Functional Groups and Substituted Hydrocarbons
Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to those molecules. Basically, they are found along the “carbon backbone” of macromolecules. This carbon backbone is formed by chains and/or rings of carbon atoms. Sometimes, it is formed with the occasional substitution of an element such as nitrogen or oxygen. In general, we call molecules with other elements in their carbon backbone, substituted hydrocarbons.
The functional groups in a macromolecule usually attach to the carbon backbone at one or several different places along its chain and/or ring structure. Each of the four types of macromolecules—proteins, lipids, carbohydrates, and nucleic acids—has its own characteristic set of functional groups. In fact, this set of functional groups contributes greatly to its differing chemical properties and its function in living organisms.
Different Kinds of Functional Groups
A functional group can participate in specific chemical reactions. Some of the important functional groups in biological molecules are shown in the image below. They include: hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl. As a matter of fact, these groups play an important role in the formation of molecules like DNA, proteins, carbohydrates, and lipids.
Functional groups are usually classified as hydrophobic or hydrophilic depending on their charge or polarity characteristics. An example of a hydrophobic group is the non-polar methane molecule.
Among the hydrophilic functional groups is the carboxyl group. This functional group is found in amino acids, some amino acid side chains, and the fatty acids that form triglycerides and phospholipids. This carboxyl group ionizes to release hydrogen ions (H+) from the COOH group which results in the negatively charged COO– group. In fact, this contributes to the hydrophilic nature of whatever molecule that contains it. Other functional groups, such as the carbonyl group, have a partially negatively charged oxygen atom. This oxygen atom may form hydrogen bonds with water molecules. Consequently, this makes the molecule more hydrophilic.
Functional Groups and Hydrogen Bonds
Hydrogen bonds between functional groups (within the same molecule or between different molecules) are important to the function of many macromolecules. Generally, they help many macromolecules fold properly into and maintain the appropriate shape for functioning.
Molecular recognition refers to the specific interaction between two or more molecules through noncovalent bonding such as hydrogen bonding, van der Waals forces. We can see the involvement of hydrogen bonds in various recognition processes. An example is DNA complementary base pairing and the binding of an enzyme to its substrate, as illustrated above.