Some Like It (Not Too) Hot: Understanding Heat and Enzyme Denaturation

Enzymes are the superheroes of the molecular world. They can catalyze reactions at rates many times faster than uncatalyzed reactions. They can be reused over and over again without being changed. (This means that a single enzyme molecule can have a huge impact on a chemical process just like how a single superhero can make an outsized difference to a world in crisis). And just like superheroes, they have their kryptonite: heat. When enzymes get too hot, they lose their powers and can’t do their job anymore. This process is called denaturation. Read on to learn everything you need to know about enzyme denaturation and heat.

What are enzymes?

Enzymes are strings of amino acids bound together and folded to form powerful proteins that act as a catalyst to bring about chemical reactions. They do this by making it easier for reactant molecules to interact. All enzymes have a structure called an active site where reactants bind. These reactants, called substrates, then go through a series of chemical reactions which transformed them into new products. By bringing these molecules together enzymes lower the activation energy needed for these transformations. Most enzymes are super-specific and also sensitive. This means that certain changes can cause them to lose their shape and their powers.

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What is enzyme denaturation?

Enzyme denaturation is the process by which enzymes lose their three-dimensional structure and consequently their biological activity. This means that the enzyme can no longer catalyze the reaction it was designed to catalyze. Temperature is the primary cause of concern when it comes to denaturation although a change in pH, mechanical forces like stirring, the introduction of certain heavy metals, or the presence of certain alcohols and ketones can also cause denaturation.  

What happens to enzymes when I add a little heat?

Thomas Shafee, CC BY 4.0, via Wikimedia Commons

Enzymes aren’t afraid of a little heat. In fact, they love it! As temperature increases so does the rate of a reaction. Raising the temperature by 1 or 2 degrees can increase the activity of most enzymes by 10% to 20%. This is because increasing the temperature of a system increases the energy of the molecules within the system and leads to more collisions between substrates and enzymes. However, this increase is only up to a certain point. Pass the “optimal temperature” additional heat is counterproductive as it begins to break the enzymes helping to catalyze a reaction.

What happens to enzymes when I add a lot of heat?

At higher temperatures, enzymes start to unfold and lose their native shape. This is because the internal bonds (hydrogen bonds, covalent bonds, ionic bonds, etc.) holding an enzyme together begin to break. When an enzyme’s native shape is lost, it can no longer chemically interact with its substrate, and, therefore, is also unable to speed up a reaction. Because each enzyme has a different structure and different internal bonds the temperature for denaturing is specific for each enzyme. However, most enzymes found in mammals rapidly become denatured at temperatures above 40°C.

Can a denatured enzyme be re-natured? 

Good question! Yes and maybe and no. Some proteins will naturally reconfigure once temperatures drop. Others can return to their original conformation with the assistance of chaperone proteins (i.e. proteins that help damaged enzymes back into their correct shapes.) Other denatured proteins are more along the lines of Humpty Dumpty and can’t be repaired. This is particularly true if any of the protein’s covalent bonds were broken. Another high heat point of no return is coagulation. This is when changes in the structure of the protein matrix not only change the protein shape but also cause binding between proteins. These stuck-together proteins are almost impossible to pull apart without permanently damaging them. 

What are the impacts of denaturation in the real (lab) world? 

Temperature-driven enzyme denaturation is one reason why each experimental component has a specific storage temperate, why many lyophilized components need to be used shortly after rehydration, and why many experiments are carried out on ice. Understanding enzyme denaturation doesn’t get rid of these experimental requirements but it can make them feel less onerous. Understanding denaturation is also important for reaction optimization – a highly valued lab skill. Scientists overseeing industrial reactions spend significant R&D time carefully calibrating and then maintaining optimal temperatures. Hint: go slow as you heat up your reactions! Enzymes tend to see a modest exponential growth of activity as they approach optimal temperatures but once above they see sharp declines.)

Interested in enzymes? Learn all about enzyme catalysis, the nature of enzyme action, and protein structure-function relationships with our classroom experiment. Or read one of our other popular enzyme posts.

Thomas Shafee, CC BY 4.0, via Wikimedia Commons
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