Enzymes as Inspiration for Industrial Catalysts
By Hilina Woldemichael
Every day, our bodies require countless chemical reactions in order to survive. Many of these reactions are aided by proteins known as enzymes. For many years, there has been great interest in harnessing the abilities of these enzymes in both industrial and research settings.
Enzymes catalyze, or speed up, nearly every chemical reaction that occurs in a living organism. Breathing, digesting food, growing new tissues—these are all complex processes that recruit several enzymes across many steps. Many of these reactions, if left to proceed with no catalysis, could not occur under normal biological temperature and pressure conditions on a timescale compatible with life [2]. In the presence of enzymes, however, such reactions can be carried out a million times faster, allowing for our bodies to function properly on a reasonable timescale. A reaction that could require years can proceed in milliseconds with the aid of an enzyme [2].
Each biochemical reaction maintains a precise ratio of starting material (substrate) and final material (product) at equilibrium. As a catalyst, enzymes do not alter this ratio but instead accelerate the rate of conversion between substrate and product. Just like inorganic catalysts, enzymes are not consumed after one use and are fully regenerated at the conclusion of the reaction.
There are several methods in which an enzyme catalyzes a specific reaction. Enzymes can orient substrates in the correct conformations to react together. They can bring substrates closer together and increase reaction rates in a proximal manner. But the key role that all enzymes fulfill is stabilizing what is known as the transition state of a reaction. This is a highly unstable intermediary state through which a substrate must first proceed to be converted to the product. The energy difference between this transition state and the substrate serves as a barrier to the reaction proceeding. The reduction of this energy barrier by enzymes through stabilization results in a significant increase in the rate of the reaction [2].
These amazing proteins are the result of billions of years of evolution, and over the past several years, scientists have been attempting to create enzyme-inspired synthetic catalysts. In a paper published in Nature Catalysis, a team consisting of researchers from Stanford University and SLAC National Accelerator Laboratory led by Matteo Cargnello successfully created a catalyst that acts very similarly to enzymes found in living organisms. They specifically wanted to mimic two key traits of enzymes: the ability to stabilize specific transition states and to transport substrates to and from the site of catalysis.
Because enzymes only function under certain reaction conditions, the authors aimed to create a synthetic enzyme that could act in more extreme conditions—such as in an industrial setting. However, there were significant challenges in achieving this. Although previous literature have demonstrated promising potential systems for this purpose, the synthesized enzymes lacked sufficient stability, as they would degrade over time. Cargnello and his team used oxidation of the dangerous compound carbon monoxide (CO) and oxygen (O2) to carbon dioxide (CO2) as the model reaction to test their catalyst. The structure that they developed consisted of a porous organic framework (POF) encasing a nanocrystal made of the metal palladium. The palladium core mimicked the trace metals, such as zinc, that are found buried in the active sites of many enzymes and participate in catalysis. The micropores of the organic polymer layer served as a confined environment for the reaction to occur.
The POFs were stable at temperatures of at least 275 degrees Celsius for over 40 hours, which suggested that they would be useful in high-temperature industrial settings. The composition of the POF layers were crucial to the catalysis of the reaction. The best results were obtained when layers of the POF structure aided in the diffusion of the CO2 product away from the palladium reaction center. The polymer layers also noticeably decreased the energy of the transition state by stabilizing a “product-like” intermediate of the reaction [3].
The findings of this paper are especially important because synthetic enzymes can be used as heterogeneous catalysts. Most natural enzymes are homogeneous catalysts, meaning that the catalyst occupies “the same phase as the reaction mixture.” In other words, if the substrates are in the liquid phase, then so too is the catalyst. The synthetic enzyme designed by Cargnello and his team can potentially serve as a heterogeneous catalyst, where a solid enzyme can accelerate the reaction between two gases. One benefit to heterogeneous catalysis over homogenous catalysis is that the catalyst can be easily removed from the reaction, enabling better recovery of often expensive compounds [1].
The ability to synthesize artificial enzymes capable of mimicking the specificity and catalytic ability of natural enzymes shows much promise in the design of industrial catalysts. The model catalyst developed in the Stanford study can be further broadened to other reactions. This not only will expand our understanding of the complexity of natural enzymes, but will also provide much more stable synthetic enzymes that can be utilized to produce important compounds.
References
Every day, our bodies require countless chemical reactions in order to survive. Many of these reactions are aided by proteins known as enzymes. For many years, there has been great interest in harnessing the abilities of these enzymes in both industrial and research settings.
