Science
The answer to a longstanding mystery suggests that proteins are far more malleable than previously thought.
Above a certain temperature, a cell will collapse and die. One of the most straightforward explanations for this lack of heat hardiness is that the proteins essential to life—the ones that extract energy from food or sunlight, fend off invaders, destroy waste products and so on—often have beautifully precise shapes. They start as long strands, then fold into helixes, hairpins and other configurations, as dictated by the sequence of their components. These shapes play a huge role in what they do. Yet when things start to heat up, the bonds that keep protein structures together break: first the weaker ones, and then, as the temperature mounts, the stronger ones. It makes sense that a pervasive loss of protein structure would be lethal, but until recently, the details of how, or if, this kills overheated cells were unknown.
Now, however, in a true tour de force, biophysicists at ETH Zurich in Switzerland have examined the behavior of every protein in cells from four different organisms as heat increases. This study and its rich deposit of data, published recently in Science, reveal that at the temperature at which a cell dies—whether it’s a human cell or one from Escherichia coli—only a handful of key proteins fall apart. Moreover, a protein’s abundance in a cell seems to show an intriguing relationship to the protein’s stability. The studies offer a glimpse into the fundamental rules that govern the order and disorder of proteins—rules that, researchers are realizing, have implications far beyond the question of why heat kills.
Paola Picotti, the biophysicist who led the study, explained that the experiments sprang from an old, thorny question: Why do some cells survive at high temperatures while others die? The bacterium Thermus thermophilus lives happily in hot springs and even in household hot water heaters, while E. coli withers above 40 degrees Celsius (104 degrees Fahrenheit). Strong evidence implies that differences in the stability of each organism’s proteins are involved. But looking at a protein’s behavior while it is still sitting in its living cell—the ideal way to understand it—is not easy. And isolating a protein in a test tube gives only partial answers, because within the organism, proteins nestle together, altering each other’s chemistry or holding each other in the right shape. To understand what is falling apart and why, you need to look at the proteins while they are still influencing each other.
To address this problem, the team devised a sprawling automated workflow in which they split open cells and heated up their contents in stages, unleashing protein-slicing enzymes on the mixtures at every stage. These enzymes are particularly good at slicing up proteins that have unfolded, so the researchers could tell by looking at the fragments which proteins fell apart at each temperature hike. In this way, they graphed an unfolding, or denaturing, curve for each of the thousands of proteins they studied, showing its arc as it moved from an intact structure at comfortable temperatures to a denatured state as the degrees ticked up. To see how these curves differed across species, they performed the process on cells from four species—humans, E. coli, T. thermophilus and yeast. “This is a beautiful study,” said Allan Drummond, a biologist at the University of Chicago, emphasizing both the scale and the delicacy of the process.
One of the clearest observations was that in each species, the proteins did not unfold en masse with a temperature boost. Instead, “we saw that only a small subset of proteins collapses very early,” Picotti said, “and these are key proteins.” In a network-style diagram of proteins’ interrelations, these fragile few are often highly connected, meaning that they influence numerous processes in the cell. “Without these the cell cannot function,” Picotti said. “When these are gone, the whole network most likely collapses.” And with it, evidently, the life of the cell.
This paradox—that some of the most important proteins seem to be the most delicate—may reflect how evolution has shaped them to do their jobs. If a protein has many roles to play, it might gain an advantage from being somewhat unstable and prone to unfolding and refolding, since this could allow it to assume various shapes appropriate to whatever its next target might be. “Many of these [key] proteins have high flexibility, which makes them more unstable,” but it may give them the versatility to bind to a variety of target molecules in the cell, Picotti explained. “That’s how they can perform their function, most likely. … It’s a trade-off.”
Looking more closely at E. coli, for which they had the cleanest data, the researchers also found a relationship between a protein’s abundance—how many copies of it are floating around the cell—and its stability. The more copies the cell made, they reported, the more heat it took to break a protein down. (Abundance, it should be noted, doesn’t necessarily correlate with being essential for life: some rare proteins are crucial.) This connection between abundance and sturdiness supports an idea that Drummond put forward a decade or so ago, concerning the cellular protein-making machinery’s tendency to make occasional errors. A mistake usually destabilizes a protein. If that protein happens to be a common one, produced by the hundreds or thousands in a cell daily, then misfolded copies made in large numbers could fatally clog the cell. It would behoove an organism to evolve versions of common proteins with extra stability built in, and the Picotti team’s data seem to reflect this.
