Protein folding is the process by which a protein folds into a three dimensional shape. Many proteins, particularly enzymes, must fold into the proper shape to be functional. In extreme cases, improperly folded proteins can cause diseases such as Creutzfeldt–Jakob disease. Folding is a complex process that involves interactions between different parts of the protein and with the surrounding environment. In some cases, the cell provides chaperone proteins, which assist proteins in folding into the proper conformation.
Protein folding is divided into four structures. Primary structure is the sequence of amino acids - at this point called a polypeptide. Secondary structure is the interactions between amino acids and hydrogen bonding between them. Tertiary structure is the shape the protein takes when fully folded, which includes interactions and bonding between amino acid residues. Quaternary structure takes into account interactions between the protein and other molecules, especially other proteins.
Several recurring shapes are often found in protein shapes. The alpha helix is a seconday structure where a segment of protein twists into a helix based on hydrogen bonding between the backbone of the amino acid chain. Beta sheets are flattened sections composed of several strands of relatively straight segments. Beta barrels are formed when beta sheets wrap around to form a barrel shape - these are often membrane proteins as the barrel shape provides a tunnel that is well suited to transporting molecules through the membrane.
Predicting protein folding
Predicting the way a protein will fold is a necessary step to creating synthetic proteins that behave as designed. Proper folding is often a consideration when producing recombinant proteins because each protein may fold differently depending on which organism it is expressed in. Currently there is no definitive means of predicting how a protein will fold, although there are many tools available that can estimate some folding properties.