3.3: Environmental Impacts on Enzyme Function

So far we've learned about how enzymes are shaped, how they function, and in the future, we'll look at how they're formed. But how do they break? How do we pause their function when they're unneeded? Topic 3.3 answers those questions.

Vocab List

Written Explanation

Denaturation:

Enzymes (which are proteins) are formed of primary, secondary, tertiary, and quaternary structures (see lesson 1.5). The bonds that form the primary structure are covalent, and thus quite strong. The secondary structure is formed of many hydrogen bonds, a little weaker, but due to their quantity, still quite strong. However, the tertiary and quaternary structures are fairly weak. Due to this fragility, those structures can be more easily broken/modified.

An enzyme which has had its tertiary or quaternary structure changed is said to be denatured. This can happen for several reasons, including the enzyme's environment having a higher or lower pH and temperature than the enzyme is built for. Each enzyme needs to be in a finely tuned environment to function properly, and otherwise, it undergoes denaturation. In some cases, enzymes will return to their original shape after being put in their proper environment, but tertiary and quaternary structures are often too complex to be simply reversed.

So if different enzymes need different environments to function, how are there multiple different enzymes in different parts of the body, as well as different parts of the cell? Well, the cell and body are compartmentalized into several parts, divided by what pH and temperature they need (see lesson 2.10).

At optimal pH levels and temperature, enzymes are able to function more efficiently and at a faster rate. Although they won't denature in temperatures immediately outside of their optimal range, sub-optimal conditions will still lower the rate of a reaction. Additionally, as temperature increases to reach the optimal range, substrates and enzymes will move around faster, resulting in a higher chance of pairing between the two.

Rate of reaction:

There are two other ways to increase the rate of a reaction: increasing the amount of enzymes, or increasing the amount of substrate. This also means there are two ways to reduce the rate of a reaction. Both methods may have a similar intent, but they do not accomplish the same thing.

Increasing the amount of enzyme clearly speeds up a reaction. Every available enzyme completes an operation, and together, more enzymes can complete more reactions per second. However, having more enzymes means that you run out of substrate more quickly.

Diagrams of product vs. time for different concentrations of enzyme

Increasing the amount of substrate means that the enzymes are functioning faster and for a longer time. When there are very few substrates, that obviously means that there will be less substrate, but it also means that it takes more time for each enzyme to "find" a substrate to pair up with. When there are more substrates, enzymes are constantly working, and don't have to wait long to "find" a substrate.

Diagrams of product vs. time for different concentrations of substrate

Helper molecules:

Cofactors are nonprotein helper molecules that bind to the active site of enzymes. They can serve several functions, namely modifying the shape of the active site to make it easier for substrates to bind. Coenzymes are a subset of cofactors that are also organic (like vitamins).

Diagram of an enzyme using a coenzyme to bind to a substrate

Inhibition:

So we've now heard of how molecules can aid enzymes, but many molecules can also suppress them.

Competitive inhibitors are molecules that compete directly with substrate molecules. The inhibitor binds directly to the active site, preventing substrates from binding in their spot. This process is often reversible, and for the most part just slows the efficiency of that specific cellular function. Competitive inhibitors are more effective the higher their concentration relative to the substrate (in much the same way that having a higher concentration of substrate speeds up reactions).

Diagram of a competitive inhibitor

Noncompetitive inhibitors are molecules that impede enzyme function by binding to the enzyme somewhere besides the active site. This binding triggers a change in the shape of the enzyme, thus making the enzyme now incompatible with its substrate. Once again, this inhibition is usually reversible. The site that a noncompetitive inhibitor binds to is called the allosteric site.

Diagram of a noncompetitive inhibitor

So far it seems like these inhibitors are all bad. They slow down the functions of enzymes, and could shut down entire cells and organs (which is why such inhibitors are often used as poisons). However, the body often intentionally inhibits itself. In feedback inhibition, the end product of an enzymatic process can act as a noncompetitive inhibitor to the start of the process. This means that once a process is completed and an adequate concentration of product has been reached, the process will stop itself! In other cases, molecules that bind to the allosteric site can actually boost the rate of a reaction.

Diagram of a feedback inhibition