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Darla has taught undergraduate Enzyme Kinetics and has a doctorate in Basic Medical Science
Ever wondered how meals you eat are digested, or how that paper cut heals? Part of the answer lies within enzymes. Enzymes are proteins that catalyze (speed up) chemical reactions. They help digest food and heal wounds. They play big roles in respiration, making proteins, and DNA replication. Enzymes function by binding to molecules called substrates to create products. These products are used by the cell in various ways, including as substrates in other reactions.
A car helps you get from place to place faster than walking. Now, imagine a car on the road with a monkey at the wheel; the consequences aren't good. The same is true for enzymes when they aren't regulated. Since enzymes are important in speeding up reactions and play major roles in cellular function, it is essential to regulate their activity.
The cell has several mechanisms to regulate enzyme activity. One mechanism is genetic regulation. Just as the number of cars produced controls how many are on the roads, the cell can regulate enzymes by controlling their production. If there's a sudden stop of Jeep production by manufacturers, the number of Jeeps being bought and used will decrease. Cells can target specific enzymes in a similar way through induction (increasing production) or repression (stopping production) of the gene producing that enzyme.
A second regulatory mechanism is compartmentalization. You have a glove compartment in your car that you use to keep various items. You may also have other compartments to hold music, equipment, sunglasses etc. Cells have their own compartments separated by membranes that restrict the movement of cellular components. Enzymes are regulated by those membranes.
For example, lysosomes are membrane-bound organelles that assist in breaking down cellular material. Enzymes inside lysosomes are bound by the membrane to stay inside the lysosome. They can't leave and can only function inside the lysosome.
Substrate concentration is another way enzymes are regulated. Your car won't go anywhere without gas. Enzymes won't be able to do their work without substrates. By controlling the amount of substrates available, a cell can effectively regulate enzyme activity.
A fourth way enzymes can be regulated is through degradation. If you take apart a car, it won't work anymore. Degradation is the breakdown of a molecule. Cells have mechanisms in place that can break down (degrade) enzymes or substrates. By degrading either, cells can regulate enzyme activity.
Yet another mechanism of regulation is through alteration of environmental factors. Just as icy roads, foggy conditions, or clear skies will change driving conditions, enzymes have certain environments where they function best. Enzymes have particular temperature and pH ranges (levels of hydrogen ions determining acidity) that control their activity. Altering the temperature or pH level will alter the function of the enzyme.
Enzymes also have inactive forms called zymogens (proenzymes). Potential drivers need to have a license before they can drive a car legally. Zymogens require a chemical modification in order to become active. Often this modification is performed by other enzymes—think of how someone has to teach you how to drive. Once modified, the enzyme is active.
Controlling production and chemical modification of zymogens is another way cells regulate enzyme activity. Lastly, the activation or inhibition of the enzyme itself is another mechanism of enzyme regulation.
There are many types of enzyme activation or inhibition. One type already discussed is genetic regulation. A second type, covalent modification, involves the breaking and formation of covalent bonds (sharing of electrons between molecules). In covalent modification, a molecular group, such as a phosphate, is covalently added or taken away from an enzyme. You may stop to drop off a friend, or you may not leave on a long trip until your friend is ready to go. The addition or subtraction of your friend controls whether your car goes or stops. The addition or subtraction of the molecular group may activate or inhibit an enzyme.
Allosteric regulation is a third type of enzyme regulation. Enzymes have a particular site where they bind their substrate called the active site. They also often have a different site where other molecules can bind. This other site is called the allosteric site. Molecules mostly bind noncovalently to the allosteric site. The binding molecules control whether an enzyme is activated or inhibited. They may change the shape of the active site so substrates can't bind, or they may make the active site more attractive to substrates.
One mechanism of allosteric regulation is feedback. It's always nice to know you passed your driving test. It's also good to know what you did wrong if you failed. Products often will give an enzyme feedback. They can bind to the enzyme that helped formed it to inhibit (feedback inhibition) or activate it (positive feedback).
A final type of enzyme regulation is strictly inhibition. Inhibition may be reversible or irreversible. There are three main types of inhibition: competitive, non-competitive, and uncompetitive.
In competitive inhibition, molecules compete with the substrate for enzyme binding. If you put water in your car instead of gasoline, it's probably not going anywhere anytime soon. With non-competitive inhibition, an inhibitor binds to a site other than the active site and prevents enzyme activity. If the cops put a boot on your car, it's not going anywhere. Finally, in uncompetitive inhibition an inhibitor binds to an enzyme after it's already attached to the substrate and prevents product formation. If you're driving your car and you get a flat tire, you are stopped from reaching your destination.
Enzymes are proteins that catalyze chemical reactions by binding substrates and turning them into products. Genetic control, compartmentalization, regulation of substrate concentration, degradation, alteration of environmental factors like pH, and processing of zymogens are all mechanisms that can be used to regulate enzyme activity (induction and repression). Allosteric regulation, genetic and covalent modification, and enzyme inhibition are all types of enzymatic regulation. Enzymes can be inhibited in three ways: competitive inhibition, non-competitive inhibition, or uncompetitive inhibition.
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