Tutor Lesson

Proteins are extraordinarily intricate molecular machines. In cells, they help maintain shape, support movement and metabolism, copy DNA, produce energy, transport substances such as cholesterol, and participate in programmed cell death.

In everyday life, protein may suggest food. A biochemist instead sees a folded structure with a particular cellular task. These structures operate at the nanometre scale—about one billionth of a metre—yet they adopt remarkably varied forms.

• A cholesterol transporter carries cholesterol through the blood.
• Actin supports cell shape, cell movement, and movement within cells.
• The flower-shaped apoptosome participates in programmed cell death.
• ATP synthase sits in a membrane and uses a rotor-like motion on a stalk to make ATP for cellular work.

Actin illustrates this link. Actin molecules fit into defined arrangements and extend into filaments whose dimensions help shape cells, enable movement, and support transport within them. ATP synthase offers another example: its structure permits the upper region to rotate on a rod and make ATP. An incorrect fold would prevent either machine from carrying out its role.

A new protein begins as a long, flexible chain, much like cooked spaghetti. Small molecules called amino acids are joined in a specific order to create this chain.

The identities and sequence of those amino acids determine what the protein becomes, how it folds, and how it functions. During folding, parts of the chain can form spirals called helices, pack into sheets, and combine into a three-dimensional structure.

Two influential ideas from the 1950s framed the protein-folding problem: Christian Anfinsen's experiment and Cyrus Levinthal's thought experiment.

The paradox rules out a search through every possible shape, like random typing that eventually produces a particular text. Proteins instead appear to follow specific routes toward their native structures.

Rather than forming the final structure in one collapse, a protein moves through a succession of partly folded intermediates. Each state leads toward another until the native fold is reached, much as separate portions of a paper crane are folded in sequence.

1. Misfolding occurs when a protein reaches a slightly incorrect shape. Because function depends on an exact fold, even a small structural error can stop the protein from working.
2. Aggregation occurs when unfolded regions remain exposed and proteins tangle into clumps. The process resembles corded earphones becoming knotted in a pocket.

• Loss of function: an aggregated or misfolded protein can no longer perform its task, such as catalysing an essential biochemical reaction.
• Direct toxicity: aggregates can capture other needed proteins and can form ordered structures that puncture cell membranes or otherwise disrupt cells.
• Vicious cycle: initial aggregation damages the cell and consumes resources needed to prevent further aggregation, so both aggregation and damage increase.

Protein aggregates are associated with Alzheimer's disease, Parkinson's disease, Huntington's disease, and many other pathologies. Cells must therefore fold tens of thousands of very different proteins efficiently enough to limit dysfunction and aggregation.

The flow of genetic information, an idea credited to Francis Crick, is expressed by the central dogma of molecular biology.

DNA stores genetic information. A gene is transcribed into messenger RNA, or mRNA, and a ribosome translates that message into protein. The ribosome is a large assembly of RNA and proteins with large and small parts positioned around the mRNA. As it advances along the message, it joins the specified amino acids into a growing chain.

• One ribosome produces approximately one protein every 80 seconds.
• A human cell contains roughly 5 to 10 million ribosomes.
• Together, they make about 4 to 8 million proteins per minute in one cell.

Electron-microscope images of a cell's edge reveal how densely cells are filled with ribosomes. Because millions of chains are made continuously, cellular folding must be both accurate and efficient.

A protein need not wait until synthesis is finished before folding. The growing chain travels through the exit tunnel in the large ribosomal part and begins to fold as it emerges into the cell.

Large proteins would fold slowly if their complete structures had to be solved at once. Instead, small regions form one after another as more of the chain is synthesized. Each manageable step contributes to the final fold.

The ribosome also assists folding directly. As a new chain exits, helices can form against its surface. Ribosomal proteins uL29 and uL23 help make up this surface; the “u” indicates that these proteins are universally present across the domains of life.

1. Directional synthesis: one domain is produced and can begin folding before the next domain exists, reducing the chance that the regions will tangle.
2. Controlled separation: even after both domains have appeared, the ribosome limits overly stable interactions between them until they can fold together correctly.

Proteins made from separate subunits face another ordering problem. Like furniture parts assembled in the wrong sequence, similar pieces may fit into incorrect arrangements. The ribosome supplies direction by allowing one subunit to contact another while the second is still being made. Away from the ribosome, alternative assemblies can form; during synthesis in cells, directed assembly favours the correct product.

Chaperones do not merely repair proteins after synthesis. They gather at the ribosome and act while a protein is still young, incomplete, and especially vulnerable.

A typical human cell has about 300 chaperones. Many have similar roles, creating backup layers, and many are essential. Researchers therefore study bacteria to uncover the basic logic more simply. The central folding problem is shared, and the ribosome-plus-chaperone solution is highly conserved from bacteria to humans.

• Ribosome—the good home: at the earliest stage, its protected environment may provide all the emerging chain needs.
• Trigger factor—the attentive parent: it supports slow-folding regions without controlling them too strongly.
• DnaK—the stricter parent: it catches small folding errors and blocks inappropriate interactions before they become larger problems.
• GroEL/GroES—the disciplinarian: it removes earlier mistakes and offers severely troubled proteins a fresh attempt inside a protected chamber.

The sequence of assistance is therefore ribosome, trigger factor, DnaK, and finally GroEL/GroES. Each level addresses difficulties that remain after the previous level.

1. Follow folding inside living cells: much current evidence comes from systems removed from cells, but cells are crowded with ribosomes, proteins, and other machinery. Researchers want to watch folding in that setting and understand its mechanism, not merely observe it.
2. Predict folding pathways: AlphaFold can predict a final structure from a sequence with high accuracy, but the result does not reveal how the chain reaches that structure. The longer-term goal is to predict the route even for previously unstudied sequences.
3. Build and engineer folding systems: studying components in cells, taking them apart, and reconstructing them can help reveal how the process works.