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Allosteric Regulation of Enzymes: Definition & Significance

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  • 0:05 Overview of Enzymes
  • 0:45 Allosteric Regulation
  • 2:48 Subunits in Enzymes
  • 3:17 Significance of…
  • 5:07 Lesson Summary
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Lesson Transcript
Instructor: Darla Reed

Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science

In this lesson, we'll conduct a brief review of enzymes, including what they are and what they do. We'll also use the analogy of a light switch to explore allosteric regulation and why the process is so important to enzymes.

Overview of Enzymes

Have you ever wondered what happens to a hamburger once it reaches your stomach, how it dissolves into pieces smaller than the eye can see? The answer to your question is enzymes. Enzymes are what change substrates into products. For example, they change the hamburger into something your body can use, as well as something small enough to get into its cells.

Enzymes are not only important in digestion, but many other bodily and cellular functions as well, like respiration. Therefore, it's also important that they are properly regulated. An unregulated enzyme may make too much of something or nothing at all, both of which cases can have profound effects on how your body performs. One key mechanism used to control enzymes is allosteric regulation.

Allosteric Regulation

Enzymes have an area called the active site, where they bind substrates, like the hamburger, and turn them into products or food for cells. Many enzymes have other areas called allosteric sites, located in a different place from the active site.

An allosteric site does not bind substrate, but instead binds another molecule that affects the enzyme's regulation. When a molecule binds an allosteric site, it alters the enzyme's shape, or conformation, which then changes how the enzyme functions.

To understand this concept more clearly, think about your kitchen lights. How do you turn them on and off? Do you screw and unscrew the light bulbs every time you want to change the lighting? No. You flick a switch. Imagine that the allosteric site represents the light switch and that by controlling the switch, you control the light bulb, or the enzyme's active site.

Allosteric enzyme regulation therefore is when a molecule binds a site other than the active site and changes the behavior of the enzyme by changing its conformation. In most cases, the binding of a molecule to the allosteric site acts like a dimmer switch that can turn a light on, making it brighter or dimmer, or turn it off. Just like the switch, allosteric molecules can activate, or turn on, the enzyme, as well as increase, or turn up, the enzyme's activity. They can also lower, or turn down, the activity of the enzyme, as well as inactivate, or turn off, the enzyme.

The activation state of an enzyme is often referred to as R, or the relaxed state, where the enzyme is on, and its activity is turned up. In the T, or the tense state, the enzyme is off, and its activity is turned down.

One molecule may bind the allosteric site and make the enzyme change from the T to R state, while a different molecule can bind the same enzyme and change it from the R to T state. The state of the enzyme will also affect its function. An example of this can be found in respiration, where a specific enzyme, phosphofructokinase-1, is activated by adenosine diphosphate (ADP), but inactivated by adenosine triphosphate (ATP).

Subunits in Enzymes

Enzymes are often made up of subunits, which can be individually or cumulatively controlled by allosteric regulation. In enzymes with many subunits, binding of an allosteric regulator to one subunit can make the other subunits more susceptible to allosteric regulatory binding, which can more quickly increase or decrease enzyme activity.

In some cases, binding to the allosteric site causes a separation of a regulatory subunit from the active enzyme section, or catalytic subunit.

Significance of Allosteric Regulation

Allosteric regulation allows for a higher degree of enzyme control than could be achieved through simply inhibiting or activating an enzyme. With allosteric regulation, the activity of an enzyme can be more tightly regulated by concentrations of, not only enzymes and substrates, but also other molecules that are not affected by the enzyme. This leads to a change in the graphic nature of enzyme activity, creating a sigmoidal or S-shaped curve rather than a simple sideways half-U or hyperbolic shape.

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