What Is an Enzyme Structure and Function?

enzymes action illustration
••• Muessig/CC/Wikimedia Commons

To understand the structure and function of enzymes, it's first important to know what they are. Simply put, enzymes are proteins that facilitate cellular metabolic processes by speeding up the rate of reaction between biomolecules. Life would not exist without the presence of enzymes.

Enzymes most commonly catalyze chemical reactions that otherwise might not occur by lowering the activation energy to a more "affordable" level for the cell. Some enzymes can even reverse a reaction (or reactants) from the direction it normally would take by reducing the activation energy (Ea) to the extent that the reaction favors the reverse direction. Part of what makes enzymes special is that they are able to facilitate these reactions without becoming altered the way fuel is burned up when used.

They can continually facilitate reaction after reaction.

Enzymes can be isolated using various protein purification methods. The purity of an enzyme preparation is measured by determining its specific activity.

How They Work

For a chemical reaction to occur, the molecules involved must collide with one another under appropriate conditions, and it is enzymes that help create appropriate conditions when introduced to these collisions. For example, under normal conditions, without the presence of an appropriate enzyme, the glucose molecules and phosphate molecules in glucose-6-phosphate will remain bonded to one another. Introduce the hydrolase enzyme, glucose-6-phosphatase, and the glucose and phosphate molecules will separate from one another.

Composition

Enzymes can have molecular weights ranging from about 10,000 to more than 1 million. Molecular weight is the sum of the atomic weights of a molecule's atoms. In chemistry, the term tends to be used interchangeably with molecular mass. Some enzymes have molecular weights that fall below the range listed above.

A small number of enzymes are not proteins but consist of small catalytic RNA molecules. Often, enzymes are multiprotein complexes made up of a number of individual protein subunits.

Many enzymes catalyze reactions without help, but some require an additional nonprotein component called a cofactor. Cofactors may be inorganic ions such as Fe2+, Mg2+, Mn2+, or Zn2+, or consist of organic or metallo-organic molecules known as co-enzymes.

Classification

Enzymes are classified into six categories according to the reactions they catalyze. The most common of the six are these three:

  • Oxidoreductases catalyze oxidation reactions in which electrons travel from one molecule to another. An example is alcohol dehydrogenase, which converts alcohols to aldehydes or ketones. This enzyme makes alcohol less toxic as it breaks it down. It also plays a key role in the fermentation process.
  • Transferases catalyze the transportation of a functional group from one molecule to another. Aminotransferases are a good example. They catalyze the amino acid degradation by removing amino groups.
  • Hydrolase enzymes catalyze hydrolysis or the process by which single bonds are broken down upon exposure to water. Glucose-6-phosphatase is an example of a hydrolase that removes the phosphate group from glucose-6-phosphate, leaving glucose and H3PO4 (phosphoric acid).

    Three less common enzymes are:

    • Lyases catalyze the breaking of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure. Pyruvate decarboxylase is an example of a lyase that removes CO2 (carbon dioxide) from pyruvate.
    • Isomerases catalyze structural shifts in molecules, causing a change in shape. An example is ribulose phosphate epimerase, which catalyzes the interconversion of ribulose-5-phosphate and xylulose-5-phosphate.
    • Ligases are in charge of the catalysis of ligation, also known as the combination of a pair of substrates, or reactants. A good example is hexokinases, a ligase that catalyzes the interconversion of glucose and ATP with glucose-6-phosphate and ADP.

      Examples in Everyday Life

      One of the most common examples of how enzymes impact elements of everyday life comes in the form of soaps and detergents. Enzymes in laundry detergents degrade stain-causing proteins, and lipases help dissolve fat stains. Thermotolerant and cryotolerant enzymes function in extreme temperatures—hot or cold—and are useful for industrial processes where high temperatures are required or for bioremediation under harsh conditions, such as those in the Arctic.

      In the food industry, enzymes are used to convert starch to sugar, saving money by allowing sweeteners to be developed from more than just sugarcane. In the clothing industry, enzymes help reduce impurities in cotton and reduce the need for potentially harmful chemicals when tanning leather.

      The plastics industry also is continually seeking ways to develop biodegradable products that can more easily be broken down.