Proteins composed of amino acids that interact with each other to hold the protein in a three-dimensional shape. Enzymes are a class of proteins that change the speed of biochemical reactions. Every protein or enzyme has a native shape it needs to perform its role in a cell. When an enzyme is heated its shape, is disrupted and it becomes inactive.
Proteins are composed of 20 amino acids -- each with a unique chemical group called a side chain. The side chains interact with each other and with the backbone of a protein in several ways, forming hydrogen bonds, ionic bonds, disulfide bonds, and other weak chemical interactions. Each single hydrogen bond, ionic bond, or other interaction is by itself weak, but cumulatively they hold the protein in its proper three-dimensional structure so it can function. Enzymes are protein catalysts that have an active site where molecules can bind and undergo chemical reactions. The active site has a particular shape and structure that is important for allowing the enzyme to bind to its substrates and function properly.
When a protein is heated beyond its normal temperature range, heat will increase the kinetic energy of atoms in the protein, causing molecular motion that breaks the various bonds that hold the protein in shape. Once the shape is disrupted and the protein cools, various bonds will re-form, but not in the previous configuration. The protein is then said to be denatured, and since it is no longer in its natural shape, it no longer functions. Denatured proteins often stick to each other in ways that normal proteins do not. For example when the egg white protein albumin is denatured by heat, it coagulates and forms a solid. Interestingly, proteins are adapted to the temperature of their natural environment. Bacteria that live in extremely hot environments, for example near hot springs, have proteins that will not denature at temperatures up to 100 degrees Celsius. whereas human proteins, adapted to life at 37 degrees Celsius, denature at 100 degrees Celsius.
Other Kinds of Denaturation
In addition to heat, other kinds of disruption can denature proteins, particularly extremes in pH or salt concentration. For example acidic conditions break the ionic bonds normally found in proteins because they add hydrogen ions to acidic side chains, neutralizing their charge so they can no longer participate in an ionic bond. High salt concentration may break ionic bonds by causing ions in the protein to partner with ions in solution, although these effects are complex and depend on the type of salts used. In addition, alcohol can disrupt hydrogen bonds in proteins, and is also a denaturing agent.
Can Proteins Renature?
Some proteins can regain their shape and function after denaturation if the denaturing agent is removed, and the presence of protein re-folding enzymes called molecular chaperones helps. However proteins denatured by heat do not renature, and heat is a reliable way to "kill" the activity of an enzyme.
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