Enzymes are a form of catalyst—they lower the activation energy needed for a particular chemical reaction. The enzyme molecules are not themselves changed by the reaction, so they return to their starting state after they have acted on their target molecules (their substrates).
Enzymes are similar to cookie cutters in that they can be used over and over to effect change on something else, without changing themselves. As they affect change, they are affected by inhibitors and activators. Inhibitors make the enzyme slow down or stop altogether; they change the shape of the enzyme so that it either doesn’t accept a molecule, or so that it accepts it more slowly. Activators, such as hormones, speed up the activity of enzymes.
A particular enzyme will generally only help one particular chemical reaction. This is because each enzyme molecule has a particular shape that only fits with one or two other target molecules. It binds to the other molecules, a change occurs, and then it breaks off again. The cycle repeats when it encounters new target molecules. Some enzymes take two molecules and recombine them, some take large molecules and split them.
The molecules that an enzyme acts upon are known as substrates, and the area of the enzyme molecule that they fit into is known as the enzyme’s active site. For example, urease is an enzyme that helps convert urea—CO(NH₂)₂—and water (H2O) into carbon dioxide (CO2) and ammonia (NH3). A molecule of urease binds to molecules of urea and water, and a reaction occurs so that when they separate they form carbon dioxide, ammonia, and the original urease molecule, which can then bind to other urea and water molecules. Plants cannot take up urea directly, so urease is one of the most important enzymes in the steps needed to make the nitrogen in urea available. Two other enzymes are used to convert the ammonia into nitrites:
- Ammonia monooxygenase is an enzyme that converts ammonia (NH3) and oxygen (O) into hydroxylamine (NH2OH).
- Hydroxylamine oxidase is an enzyme used to convert hydroxylamine (NH2OH) into nitrite (NO2) and water (H2O).
Additionally, while some plants can feed directly from nitrites, some prefer nitrates. Nitrite oxidoreductase is an enzyme that converts the nitrite (NO2) into nitrate (NO3).
Each reaction has its own enzyme, which is why having the correct enzymes present for the desired reactions is critical. Enzymes are used not only for nitrogen availability, but for a wide variety of chemical reactions, both internal and external to plants (and all other forms of life). Internally, enzymes in plants are used for processes such at photosynthesis.
Externally, plant roots naturally express enzymes to assist with nutrient uptake. There are other garden uses for enzymes as well. For example, some enzymes will act as insecticides, dissolving the insect’s waxy protective coating known as its cuticle and exposing its exoskeleton. Others can help protect plants from molds and bacteria.
While most enzymes will only accept a particular set of molecules to act on, they can occasionally be fooled by similarly shaped molecules. For example, glyphosate is an herbicide that kills by binding to needed enzymes, interfering with their function. Since the glyphosate is bound to the enzyme molecules, they cannot bind normally to other molecules, inhibiting chemical reaction.
Other factors that can be used to inhibit enzyme function are temperature (enzymes cannot tolerate the high temperatures used in cooking, which begins to explain the raw food diet movement in humans) and pH. Fortunately, enzymes can tolerate a broad pH range of 3.0 to 9.0).
The developers of today’s plant supplements have cultivated specific enzymes that can boost plant growth and health depending on the growth cycle. For help selecting the right type for your set-up, as well as dosage instructions, ask the people behind the counter at your local hydroponics shop, or check out past issues of Maximum Yield.