Protein folding

Proteins are built as long, linear chains of amino acids on ribosomes. Through a physical process called protein folding, these unstable, random coils transform into a specific, ordered three-dimensional shape. This final shape is what makes the protein biologically active and able to do its job.

Folding often begins while the protein is still being assembled. The amino acids in the chain interact with one another, and their exact sequence—the protein’s primary structure—determines the final, well-defined shape, known as the native state. While the correct three-dimensional structure is crucial for function, some parts of a working protein may remain unfolded, showing that protein dynamics matter. If a protein fails to fold into its native state, it usually becomes inactive. In some cases, misfolded proteins can take on a modified or even toxic function. Several neurodegenerative and other diseases are thought to stem from the buildup of amyloid fibrils created by misfolded proteins; the infectious forms of these are called prions. Many allergies arise when the immune system misidentifies incorrectly folded proteins because it hasn’t produced antibodies for those particular structures.

Denaturation is the process where a folded protein unfolds. This happens during cooking, burns, and in protein-related diseases. Any leftover structure in the unfolded state can act as a folding initiation site, guiding the protein back into shape. The time it takes to fold varies a lot. Outside a cell, the slowest proteins can take minutes or hours, mainly due to proline isomerization, and they pass through several intermediate states—like checkpoints—before finishing. In contrast, very small proteins made of up to about a hundred amino acids often fold in a single step, usually within milliseconds. The fastest known folding reactions finish in just a few microseconds. A protein’s folding speed depends on its size, contact order, and circuit topology. Since the late 1960s, understanding and simulating protein folding has been a major challenge for computational biology.

The primary structure—the linear sequence of amino acids—determines the native conformation. Which specific amino acids appear and where they sit in the chain decide which parts fold closely together to form the three-dimensional shape. The sequence matters more than the overall composition. Still, the key point is that the amino acid sequence holds all the information needed to specify both the final structure and the path to get there. That said, nearly identical sequences don’t always fold the same way; environmental factors can cause similar proteins to fold differently.

The first step in folding is the formation of secondary structures, like alpha helices and beta sheets. These fold quickly because they’re stabilized by intramolecular hydrogen bonds, first described by Linus Pauling. In alpha helices, the backbone hydrogen-bonds into a spiral shape. In beta pleated sheets, the backbone bends over itself to form hydrogen bonds between the amide hydrogen and carbonyl oxygen of the peptide bond. There are both antiparallel and parallel beta sheets; the antiparallel form has stronger hydrogen bonds because they form at an ideal 180-degree angle, unlike the slanted bonds in parallel sheets.

Alpha helices and beta sheets are often amphipathic—they have both hydrophilic and hydrophobic parts. This helps form the tertiary structure, where the protein folds so that hydrophilic sides face the watery environment and hydrophobic sides face the protein’s core. Secondary structure leads hierarchically to tertiary structure. Once the tertiary structure is stabilized by hydrophobic interactions, covalent disulfide bridges can form between two cysteine residues. These non-covalent and covalent contacts create a specific topological arrangement in the native structure. Tertiary structure involves a single polypeptide chain, but additional interactions between folded chains can lead to quaternary structure.

In some proteins, tertiary structure gives way to quaternary structure, which involves the assembly or coassembly of already-folded subunits. Multiple polypeptide chains can interact to form a fully functional quaternary protein. Folding is a spontaneous process driven mainly by hydrophobic interactions, intramolecular hydrogen bonds, and van der Waals forces, and it is opposed by conformational entropy. The folding time scale of an isolated protein depends on its size, contact order, and circuit topology. Inside cells, the folding process continues.