Proteins are large, complex molecules that fulfill many important functions in the body. They do most of the work in the cells and are needed for the structure, function and regulation of the tissues and organs of the body.
Proteins are made up of hundreds or thousands of smaller units called amino acids that are linked together in long chains. There are 20 different types of amino acids that can be combined to form a protein. The amino acid sequence determines the unique three-dimensional structure of each protein and its specific function.
Proteins can be described by their broad spectrum of functions in the body, arranged alphabetically:
Function Description Example
#1. Antibody Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body.
e.g Immunoglobulin G (IgG)
#2. Enzyme Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA. e.g Phenylalanine hydroxylase
#3. Messenger Messenger proteins, such as some types of hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs. e.g Growth hormone
#4. Structural component These proteins provide structure and support for cells. On a larger scale, they also allow the body to move. e.g Actin
#5. Transport/storage These proteins bind and carry atoms and small molecules within cells and throughout the body. e.g Ferritin
A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides containing less than 20-30 residues are rarely considered proteins and are commonly called peptides or sometimes oligopeptides. The individual amino acid residues are linked together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene encoded in the genetic code. In general, the genetic code indicates 20 standard amino acids; However, in some organisms the genetic code may contain selenocysteine and in some archaea pyrrolysin. Shortly after or even during synthesis, the residues in a protein are often chemically modified by post-translational modification which alters the physical and chemical properties, folding, stability, activity and ultimately the function of the proteins. Sometimes proteins have bound non-peptidic groups, which may be termed prosthetic groups or cofactors. Proteins can also work together to achieve a particular function and are often combined to form stable protein complexes.
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