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 the reason that 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.
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.
Both feedback inhibition and covalent modification of enzymes are forms of allosteric regulation. Feedback inhibition allows the cell to tell itself when to stop making something. Covalent modification is the alteration of enzyme activity through changes in covalent bonds, such as the addition or subtraction of a chemical group like a phosphate, known as phosphorylation or dephosphorylation.
Because enzymes can be allosterically regulated, the cell can use a similar amino acid sequence. This produces an almost identical enzyme with the same function, binding the same substrates to produce the same products, but with different molecules to control it. As a result, the cell is able to conserve its resources while deferentially regulating how the enzyme functions.
An example of this can be seen in the liver and muscles, where an almost identical enzyme (glycogen phosphorylase), with the same function (conversion of glycogen to glucose-1-phosphate) is allosterically regulated. In the liver, glucose binding to the enzyme inactivates it. However, in muscle cells, the main inactivator of the almost identical enzyme with the same function is not glucose, but ATP. Along the same lines, adenosine monophosphate (AMP) activates the enzyme in the muscle, but has no effect on the liver enzyme.
Enzymes bind substrates at the active site and form products. Molecules bind to and affect enzyme behavior at allosteric sites by altering enzyme conformation. Allosteric enzyme regulation is where a molecule binds an allosteric site, altering enzyme conformation and thereby activating or deactivating the enzyme, or increasing and decreasing its activity. Allosteric regulation is important because it permits a more dynamic and complex control of enzyme activity, while allowing the cell to use almost identical enzymes, thereby conserving its resources. In most cases, allosteric regulation changes enzyme conformation into one of two states: a relaxed (R) active state or tense (T) inactive state.
Confirm your ability to do the following when the lesson concludes:
- Recall the function of enzymes
- Discuss the occurrence of allosteric enzyme regulation
- Note the importance of allosteric regulation
- Discuss subunits within enzymes
- Differentiate between the ‘R’ state and the ‘T’ state