What Is an Enzyme Structure and Function?

What are enzymes? and how do they function?

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

To understand the structure and function of enzymes, it's first important to know what enzymes are exactly. Simply put, enzymes are proteins that participate in cellular metabolic processes with the ability to enhance the rate of reaction between bio-molecules. Life would not exist without the presence of enzymes.

What are enzymes and what makes them special?

Enzymes most commonly catalyze chemical reactions that might not otherwise 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 would normally take by reducing the activation energy (Ea) to the extent that the reaction favors the reverse direction. 

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

The Composition of Enzymes

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 multi-protein complexes made up of a number of individual protein sub-units.

Many enzymes catalyze reactions without help, but some require an additional non-protein component called a co-factor.

Co-factors may be inorganic ions such as Fe2+, Mg2+, Mn2+, or Zn2+, or consist of organic or metallo-organic molecules known as co-enzymes.

How Enzymes Are Classified

Enzymes are classified according to the reactions they catalyze. The six classes are as follows: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases.

Hydrolases, oxidoreductases, and transferases are the most numerous forms of enzymes, while the remaining forms of enzymes are less common.

Oxidoreductases catalyze oxidation reactions in which electrons travel from one molecule to another. Transferases catalyze the transportation of a functional group from one molecule to another. Hydrolase enzymes catalyze hydrolysis or the process by which single bonds are broken down upon exposure to water.

Lyases are enzymes that 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. Isomerases catalyze structural shifts in a molecule, causing a change in shape. Ligases are in charge of the catalysis of ligation, also known as the combination of a pair of substrates, or reactants.

Examples of Enzymes

Alcohol dehydrogenase is an oxidoreductase enzyme that 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.

Aminotransferases are transferases that catalyze the amino acid degradation by removing amino groups.

Glucose-6-phosphatase is a hydrolase that removes the phosphate group from glucose-6-phosphate, leaving glucose and H3PO4.

Pyruvate decarboxylase is a lyase that removes CO2 from pyruvate.

Ribulose phosphate epimerase is an isomerase that catalyzes the interconversion of ribulose-5-phosphate and xylulose-5-phosphate. And, finally, hexokinaseis a ligase that catalyzes the interconversion of glucose and ATP with glucose-6-phosphate and ADP.