Enzymes catalyze, or speed up, nearly every chemical reaction that occurs in a living organism. Breathing, digesting food, growing new tissues—these are all complex processes that recruit several enzymes across many steps. Many of these reactions, if left to proceed with no catalysis, could not occur under normal biological temperature and pressure conditions on a timescale compatible with life [2]. In the presence of enzymes, however, such reactions can be carried out a million times faster, allowing for our bodies to function properly on a reasonable timescale. A reaction that could require years can proceed in milliseconds with the aid of an enzyme [2].
Each biochemical reaction maintains a precise ratio of starting material (substrate) and final material (product) at equilibrium. As a catalyst, enzymes do not alter this ratio but instead accelerate the rate of conversion between substrate and product. Just like inorganic catalysts, enzymes are not consumed after one use and are fully regenerated at the conclusion of the reaction.
There are several methods in which an enzyme catalyzes a specific reaction. Enzymes can orient substrates in the correct conformations to react together. They can bring substrates closer together and increase reaction rates in a proximal manner. But the key role that all enzymes fulfill is stabilizing what is known as the transition state of a reaction. This is a highly unstable intermediary state through which a substrate must first proceed to be converted to the product. The energy difference between this transition state and the substrate serves as a barrier to the reaction proceeding. The reduction of this energy barrier by enzymes through stabilization results in a significant increase in the rate of the reaction [2].
These amazing proteins are the result of billions of years of evolution, and over the past several years, scientists have been attempting to create enzyme-inspired synthetic catalysts. In a paper published in Nature Catalysis, a team consisting of researchers from Stanford University and SLAC National Accelerator Laboratory led by Matteo Cargnello successfully created a catalyst that acts very similarly to enzymes found in living organisms. They specifically wanted to mimic two key traits of enzymes: the ability to stabilize specific transition states and to transport substrates to and from the site of catalysis.
Because enzymes only function under certain reaction conditions, the authors aimed to create a synthetic enzyme that could act in more extreme conditions—such as in an industrial setting. However, there were significant challenges in achieving this. Although previous literature have demonstrated promising potential systems for this purpose, the synthesized enzymes lacked sufficient stability, as they would degrade over time. Cargnello and his team used oxidation of the dangerous compound carbon monoxide (CO) and oxygen (O2) to carbon dioxide (CO2) as the model reaction to test their catalyst. The structure that they developed consisted of a porous organic framework (POF) encasing a nanocrystal made of the metal palladium. The palladium core mimicked the trace metals, such as zinc, that are found buried in the active sites of many enzymes and participate in catalysis. The micropores of the organic polymer layer served as a confined environment for the reaction to occur.
The POFs were stable at temperatures of at least 275 degrees Celsius for over 40 hours, which suggested that they would be useful in high-temperature industrial settings. The composition of the POF layers were crucial to the catalysis of the reaction. The best results were obtained when layers of the POF structure aided in the diffusion of the CO2 product away from the palladium reaction center. The polymer layers also noticeably decreased the energy of the transition state by stabilizing a “product-like” intermediate of the reaction [3].
The findings of this paper are especially important because synthetic enzymes can be used as heterogeneous catalysts. Most natural enzymes are homogeneous catalysts, meaning that the catalyst occupies “the same phase as the reaction mixture.” In other words, if the substrates are in the liquid phase, then so too is the catalyst. The synthetic enzyme designed by Cargnello and his team can potentially serve as a heterogeneous catalyst, where a solid enzyme can accelerate the reaction between two gases. One benefit to heterogeneous catalysis over homogenous catalysis is that the catalyst can be easily removed from the reaction, enabling better recovery of often expensive compounds [1].
The ability to synthesize artificial enzymes capable of mimicking the specificity and catalytic ability of natural enzymes shows much promise in the design of industrial catalysts. The model catalyst developed in the Stanford study can be further broadened to other reactions. This not only will expand our understanding of the complexity of natural enzymes, but will also provide much more stable synthetic enzymes that can be utilized to produce important compounds.
References
- Boundless. “Boundless Chemistry.” Lumen, 2020, courses.lumenlearning.com/boundless-chemistry/chapter/catalysis/.
- Cooper, Geoffrey M. (2000). “The Central Role of Enzymes as Biological Catalysts.” The Cell: A Molecular Approach. 2nd Edition. http://www.ncbi.nlm.nih.gov/books/NBK9921/.
- Riscoe, Andrew R., et al. “Transition State and Product Diffusion Control by Polymer–Nanocrystal Hybrid Catalysts.” Nature Catalysis, vol. 2, no. 10, 2019, pp. 852–863., doi: 10.1038/s41929-019-0322-7.
