A chemical reaction relies on a separate substance, a catalyst, to increase its speed without the substance itself being changed. An autocatalytic reaction is where one of the reaction’s own products fulfills the role of the catalyst. The process is similar to how a rumor spreads; as more people hear the rumor (the product), they rapidly tell others (the reactants), causing the rumor to spread faster.
The Self-Accelerating Process
An autocatalytic reaction starts slowly because there is little to no product present to act as a catalyst. As the reaction proceeds, it produces the catalytic product, initiating a positive feedback loop. The increasing concentration of the product accelerates the conversion of reactants at an ever-faster pace.
This period of rapid acceleration is a hallmark of autocatalysis. Eventually, the process slows down as the initial reactants are used up. With fewer reactants available, the rate of product formation declines and eventually stops, causing the reaction to plateau.
This dynamic—a slow beginning, rapid acceleration, and a final slowdown—is visually captured by a sigmoidal, or “S-shaped,” curve when plotting the concentration of the product over time. This curve graphically represents the positive feedback mechanism.
Examples of Autocatalysis
Autocatalytic processes are found in both biological systems and simple chemical reactions. In human biology, the blood clotting cascade is an example. When a blood vessel is injured, clotting factors at the site are activated. These activated factors, which are products of the reaction, then trigger the activation of more clotting factors, leading to the rapid formation of a blood clot.
Another biological example occurs during digestion. The stomach lining secretes a substance called pepsinogen, which is converted into the active digestive enzyme pepsin. This newly formed pepsin then acts as a catalyst, speeding up the conversion of more pepsinogen into pepsin, ensuring a robust supply for protein digestion.
A non-biological case is the phenomenon known as “tin pest.” Below 13.2 degrees Celsius, stable white tin can begin to transform into brittle, grey tin. This transformation is catalyzed by the presence of grey tin itself. If a piece of white tin is exposed to the grey form, the reaction will accelerate, causing the metal to crumble.
Significance in the Origin of Life
The concept of autocatalysis provides a framework for understanding how simple molecules could have organized into complex, self-sustaining systems. Scientists studying abiogenesis, the origin of life from non-living matter, use autocatalysis to explain the transition from simple chemistry to self-replicating entities.
One theory is the idea of “autocatalytic sets,” where a network of different molecules collectively catalyzes each other’s formation. In such a system, a group of molecules can create copies of each other from simple starting materials. This forms a self-replicating cycle that allows molecular complexity to increase over time.
This model is linked to the “RNA World” hypothesis, which suggests that early life may have been based on ribonucleic acid (RNA). This hypothesis proposes that RNA molecules, known as ribozymes, could have acted as both carriers of genetic information and as enzymes to catalyze their own replication. Such an RNA-based autocatalytic system offers a plausible mechanism for the emergence of the first protocells.