Proteins

Sections

Structural levels

Roles of proteins

Enzymes and their roles

Activities

Additional Resources and Links

References

Structural levels

Proteins can be found in four different forms, or structures. The first is called the primary structure, which is described as long polypeptide chains. In each polypeptide, the arrangement of amino acids resembles a string of beads, and the sequence of amino acids forms the primary structure. The secondary structure occurs when the polypeptide twists, usually into the shape of a spiral. When the secondary structure folds back on itself, it forms the tertiary structure, which is usually spherical (Light, n.d.). The quaternary structure forms when several polypeptides in the tertiary structure combine (“Biochemistry,” n.d.).

An illustration of the peptide bonds that bind amino acids together in the primary structure. The red sphere are "r" groups, or variable groups. (Rusconi)

Roles of proteins

There are many different types of proteins. Structural proteins, for example, provide support in animal tissues. The proteins collagen and elastin compose the fibrous framework in connective tissues, while keratin can be found in hair, feathers, and other skin appendages. Transport proteins, such as hemoglobin, transport substances from one place to another. Receptor proteins control the response of cells to chemical stimuli in the body. Defensive proteins, like antibodies, protect the body against bacteria and viruses. One of the most important proteins is the enzyme, which catalyzes and accelerates certain chemical reactions (Campbell and Reece, 2002).

Enzymes and their roles

Enzymes are used as catalysts in specific biochemical reactions (“Biological Science,” 1996). In enzymes, the way that the polypeptides are folded causes different enzymes to catalyze different chemical reactions (Merrick, n.d.).

Enzymes are important because they are necessary for chemical reactions to take place in the body. Chemical reactions must occur within a narrow range of temperatures. Those temperatures are unable to supply the energy needed to start a reaction. Enzymes are used to lower the amount of activation energy needed (“Biological Science,” 1996).

The specific reaction that an enzyme controls depends on an area of its tertiary structure. This area is called the active site. The active site is where specific binding and activities with other molecules occur. Because of this, the active site can hold only certain molecules. An enzyme and the specific molecule it reacts with, called reactants, are said to fit together in a lock-and-key fashion. This is called the lock-and-key hypothesis.

The large blue molecule is an enzyme. The reactant binds at the active site, catalyzing it's reaction with another reactant (Rusconi).