2.5 Additional Enzymatic Reactions

Additional Enzymatic Reactions

The ability of a microorganism to generate energy, while essential, is only part of the requirement to sustain life. Cells must also be able to maintain a functional equilibrium—that is, they must operate within the bounds of their limitations. There are numerous variants that, if not accurately and quickly controlled, could be detrimental to the cell. In order to help maintain this balance, cells produce proteins known as enzymes.

Common Enzymes

Enzymes accelerate or catalyze biochemical reactions. Since these enzymes, like all catalysts, are not consumed or destroyed during the chemical reaction, they can be readily detected in the laboratory and are useful in various diagnostic tests. In order to maintain homeostasis, the optimal level of activity for a stable environment, enzymes are very specific and will only effectively catalyze a specific chemical reaction. Enzymes are often named based on its substrate, the starting molecule to be acted on, and end with the suffix –ase. Examples of common enzymes are listed below:

Protease (proteinase) catalyzes the breakdown of proteins. This enzyme is essential anytime we eat. If we can’t break down the steak (protein) we just consumed into smaller subunits, its nutritional and energetic potential cannot be used.

Catalase accelerates the decomposition of hydrogen peroxide (H₂O₂) into water and oxygen. Without catalase, a cell would be unprotected from oxidative damage by reactive oxygen species (ROS) and could not survive.

Lipase catalyzes the breakdown of lipids (fats). Lipases provide an essential role in the metabolism of fats especially when glucose is not available.

Six Major Categories of Enzymes

As enzymes serve to maintain equilibrium, inhibiting even a single enzyme can drastically alter the outcome of microbial survival. There are six major categories of enzymes, each based on the type of reaction being catalyzed.

Hydrolases catalyze hydrolysis—the cleavage or breaking apart of chemical bonds by the addition of water. (i.e. A-B + H2O → A-OH + B-H)

Isomerases promote intramolecular rearrangements by altering bonds and/or confirmations within the same molecule producing a different molecule having the same molecular formula, also known as an isomer. (i.e. A-B→ B-A)

Ligases cause covalent bonds to be formed between molecules. (i.e. A + B → A-B)

Lyases cause the cleavage (break) of bonds by means other than hydrolysis or oxidation. (i.e. A-B → A + B)

Oxidoreductases catalyze the transfer of electrons from the reductant (electron donor) to the oxidant (electron acceptor). (i.e. A— + B →A + B—)

Transferases act to transfer a specific functional group from one molecule to another. (i.e. Ab + C → A + Cb)

Active Site

The specificity of an enzyme is heavily influenced by its active site — a unique chemical structure bound only by select target molecules. The molecule undergoing the reaction, the substrate, will bind to the active site found within the enzyme molecule. Again, only select substrate molecules will fit within the active site of an appropriate enzyme. The enzyme protein may also have two active sites to bring two different substrates within close proximity to one another. Once the substrate/s attaches to the active site of an enzyme, it is then converted accordingly. The product is then released from the enzyme, and the reaction is reset, meaning the enzyme is available to be used again as needed. Many of these enzymatic reactions are now readily detectable and have thus become very important in the clinical laboratory where they are used for the diagnosis of numerous medical conditions. Further, the biochemical process a particular microbe uses is beneficial in its classification and identification, as we will see in subsequent modules.