Proteins in the Cell

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 that are essential for all living organisms. By dry weight, proteins are the largest unit of cells. Proteins are involved in virtually all cell functions and a different type of protein is devoted to each role, with tasks ranging from general cellular support to cell signaling and locomotion. In total, there are seven types of proteins.

Proteins

  • Proteins are biomolecules composed of amino acids that participate in nearly all cellular activities.
  • Occurring in the cytoplasm, translation is the process through which proteins are synthesized.
  • The typical protein is constructed from a single set of amino acids. Every protein is specially equipped for its function.
  • Any protein in the human body can be created from permutations of only 20 amino acids.
  • There are seven types of proteins: antibodies, contractile proteins, enzymes, hormonal proteins, structural proteins, storage proteins, and transport proteins.

Protein Synthesis

Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves converting genetic codes into proteins. Genetic codes are assembled during DNA transcription, where DNA is decoded into RNA. Cell structures called ribosomes then help transcribe RNA into polypeptide chains that need to be modified to become functioning proteins.

Amino Acids and Polypeptide Chains

Amino acids are the building blocks of all proteins, no matter their function. Proteins are typically a chain of 20 amino acids. The human body can use combinations of these same 20 amino acids to make any protein it needs. Most amino acids follow a structural template in which an alpha carbon is bonded to the following forms:

  • A hydrogen atom (H)
  • A carboxyl group (-COOH)
  • An amino group (-NH2)
  • A "variable" group

Across the different types of amino acids, the "variable" group is most responsible for variation as all of them have hydrogen, carboxyl group, and amino group bonds.

Amino acids are joined through dehydration synthesis until they form peptide bonds. When a number of amino acids are linked together by these bonds, a polypeptide chain is formed. One or more polypeptide chains twisted into a 3-D shape forms a protein.

Protein Structure

The structure of a protein may be globular or fibrous depending on its particular role (every protein is specialized). 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 structures. 

There are four structural levels of protein: primary, secondary, tertiary, and quaternary. These levels determine the shape and function of a protein and are distinguished from one another by the degree of complexity in a polypeptide chain. The primary level is the most basic and rudimentary while the quaternary level describes sophisticated bonding.

A single protein molecule may contain one or more of these protein structure levels and the structure and intricacy of a protein determine its function. Collagen, for example, has a super-coiled helical shape that is long, stringy, strong, and rope-like—collagen 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.

Types of Proteins

There is a total of seven different protein types under which all proteins fall. These include antibodies, contractile proteins, enzymes, hormonal proteins, structural proteins, storage proteins, and transport proteins.

Antibodies

Antibodies are specialized proteins that defend the body against antigens or foreign invaders. Their ability to travel through the bloodstream enables them to be utilized by the immune system to identify and defend against bacteria, viruses, and other foreign intruders in blood. One way antibodies counteract antigens is by immobilizing them so that 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. Eukaryotes tend to possess copious amounts of actin, which controls muscle contraction as well as cellular movement and division processes. Myosin powers the tasks carried out by actin by supplying it with energy.

Enzymes

Enzymes are proteins that facilitate and speed up biochemical reactions, which is why they are often referred to as catalysts. Notable enzymes include lactase and pepsin, proteins that are familiar for their roles in digestive medical conditions and specialty diets. Lactose intolerance is caused by a lactase deficiency, an enzyme that breaks down the sugar lactose found in milk. Pepsin is a digestive enzyme that works in the stomach to break down proteins in food—a shortage of this enzyme leads to indigestion.

Other examples of digestive enzymes are those present in saliva: salivary amylase, salivary kallikrein, and lingual lipase all perform important biological functions. Salivary amylase is the primary enzyme found in saliva and it breaks down starch into sugar.

Hormonal Proteins

Hormonal proteins are messenger proteins that help coordinate certain bodily functions. Examples include insulin, oxytocin, and somatotropin.

Insulin regulates glucose metabolism by controlling blood-sugar concentrations in the body, oxytocin stimulates contractions during childbirth, and somatotropin is a growth hormone that incites protein production in muscle cells.

Structural Proteins

Structural proteins are fibrous and stringy, this formation making them ideal for supporting various other proteins such as keratin, collagen, and elastin.

Keratins strengthen protective coverings such as skin, hair, quills, feathers, horns, and beaks. Collagen and elastin provide support to connective tissues like tendons and ligaments.

Storage Proteins

Storage proteins reserve amino acids for the body until ready for use. Examples of storage proteins 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 that move molecules from one place to another in the body. Hemoglobin is one of these and is responsible for transporting oxygen through the blood via red blood cells. Cytochromes, another type of transport protein, operate in the electron transport chain as electron carrier proteins.