Polypeptides are the molecules that make up proteins.
The molecules that make up proteins are called polypeptides . These are peptides composed of at least ten amino acids (a type of organic molecule).
A polypeptide, in other words, is a sequence of amino acids that are linked through peptide bonds . If the chained amino acids are more than a hundred, we can now speak of a protein.
Proteins, on the other hand, can be made up of one or more polypeptide chains. Those that have a single chain are classified as monomeric proteins , while those that have two or more chains are called multimeric proteins .
It should be noted that, just as the type of molecule that has more than one hundred amino acids falls within the group of proteins, the one that has less than ten is called an oligopeptide .
Examples of polypeptides
Insulin is an example of a polypeptide. This hormone, which is produced in the pancreas , is essential for the proper functioning of the metabolism. The disease known as diabetes mellitus implies that the person suffers from a deficiency of this polypeptide. On the other hand, if the subject produces an excessive amount of insulin, they will experience hyperglycemia .
Another polypeptide is gastrin , a hormone that participates in the actions that take place in the digestive system. Among the functions of gastrin is the stimulation of the movement of blood and muscles in the stomach.
During bone development, on the other hand, osteoblasts produce a polypeptide called osteocalcin . Abnormal levels of this hormone can be an indicator of cirrhosis, osteoporosis or osteomalacia, among other disorders and diseases .
Polypeptides are composed of ten or more amino acids.
The order of amino acids
In protein, the order of amino acids has been studied and follows certain rules. Conventionally, the N-terminus of the polypeptide chain (the NH3+ amino terminus) should be written to the left of the sequence; therefore, the C-terminal (or carboxyl group) should be written to its right. Any given sequence must be read from its N-terminus to its C-terminus.
In nature, the enzyme that forms peptide bonds (between a caroxyl group of one amino acid and an amino group of another, and the type of bond that joins proteins and peptides) is the ribosome ; In it, we find the explanation of the convention explained in the previous paragraph, since the growth of the chain is produced by adding an amino acid to the carboxyl terminus, so that the first end that emerges is the N-terminus.
The characteristics of a peptide or protein vary according to the quality and quantity of ionizable groups found in a molecule. Just like free amino acids, peptide and protein have isoelectric pH (pl) and titration curves; Furthermore, its pH does not vary in an electric field .
Nomenclature of polypeptides
To name a polypeptide, the suffixes of amino acids ending in -ico (such as aspartic acid), -ane (such as tryptophan), and -ine (such as glycine) must be changed to -yl ; The only exception to this rule occurs with the carboxyl terminus. To cite an example, to obtain the dipeptide valylalanine through a peptide bond, we start from valine and alanine; On the other hand, valylglycilleucine is a tripeptide that is formed with a valine in the N-terminal position, a glycine, and a leucine in the C-terminal position.
GIP or gastric inhibitory polypeptide belongs to the secretin family of hormones, a type of molecules called incretins, which have the task of preparing the body to carry out the storage of the food it receives. Its discovery took place in the 1920s and it was initially believed that it was responsible for protecting the small intestine against acid, although its function is currently considered to be to stimulate insulin secretion.