To explore what qualities make a protein heat stable, the researchers compared the data from E. coli and T. thermophilus. E. coli proteins began to fall apart at 40 degrees Celsius and had mostly degraded by 70 degrees Celsius. But at that temperature, T. thermophilus proteins were just starting to get uncomfortable: Some of them continued to hold their shape up to at least 90 degrees Celsius. The team found that the T. thermophilus proteins tended to be shorter, and certain kinds of shapes and components cropped up more often in the most stable proteins.
These findings could help researchers design proteins with stabilities carefully tuned to their needs. In many industrial processes that involve bacteria, for instance, raising the temperature increases yield—but before too long the bacteria die from the trauma of heat. It will be interesting to see if we can stabilize a bacterium by making those few proteins that disintegrate early more resistant to temperature, Picotti said.
Beyond all these observations, however, the group’s wealth of information about how easily each protein unfolds has some biologists especially excited. A protein’s stability is a direct measure of how likely it is to form aggregates: clumps of unfolded proteins that stick to each other. Aggregates, often a nightmare for the cell, can interfere with essential tasks. For instance, they are implicated in some serious neurological conditions, such as Alzheimer’s disease, in which plaques of denatured proteins gum up the brain.
But that doesn’t mean aggregation occurs only in individuals suffering from these conditions. On the contrary, investigators are realizing that it may be happening all the time, without obvious stressors, and that a healthy cell has ways of dealing with it. “I think this is increasingly recognized as a very common phenomenon,” said Michele Vendruscolo, a biochemist at the University of Cambridge. “Most proteins actually misfold and aggregate in the cellular environment. The most fundamental information obtained by Picotti is about the fraction of time in which any given protein is in its unfolded state. This fraction determines the degree to which it will aggregate.” Some proteins almost never unfold and aggregate, others do it only in certain situations, and still others do it constantly. The new paper’s detailed information will make it much easier to study why these differences exist and what they mean, he said. Some of the denaturing curves even show patterns that suggest the proteins were aggregating after they unfolded. “They’ve been able to monitor both steps—both the unfolding and the subsequent aggregations,” Vendruscolo said. “That’s the excitement of this study.”
While many scientists are interested in aggregates because of the damage they cause, some are thinking about the phenomenon from another angle. Drummond said it has become clear that some aggregates are not just wads of trash floating around the cell; rather, they contain active proteins that continue to do their jobs.
Imagine that from a distance, you see smoke billowing out of a building, he said. All around it are forms that you take to be bodies, dragged from the wreckage. But if you get closer, you may find that they’re actually living people, who escaped from the burning building and are waiting for the emergency to pass. That’s what’s happening in the study of aggregates, Drummond said: Researchers are finding that instead of being casualties, proteins in aggregates may sometimes be survivors. “In fact, there is a whole field that is now exploding,” he said.
Rather than being just a sign of damage, the clumping may serve as a way for proteins to preserve their function when the going gets tough. It might help protect them from the surrounding environment, for instance. And when conditions improve, the proteins could leave the aggregates and refold themselves. “They have temperature-sensitive [shape] changes that, if you don’t look too closely, look like misfolding,” Drummond said. “But there’s something else going on.” In a 2015 Cell paper, he and collaborators identified 177 yeast proteins that seem to regain function after being cloistered in aggregates. In a paper that appeared this past March, his team found that altering one of these proteins so that it couldn’t aggregate actually caused serious problems for the cell.
All in all, this work suggests that proteins are curiously dynamic structures. At first they might look like rigid machines, at work on fixed tasks for which one specific shape suits them. But in fact, proteins may morph into several different forms in the course of their normal duty. And in times of need, their shapes may alter so radically that they look as though they are expiring, when they are really fortifying themselves. At the molecular level, life may consist of constantly coming together and falling apart.
