Proteins

By Regina Bailey

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Proteins

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Immunoglobulin G Antibody - Laguna Design/Science Photo Library/Getty Images

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. Photo Credit: Laguna Design/Science Photo Library/Getty Images

Updated February 24, 2016.

What Are Proteins?

Proteins are very important 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. While proteins have many diverse functions, all are typically constructed from one set of 20 amino acids. Examples of proteins include antibodies, enzymes, and some types of hormones (insulin).

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.

Polypeptide Chains

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

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