The Function and Structure of Proteins

protein hemoglobin
This is a molecular model of the protein hemoglobin. This molecule transports oxygen around the body in red blood cells. It consists of four globin proteins (amino acid chains; green, yellow, blue and pink). Laguna Design / Science Photo Library / Getty Images

Proteins are very important molecules in our cells. They are involved in virtually all cell functions. Each protein within the body has a specific function. Some proteins are involved in structural support, while others are involved in bodily movement, or in defense against germs. Proteins vary in structure as well as function. They are constructed from a set of 20 amino acids, which are linked to form polypeptide chains.

Polypeptide chains that form distinct three-dimensional shapes constitute a protein. Below is a list of several types of proteins and their functions.


  • Antibodies - are specialized proteins involved in defending the body from antigens (foreign invaders). They can travel through the bloodstream and are utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders. One way antibodies counteract antigens is by immobilizing them so that they can be destroyed by white blood cells.
  • Contractile Proteins - are responsible for movement. Examples include actin and myosin. These proteins are involved in muscle contraction and movement.
  • Enzymes - are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Examples include the enzymes lactase and pepsin. Lactase breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food.
  • Hormonal Proteins - are messenger proteins which help to coordinate certain bodily activities. Examples include insulin, oxytocin, and somatotropin. Insulin regulates glucose metabolism by controlling the blood-sugar concentration. Oxytocin stimulates contractions in females during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells.
  • Structural Proteins - are fibrous and stringy and provide support. Examples include keratin, collagen, and elastin. Keratins strengthen protective coverings such as skin, hair, quills, feathers, horns, and beaks. Collagens and elastin provide support for connective tissues such as tendons and ligaments.
  • Storage Proteins - store amino acids. Examples include ovalbumin, casein, ferritin. Ovalbumin is found in egg whites and casein is a milk-based protein. Ferritin stores iron in hemoglobin.
  • Transport Proteins - are carrier proteins which move molecules from one place to another around the body. Examples include hemoglobin and cytochromes. Hemoglobin transports oxygen through the blood via red blood cells. Cytochromes operate in the electron transport chain as electron carrier proteins.


There are four levels of protein structure. These levels are distinguished from one another by the degree of complexity in the polypeptide chain (linked amino acids). The four levels of protein structure are primary, secondary, tertiary, and quaternary structure. A single protein molecule may contain one or more of these protein structure types. The structure of a protein determines its function.

For example, collagen has a super-coiled helical shape. It is long, stringy, strong, and resembles a rope. This structure is great for providing support. Hemoglobin, on the other hand, is a globular protein that is folded and compact. Its spherical shape is useful for maneuvering through blood vessels.


Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves the translation of genetic codes into proteins. These gene codes are assembled during DNA transcription, where DNA is transcribed into an RNA transcript. Cell structures called ribosomes help translate the gene codes in RNA into polypeptide chains that undergo several modifications before becoming fully functioning proteins.