This article appears courtesy of Quanta.
FAQs
Why does heat kill cells? ›
Direct heat exposure to cells causes protein degradation and DNA damage, which can lead to genetic alteration and cell death, but little is known about heat-induced effects on the surrounding tissue.
What happens to a cell when its too hot? ›Above a certain temperature, a cell will collapse and die. One of the most straightforward explanations for this lack of heat hardiness is that the proteins essential to life—the ones that extract energy from food or sunlight, fend off invaders, destroy waste products and so on—often have beautifully precise shapes.
Can high heat kill cells? ›It has been long recognized that hyperthermia in the 40–47°C temperature range kills cells in a reproducible time and temperature dependent manner.
WHY CAN T cells use heat? ›Answer and Explanation: The correct answer here is b) as the heat increases the cell's temperature, it must be released to prevent cell damage. The cell is a machine that consists in large part of water, and as such, most of its organelles rely on water in order to function.
How does heat kill cells quizlet? ›Moist heat kills microbes by denaturing enzymes (coagulation of proteins; caused by breakage of the hydrogen bonds that hold the proteins in their three-dimensional structure).
How does extreme heat kill bacterial cells? ›Heat kills bacteria by denaturing these essential proteins. As the temperature rises, the weakest bonds that keep protein structures together start to break, followed by the stronger bonds with rising temperatures.
How does temperature affect the cell cycle? ›They showed that temperature controls the rate and speed at which a cell divides. They further show that of all phases of the cell cycle, mitosis was very sensitive to temperature, and that cell cuture subjected to temperature between 24-31°C exhibited an accumulation of cells in mitosis.
What temperature can cells survive at? ›Most mammalian cells thrive at around 37 °C. Insect cells require lower temperatures of approximately 27 °C for optimal growth. Avian cell lines normally require 38.5 °C for maximum growth. 'Cold-blooded' animals (e.g., amphibians, cold-water fish) can be cultured anywhere between 15 °C and 26 °C.
Does temperature affect cell potential? ›The Nernst equation shows the relation between cell potential and temperature. According to the equation, cell potential increases as temperature decreases.
What temperature will kill cells? ›Temperatures between 46°C and 60°C are associated with irreversible cellular damage, proportional to the exposure time (8, 9). Between 60°C and 100°C, protein coagulation occurs instantly with irreversible damage of key cytosolic and mitochondrial enzymes and nucleic acid-histone complexes (9).
Does boiling kill cells? ›
Boiling water kills or inactivates viruses, bacteria, protozoa and other pathogens by using heat to damage structural components and disrupt essential life processes (e.g. denature proteins). Boiling is not sterilization and is more accurately characterized as pasteurization.
How does heat destroy DNA? ›Heat stress not only inhibits DNA repair systems, but can also act as a DNA damaging agent. It is known that heat stress can lead to the accumulation of 8-oxoguanine, deaminated cytosine, and apurinic DNA sites (AP-sites) in a cell [18-20].
Does heat break cell walls? ›The evidence of this study indicates that cell walls are not broken down by heat and moisture in the cooking process. Hence, it is presumable that the cellulose skeletal network is likewise unimpaired.
Do cells give off energy as heat? ›Our body temperature might not ever get much hotter than 37°C. But it turns out that the insides of our cells can reach a scorching 50°C. Our cells effectively burn food in oxygen to produce energy. Unlike a fire, this is a controlled process involving several steps, but it still generates a lot of heat.
Why does heat kill? ›Exposure to extreme heat can cause exhaustion and heatstroke, a severe condition that occurs when body temperature rises to 40°C or higher and if untreated can quickly damage the brain, heart, kidneys, and muscles,3 being fatal in 10-50% of all cases.
What process kills cells? ›Definition. Apoptosis is the process of programmed cell death. It is used during early development to eliminate unwanted cells; for example, those between the fingers of a developing hand. In adults, apoptosis is used to rid the body of cells that have been damaged beyond repair.
Why do we use heat to kill bacteria? ›After inactivation of bacteria, mainly by heat treatment, dead cells can release bacterial components with key immunomodulating effects and with antagonizing properties against pathogens.
