Proteins are important to a vast array of functions in living things, from catalyzing reactions to providing structure and conveying messages within the body. They are absolutely vital to life and are produced by transcription and translation of DNA and RNA.Proteins are made up of long, polymerized chains of amino acids. An amino acid is a protein-building molecule that contains an amino group (), a carboxyl group (), and between them a carbon atom attached to a side chain. The side chain, known as "the R group," gives the amino acid its identity and contributes to its structure and function of the protein.
Amino Acid Structure
Amino acids are joined together by peptide bonds that result from a dehydration synthesis reaction. A peptide bond is a covalent amide () bond that forms between two adjacent amino acids along the same peptide chain. The bond forms between the amine () group, or amino group, of one amino acid and the carboxyl () group of another. In forming the bond, the group loses a hydroxide ion (OH–) while the amine group loses a hydrogen ion (H+), producing a molecule of water. The nitrogen atom of the amine group then bonds to the carbon atom of the group, creating a peptide bond. The process is repeated for every amino acid added to the chain.
Structure and Function of Amino Acids
For all proteins, the shape of the molecule determines how it interacts with other molecules and therefore determines its function. There are four structural levels to a protein. A protein's primary structure is the sequence of amino acids linked by peptide (amide) bonds to form a polypeptide chain. Proteins tend to be large molecules, and each protein has its own unique primary structure. Many proteins are between 200 and 400 amino acids long but can have up to several thousand.The secondary structure of a protein is the initial folding of the amino acid polymer that arises from hydrogen bonding between the amine and carboxyl groups of amino acids in neighboring parts of the chain. This secondary structure appears in two variations. One type of secondary structure is the alpha helix, a helical structure of proteins that adopts a right-handed-spiral conformation, where chain backbone groups form hydrogen bonds with groups of nearby amino acids along the protein chain. Hydrogen bonds form between the amine group of one amino acid and the carboxyl group of the amino acid four spaces past it on the same chain. The second variation of a protein's secondary structure is a beta pleated sheet, a structure of amino acids that takes the shape of a pleated sheet folded at regular intervals. The shape is held in place by hydrogen bonds that form between adjacent amino acids on different chains. To picture secondary structures, it's important to remember that proteins (and all molecules) exist as three-dimensional structures in space, even though they are commonly represented as two-dimensional structures on paper. A protein may have a secondary shape that is entirely composed of one of these structures or a mixture of both.
Levels of Protein Structure
Quaternary Structure of Hemoglobin
Many important chemical reactions in the human body, such as extracting energy from food or copying DNA, would happen extremely slowly if it weren't for enzymes. Enzymes are proteins with precise shapes that allow them to catalyze biological reactions. Each reaction requires its own specific enzyme, and each enzyme has its own unique shape.
Each enzyme contains an area called the active site, which is the place on an enzyme where the substrate binds and the reaction occurs. A substrate is a molecule that is acted upon by an enzyme by binding to the enzyme's active site. The binding of the enzyme to the substrate can change the substrate in a number of ways, allowing it to carry out a function. The binding of the enzyme may pull the substrate apart, bring molecules close enough together to react, or affect the configuration of electrons on the substrate, altering its ability to bond with particular nearby molecules.
The function of an enzyme can be affected by factors that influence the hydrogen bonding or disulfide bridges, which are bonds between two sulfur atoms forming the general structure , that stabilize the shape of the enzyme. These bonds are very sensitive to changes in temperature and pH. Enzymes in conditions other than their optimal temperature or pH may become denatured (structurally altered), losing their shape and therefore becoming unable to carry out their functions.