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Enzymes are almost everywhere in your body. Discover where they are found, how they work and why they are important. Learn two models for how the enzyme-substrate complex is formed as well as the changes that could cause this bonding to fail.

What Are Enzymes?

Examples of enzymes are found pretty much everywhere in your body. Amylase is in your saliva and starts the digestion of food. If you can’t drink milk without getting sick, that means you’re lactose intolerant, which means you are lacking the enzyme lactase.Lactic acid is a byproduct of hard exercise and is responsible for that burning feeling in your muscles when you work out.

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Your body removes lactic acid with the assistance of lactate dehydrogenase. DNA polymerase helps your cells copy DNA so that the cells can multiply and you can grow.On a side note, did you notice that all the enzymes mentioned end with the suffix -ase? This is a good thing to keep in mind when studying biology.Enzymes are proteins that catalyze, or speed up, chemical reactions, allowing for greater cellular activity in a shorter period of time. Enzymes do this by lowering the activation energies of the chemical reactions. In other words, they lower the amount of energy required for the reaction to start. Almost all biochemical reactions require the aid of an enzyme.

Without enzymes, the chemical reactions in cells would be much too slow to maintain life.

How Do Enzymes Work?

The molecule (or molecules) with which the enzyme bonds is referred to as the substrate; the molecule (or molecules) that are created at the end of the reaction is referred to as the product. The substrate bonds to a small area of the enzyme termed the active site. When the enzyme is bonded to the substrate, we call this the enzyme-substrate complex. Once the reaction is complete, the enzyme releases the product and is ready to bond with another substrate.Enzymes are only able to function if they bond with the correct substrate. And enzymes are highly selective; they’ll activate with one and only one particular substrate.

So how do they know whether or not the molecule next to them is the substrate that they want? Two models attempt to explain this.

Lock and Key Model

In the lock and key model, the substrate and the enzyme’s active site are exact matches for each other, similar to puzzle pieces fitting together. Only a single substrate is the ‘key’ that matches the ‘lock’ of the active site.

Once the enzyme locates the molecule that fits it exactly, the chemical reaction begins. After the products are released, the enzyme searches for another molecule that is an exact match.

Induced Fit Model

The induced fit model is a modification of the lock and key model and is generally thought to be the more accurate version. In this representation, the active site is not an exact fit for the substrate.

Rather, the substrate induces a change in the enzyme, causing it to modify its shape until the binding is complete.This model explains why some molecules are able to bond with the enzyme but are unable to produce a reaction. Only the proper substrate is able to induce the correct shape to bring about the reaction.

The Effect of pH and Temperature

Enzymes work best at very specific temperatures and pH levels. If these factors are altered, the enzymes denature, or change shape. Denaturation makes it more difficult, if not impossible for the substrate to bind with the enzyme’s active site. Remember, if an enzyme can’t bond with its substrate, the biochemical reactions necessary for a cell’s life processes will cease and the cell will die.

In humans, the ideal temperature for enzymes is right around human body temperature. If your core body temperature gets too hot or too cold, your enzymes stop functioning correctly. This is one of the reasons why severe hypothermia, an extreme drop in body temperature, or a high fever, a critical increase in body temperature, can be so dangerous.The optimal pH for an enzyme depends on where that enzyme is located. For example, the enzymes in your acidic stomach work best at a much lower pH than the enzymes located inside the neutral pH of your mouth. An ulcer occurs when there is a breach in the lining of your stomach, and the acids inside leak out into the surrounding tissue.

Aside from being extremely painful, this can lead to the death of the adjacent cells as the resulting change in pH causes the cellular enzymes to cease functioning.

Lesson Summary

Enzymes are essential elements of biochemistry. Without them, cellular reactions within cells would not take place quick enough to sustain life. Enzymes catalyze reactions by lowering the activation energy required for the reaction to start.

The substrate binds to the enzyme at the active site. Two models have been proposed to explain this binding: the lock and key model and the induced fit model. Once the reaction is complete, the enzyme releases the product and is ready to bind with another substrate.

Enzymes are active at very narrow pH and temperature ranges. If either of these factors are changed too much, the enzyme loses its shape and is no longer able to bind with a substrate.

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