Why is heat a highly effective sterilizing agent? ›Dry heat helps kill the organisms using the destructive oxidation method. This helps destroy large contaminating bio-molecules such as proteins. The essential cell constituents are destroyed and the organism dies. The temperature is maintained for almost an hour to kill the most difficult of the resistant spores.
How do bacteria survive in heat? ›Thermophilic bacteria can thrive in extreme heat because their proteins have an abundance of disulfides (yellow, above), covalent bonds between sulfur atoms that improve stability and likely boost heat-tolerance.
Does heat really kill bacteria? ›Cooking and reheating are the most effective ways to eliminate bacterial hazards in food. Most foodborne bacteria and viruses can be killed when food is cooked or reheated long enough at sufficient high temperature. The core temperature of food should reach at least 75℃.
How does temperature affect cell capacity? ›
Battery life reduces at higher temperatures
Battery capacity is reduced by 50% at -22 degrees F – but battery LIFE increases by about 60%. Battery life is reduced at higher temperatures – for every 15 degrees F over 77, battery life is cut in half.
Heat shock is performed at 37–42°C for 25–45 seconds as appropriate for the bacterial strain and DNA used. For smaller volumes of cells in smaller tubes, the heat-shock interval, which depends on the surface-to-volume ratio of the cell suspension, should be reduced.
Why do cells need temperature to survive? ›In particular, enzymes in a body's cells must have the correct temperature to be able to catalyse chemical reactions. Extremes of body temperature are dangerous: high temperatures can cause dehydration, heat stroke and death if untreated.
Can cells survive freezing? ›Cells can endure storage at low temperatures such as--196 degrees C for centuries. The challenge is to determine how they can survive both the cooling to such temperatures and the subsequent return to physiological conditions. A major factor is whether they freeze intracellularly.
How does temperature affect cell voltage? ›From the experiment performed using the Nernst equation, it was hypothesized that the voltage produced by the galvanic cell would decrease as the temperature increases. The voltage and the temperature is inversely proportional to each other.
How does heat affect potential energy? ›Explanation: When a substance is heated at a constant temperature (i.e. during its phase change state), the heat supplied makes the vibrating molecules gain potential energy to overcome the intermolecular force of attraction and move about freely. This means that the potential energy increases.
What factor causes a cell to modify due to temperature? ›As temperatures increases, the kinetic energy of the phospholipids also increases, which increases their movement. This increase in movement leads to an increase in permeability of the membrane.
What temperature is enough to kill? ›At an internal temperature of 95 degrees, humans can experience hypothermia, shivering and pale skin. At 86 degrees, they become unconscious and, at 77 degrees, cardiac arrest can occur. Most people cannot survive if their core temperature drops to 75 degrees.
What temperature is hot enough to kill most bacteria? ›Danger Zone! Bacteria multiply rapidly between 40 and 140 degrees. Bacteria will not multiply but may start to die between 140 and 165 degrees. Bacteria will die at temperatures above 212 degrees.
Why does denaturation kill cells? ›Denatured proteins initiate the formation of protein aggregates which subsequently disrupt normal cellular structure and function. Ultimately denaturation aggregation related damage prevents cell from undergoing a successful mitosis and they are rendered clonogenically dead.
Can cells be killed? ›
Cells can die because they are damaged, but most cells die by killing themselves. There are several distinct ways in which a cell can die. Some occur by an organised, 'programmed' process.
Do microwaves kill cells? ›Microwaves can be very effective in the reheating process if used correctly. Here's the deal, microwaves don't actually kill bacteria. The microwaves instead, create heat that is able to kill bacteria in foods. But microwaved foods can be cooked unevenly because of irregular shapes or differences in thickness.
Does heat kill skin cells? ›Numerous in vitro studies show that the rate of cell killing during exposure to heat is exponential and dependent on the temperature and length of exposure.
Does DNA get destroyed in heat? ›Blood and DNA are believed to be no longer traceable after exposure to a temperature of 1000 °C. This study exposed different objects of a standardized procedure to temperatures of 300, 700, and 1000 °C. It documented the influence of heat on blood traces through the use of luminol.
