What Are Enzymes?
Enzymes are catalytic proteins that speed up chemical reactions. Without enzymes, these reactions would occur at a much slower rate or not at all. Like other proteins, enzymes consist of long chains of amino acids held together by peptide bonds.
Enzyme Substrate Complex
Unlike inorganic catalysts such as acids, bases and metals, enzymes are very specific. Each type of enzyme can break down or synthesize one particular compound, its substrate.
Lock and Key Model
Each enzyme has a unique 3-D shape, including a surface groove called an active site which fits its target substrate much like a lock fits a key. Other substances that don’t fit can't enter the active site and no reaction occurs.
Activation Energy
Enzymes work by weakening substrate bonds which lowers activation energy (amount of energy necessary for the reaction to occur). The enzyme then releases the product and is ready to begin the process again. One enzyme molecule can act over and over, transforming many substrate molecules.
In summary, enzymes are…
- proteins
- catalysts that accelerate cellular reactions
- not permanently changed in the reactions they mediate
- specific to what they will catalyze
- reusable
- usually named with an –ase suffix
Enzymes and Digestion
The food that you eat is exposed to enzymes from beginning to end. Amylase works in your mouth while you chew, breaking down starch (a big sugar) into smaller sugars.
In your stomach, food is exposed to acidic gastric juices which contain the enzyme pepsin. Even in under highly acidic conditions, pepsin functions to split proteins.
Pancreatic juice released into the duodenum neutralizes the acid from the stomach, and contains an enzyme that continues to break down sugars; another that breaks down proteins; and a third, that digests fat. Once the food moves on to the small intestine, it has been broken down into tiny nutrients that the body can absorb and use for energy, repair and growth.
Why Does the Flesh of Apples Turn Brown?
Catecholase is the enzyme present in most fruits and vegetables. It facilitates the browning of cut or bruised produce by catalyzing a reaction between the molecule catechol and the oxygen in the atmosphere. The product of this reaction is polyphenol, the brown substance that we see accumulating when, for example apples or potatoes are exposed to air.
Fresh Pineapple and Jell-O™
Have you even tried to make Jell-O with fresh pineapple in it? If so, you’ve been disappointed. Here’s why.
Jell-O is made of gelatin, a processed version of a structural protein called collagen that is found in many animals, including humans. The gelatin you eat in Jell-O comes from the collagen in cow or pig bones, hooves, and connective tissues.
Pineapple contains the enzyme bromelain, which can digest protein. If fresh pineapple is added while you make Jell-O, the bromelain breaks down the gelatin’s protein, and the Jell-O will not solidify upon cooling.Heat denatures, or changes the shape of proteins, and enzyme shape is crucial for the substrate to precisely fit into the active site.
If instead of fresh, canned pineapple is used (cooked as part of the canning process) the bormelain is denatured and cannot facilitate the breakdown of gelatin. Using canned pineapple allows the gelatin to firm and set.
More Information on Enzymatic Activity and Organic Molecules
More Chemistry Resources
For more information on organic chemistry and biochemistry and cell biology, see the websites:
- Science Prof Online
- McGraw Hill Enzyme animations
- Chem4Kids (Even though this site is designed for children, it is a great resource for chemistry neophytes of any age.)
About the Photo
Apple browning due to the reaction of catechol and oxygen. Lemon juice has been applied to the apple on the left, changing the pH and denaturing the enzyme, catecholase, which is responsible for the accelerating the reaction that causes apples (and many other fruits and vegetables) to brown.
Sources
Campbell, NA and Reece JB (2005) Biology. Seventh Edition. Pearson Education, Inc.
Thorpe, P. ed. (2007) Biology 120 General Biology I: Laboratory Experiments & Exercises. Grand Valley State University.
*This article first appeared in Suite101 online magazine.
