Most of the chemical reactions that take place within a cell involve protein catalysts called enzymes. Enzymes, like other catalysts, speed up the rates of chemical reactions by lowering the activation energy of that reaction (i.e., the amount of energy needed to start a chemical reaction). They do so by binding reactants (hereafter referred to as substrates) and holding them in a particular orientation that maximizes the chances that a particular chemical reaction will occur, converting the substrates into products. Like other catalysts, enzymes themselves are not permanently altered in the chemical reaction they catalyze—enzymes return to their original form at the end of the reaction. Also, like other catalysts, the enzyme does not provide the free energy necessary to drive otherwise energetically unfavorable reactions, but simply facilitates energetically favorable ones.
The ability of enzymes to function as catalysts depends on the three dimensional shape of the protein. Recall that all proteins have a particular shape which is due to various types of chemical interactions that occur among amino acid side chains (e.g., ionic interactions among charged side chains, hydrogen bonds, polar/nonpolar interactions, disulfide bonds, etc.) and between amino acid side-chains and the surrounding environment. In the case of enzymes, some of the amino acids of the polypeptide are arranged in such a way that they form a pocket-like structure called an active site.
The amino acids in the active site are arranged in such a way that they can a) form a number of non-covalent bonds with the substrate(s), thus temporarily binding thesubstrate(s) and b) help to destabilize certain chemical bonds within the substrate(s), increasing the chances that a particular chemical reaction will take place. There are many different types of enzymes, which perform a variety of different chemical interactions. However, the process of enzyme catalysis is similar among different enzymes.
Enzyme + Substrates ---> Enzyme-Substrate ---> Complex ---> Enzyme-Product ---> Complex Enzyme + Product(s)
An example of an enzyme catalyzed reaction. In this case, the substrate binds to the active site of the enzyme. The reaction takes place, cleaving the substrate into two products. The products are then subsequently released.
First, the substrate(s) binds to the active site to form an enzyme-substrate complex. Secondly, the reaction occurs converting the substrate(s) into product(s), forming an enzyme-product complex. Finally, the products are released from the active site, leaving the enzyme in its original, unaltered form. Because the active sites have a particular orientation of specific amino acid side chains (and their respective chemical properties) there is usually only one molecule or at most a few types of molecules that can bind to the active site for a long enough period of time for a chemical reaction to take place. Thus most enzymes show a very high degree of specificity—they bind specific substrates, catalyze specific reactions involving those substrates, and thus produce specific products.
Environmental effects on enzyme function
Because active sites are finely tuned to help a chemical reaction happen, they can be very sensitive to changes in the enzyme’s environment. Factors that may affect the active site and enzyme function include:
Temperature. A higher temperature generally makes for higher rates of reaction, enzyme-catalyzed or otherwise. However, either increasing or decreasing the temperature outside of a tolerable range can affect chemical bonds in the active site, making them less well-suited to bind substrates. Very high temperatures (for animal enzymes, above 40°C or 104°F) may cause an enzyme to denature, losing its shape and activity.
pH. pH can also affect enzyme function. Active site amino acid residues often have acidic or basic properties that are important for catalysis. Changes in pH can affect these residues and make it hard for substrates to bind. Enzymes work best within a certain pH range, and, as with temperature, extreme pH values (acidic or basic) can make enzymes denature.
