Proteins and Their Components

Antibody Immunoglobulin G
Immunoglobulin G is a type of protein known as an antibody. This is the most abundant immunoglobulin and is found in all body fluids. Each Y-shaped molecule has two arms (top) that can bind to specific antigens, for instance bacterial or viral proteins.

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Proteins are very important biological molecules in cells. By weight, proteins are collectively the major component of the dry weight of cells. They can be used for a variety of functions from cellular support to cell signaling and cellular locomotion. Examples of proteins include antibodies, enzymes, and some types of hormones (insulin). While proteins have many diverse functions, all are typically constructed from one set of 20 amino acids. We obtain these amino acids from the plant and animal foods we eat. Foods high in protein include meats, beans, eggs, and nuts.

Amino Acids

Most amino acids have the following structural properties:

A carbon (the alpha carbon) 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.

The sequence of the amino acids in an amino acid chain determines a protein's 3-D structure. Amino acid sequences are specific to specific proteins and determines a protein's function and mode of action. A change in even one of the amino acids in an amino acid chain can alter protein function and result in disease.

Key Takeaways: Proteins

  • Proteins are organic polymers composed of amino acids. Examples of proteins antibodies, enzymes, hormones, and collagen.
  • Proteins have numerous functions including structural support, storage of molecules, chemical reaction facilitators, chemical messengers, transport of molecules, and muscle contraction.
  • Amino acids are linked by peptide bonds to form a polypeptide chain. These chains can twist to form 3-D protein shapes.
  • The two classes of proteins are globular and fibrous proteins. Globular proteins are compact and soluble, while fibrous proteins are elongated and insoluble.
  • The four levels of protein structure are primary, secondary, tertiary, and quaternary structure. A protein's structure determines its function.
  • Protein synthesis occurs by a process called translation where genetic codes on RNA templates are translated for the production of proteins.

Polypeptide Chains

Encephalin Polypeptide
This is a computer molecular graphic of part of a molecule of encephalin, a polypeptide found in the human brain.

 Alfred Pasieka/Science Photo Library/Getty Images

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

Polypeptide chains have some flexibility but are restricted in conformation. These chains have two terminal ends. One end is terminated by an amino group and the other by a carboxyl group.

The order of amino acids in a polypeptide chain is determined by DNA. DNA is transcribed into an RNA transcript (messenger RNA) that is translated to give the specific order of amino acids for the protein chain. This process is called protein synthesis.

Protein Structure

Hemoglobin Molecule
Hemoglobin is a protein that transports oxygen around the body in red blood cells.

 Laguna Design/Science Photo Library/Getty Images

There are two general classes of protein molecules: 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 of four types of protein structure. The four structure types are primary, secondary, tertiary, and quaternary structure.

A protein's structure determines its function. For instance, structural proteins such as collagen and keratin are fibrous and stringy. Globular proteins like hemoglobin, on the other hand, are folded and compact. Hemoglobin, found in red blood cells, is an iron-containing protein that binds oxygen molecules. Its compact structure is ideal for traveling through narrow blood vessels.

Protein Synthesis

Proteins are synthesized in the body through a process called translation. Translation occurs in the cytoplasm and involves the rendering of genetic codes that are assembled during DNA transcription into proteins. Cell structures called ribosomes help translate these genetic codes into polypeptide chains. The polypeptide chains undergo several modifications before becoming fully functioning proteins.

Organic Polymers

Biological polymers are vital to the existence of all living organisms. In addition to proteins, other organic molecules include:

  • Carbohydrates are biomolecules that include sugars and sugar derivatives. They not only provide energy but are also important for energy storage.
  • Nucleic Acids are biological polymers, including DNA and RNA, that are important for genetic inheritance.
  • Lipids are a diverse group of organic compounds inclusive of fats, oils, steroids, and waxes.