Why does DNA denature at high temperature? ›DNA secondary structure, the double helix, is held together by hydrogen bonds between base pairs. Specifically, adenine bases pair with thymine bases and guanine bases pair with cytosine bases. Heating a DNA sample disrupts these hydrogen bonds, thus “unwinding” the double helix and denaturing the DNA.
What temperature does DNA damage? ›At 38 °C (body core temperature), meiotic prophase I is damaged, showing increased DNA double-strand breaks (DSBs) and compromised DSB repair.
Why does heat damage cell membranes? ›Prolonged exposure to extreme temperature ranges can kill the cell. At temperatures slightly higher than the physiological temperature, the fatty acid tails of the phospholipids become less rigid and the phospholipids have enough kinetic energy to overcome the intermolecular forces that hold the membrane together.
Why is energy lost as heat? ›When energy is transformed from one form to another, or moved from one place to another, or from one system to another there is energy loss. This means that when energy is converted to a different form, some of the input energy is turned into a highly disordered form of energy, like heat.
Can cells survive without energy? ›As we have just seen, cells require a constant supply of energy to generate and maintain the biological order that keeps them alive. This energy is derived from the chemical bond energy in food molecules, which thereby serve as fuel for cells.
Which cells produce heat? ›Mitochondria generate most of the heat in endotherms. Given some impedance of heat transfer across protein-rich bioenergetic membranes, mitochondria must operate at a higher temperature than body temperature in mammals and birds.
Why can heat destroy a protein? ›
During cooking the applied heat causes proteins to vibrate. This destroys the weak bonds holding proteins in their complex shape (though this does not happen to the stronger peptide bonds). The unraveled protein strands then stick together, forming an aggregate (or network).
What happens when a cell is denatured? ›During the denaturation of proteins, the secondary and tertiary structures get destroyed and only the primary structure is retained. Covalent bonds are broken and interaction between amino-acid chains gets disrupted. This results in the loss of biological activity of the proteins.
Why does heating a protein cause it to denature? ›A protein becomes denatured when its normal shape gets deformed because some of the hydrogen bonds are broken. Weak hydrogen bonds break when too much heat is applied or when they are exposed to an acid (like citric acid from lemon juice).
Why does heat cause denaturation? ›This occurs because heat increases the kinetic energy and causes the molecules to vibrate so rapidly and violently that the bonds are disrupted. The proteins in eggs denature and coagulate during cooking. Other foods are cooked to denature the proteins to make it easier for enzymes to digest them.
Is protein killed by heat? ›When a protein is denatured by heat, most of the original tertiary structure is lost, so that many of the sites recognized by antibodies on the native molecule are destroyed. There are many examples of allergenicity being reduced, but not eliminated, by heating.
At what temperature does cell death occur? ›Cell death is induced differently by temperature. Necrosis occurs at temperatures of 50 °C or higher, while apoptosis occurs in the range of 43‒50 °C [20,21,22]. However, even if cancer cells are heated to induce selective apoptosis, it is impossible to evenly heat whole tumors due to thermal diffusion.
Are proteins easily destroyed by heat? ›Most chemistry courses teach that heat breaks down proteins, whether the reaction is the heat from a pan breaking down the protein in eggs or the heat from a straightening iron breaking down the protein in hair. Heat causes many changes to the structure and function of protein, especially in foods.
Can most bacteria be killed by heat? ›Bacteria multiply rapidly between 40 and 140 degrees. Bacteria will not multiply but may start to die between 140 and 165 degrees. Bacteria will die at temperatures above 212 degrees.
Does heat always kill bacteria? ›Proper heating and reheating will kill foodborne bacteria. However, some foodborne bacteria produce poisons or toxins that are not destroyed by high cooking temperatures if the food is left out at room temperature for an extended period of time.
What happens to DNA when it is heated and then cooled? ›DNA Denaturation through Heat
DNA can be denatured through heat in a process that is very similar to melting. Heat is applied until the DNA has unwound itself and separated into two single strands. Once the strands have been separated, the DNA will then be cooled back down to a stable temperature.