The Function and Structure of Proteins

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 and are essential for all living organisms. By weight, proteins are collectively the major component of the dry weight of cells and are involved in virtually all cell functions.

Each protein within the body has a specific function, from cellular support to cell signaling and cellular locomotion. In total, there are seven types of proteins, including antibodies, enzymes, and some types of hormones, such as insulin.

While proteins have many diverse functions, all are typically constructed from one set of 20 amino acids. The structure of a protein may be globular or fibrous, and the design helps each protein with their particular function.

In all, proteins are absolutely fascinating and a complex subject. Let's explore the basics of these essential molecules and discover what they do for us.

Antibodies

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 they can be destroyed by white blood cells.

Contractile Proteins

Contractile proteins are responsible for muscle contraction and movement. Examples of these proteins include actin and myosin.

Enzymes

Enzymes are proteins that facilitate biochemical reactions. They are often referred to as catalysts because they speed up chemical reactions. Enzymes include lactase and pepsin, which you might hear of often when learning about specialty diets or digestive medical conditions.

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

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 during childbirth. Somatotropin is a growth hormone that stimulates protein production in muscle cells.

Structural Proteins

Structural proteins are fibrous and stringy and because of this formation, they provide support for various body parts. 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

Storage proteins store amino acids for the body to use later. Examples include ovalbumin, which is found in egg whites, and casein, a milk-based protein. Ferritin is another protein that stores iron in the transport protein, hemoglobin.

Transport Proteins

Transport proteins are carrier proteins which move molecules from one place to another around the body.

Hemoglobin is one of these and is responsible for transporting oxygen through the blood via red blood cells. Cytochromes are another that operate in the electron transport chain as electron carrier proteins.

Amino Acids and Polypeptide Chains

Amino acids are the building blocks of all proteins, no matter their function. Most amino acids follow a particular structural property in which a carbon (the alpha carbon) is bonded to four different groups:

  • A hydrogen atom (H)
  • A Carboxyl group (-COOH)
  • An Amino group (-NH2)
  • A "variable" group

Of the 20 amino acids that typically make up proteins, the "variable" group determines the differences among the amino acids. All amino acids have the hydrogen atom, carboxyl group, and amino group bonds.

Amino acids are joined together through dehydration synthesis to form a peptide bond.

When a number of amino acids are linked together by peptide bonds, a polypeptide chain is formed. One or more polypeptide chains twisted into a 3-D shape forms a protein.

Protein Structure

We can divide the structure of protein molecules into two general classes: globular proteins and fibrous proteins. Globular proteins are generally compact, soluble, and spherical in shape. Fibrous proteins are typically elongated and insoluble. Globular and fibrous proteins may exhibit one or more types of protein structure. 

There are four levels of protein structure: primary, secondary, tertiary, and quaternary. These levels are distinguished from one another by the degree of complexity in the polypeptide chain.

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, which 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.

In some cases, a protein may contain a non-peptide group. These are called cofactors and some, such as coenzymes, are organic. Others are an inorganic group, such as a metal ion or iron-sulfur cluster.

Protein Synthesis

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.

The 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.