
- Community College of Baltimore County: Microbial Genetics
- Biochemistry: Negative Cooperativity in Enzyme Action
- University of California, Davis: Sigmoid Kinetics
- Biochemistry: Enzymes Can Be Inhibited by Specific Molecules
- BananaStock/BananaStock/Getty Images