Note: This article is an extension of the Protein Folding Problem
“Knotting in proteins was once considered exceedingly rare. However, systematic analyses of solved protein structures over the last two decades have demonstrated the existence of many deeply knotted proteins.”
“Systematic reviews of the ever-growing Protein Data Bank (PDB) and the development of specialized servers for detecting protein knots have led to the identification of hundreds of knotted proteins, and it is now generally accepted that a small but significant fraction of proteins contains knots. However, exactly how and why such knots form are still unknown.”
So far, five distinct types of knots have been discovered. Given that the physical shape of a protein is crucial to its biological functions, it's likely that the knotting will have a profound impact on its properties.
See: Topological descriptions of protein folding, PNAS, April 2019