An Artificial Metalloenzyme (ArM) is a designer metalloprotein, not found in nature, which can catalyze desired chemical reactions. [1] [2] Despite fitting into classical enzyme categories, ArMs also have potential in new-to-nature chemical reactivity like catalysing Suzuki coupling, [3] Metathesis [4] etc., which were never reported among natural enzymatic reactions.
ArMs have two main components: a protein scaffold and an artificial catalytic moiety, which, in this case, features a metal center. This class of designer biocatalysts is unique because of the potential to improve the catalytic performance through chemogenetic optimization, a parallel improvement of both the direct metal surrounding (first coordination sphere) and the protein scaffold (second coordination sphere).The second coordination sphere (protein scaffold) is easily evolvable and, in the case of ArMs, responsible for very high (stereo)selectivity. [5] With the progress in organometallic synthesis and protein engineering, more and more new kind of design of ArMs were developed, showing promising future in both academia and industrial aspects. [6]
In 2018, one-half of the Nobel Prize in Chemistry was awarded to Frances H. Arnold "for the directed evolution of enzymes", who elegantly evolved artificial metalloenzymes to realize efficient and highly selective new-to-nature chemical reactions in vitro and in vivo .
Dated back to 1956, the first protein modified transition metal catalyst was documented. [7] The Palladium(II) salt was absorbed onto silk fibroin fiber, reduced by hydrogen to get the first reported ArM, which can catalyze asymmetric hydrogenation. This work was not reproducible, but it is considered to be the first work in the field of artificial metalloenzymes. [5] At that time, the major challenge that blocked further studies was underdeveloped protein production and purification technology. The first attempt to anchor an abiotic metal center onto a protein was reported by Whitesides et al. using biotin-avidin interaction, making an artificial hydrogenase. [8] The presence of avidin can significantly increase the catalytic capacity of Rhodium(I) cofactor in aqueous phosphate buffer. Another pioneering work was conducted by Kaiser et al. where carboxypeptidase A (CPA) was repurposed into an oxidase by substituting Zn(II) center by Cu(II), for the oxidation of ascorbic acid. [9]
The real potential of ArMs was unleashed when recombinant protein production was developed, namely in 1997 Distefano and Davies reported a scaffold modification of a recombinant adipocyte lipid-binding protein (ALBP) with iodoacetamido-1,10-phenanthroline coordinating Cu(II) for the stereoselective hydrolysis of racemic esters. [10]
Four strategies have been used to assemble ArMs: [6]
These four strategies led to a great progress in the field of artificial metalloenzymes since the beginning of the 21st century, unlocking exceptional selectivity for new-to-nature reactions.
With the development of bioconjugation technology, there are plenty of strategies to covalently bind an artificial metallocofactor onto a protein scaffold:
Streptavidin or avidin in combination with biotinylated artificial metal cofactors is the most commonly used supramolecular strategy to make ArMs. [16] In the early example from Ward et al. shown below, the ligand of Ru(I) complex was covalently linked to biotin and than the whole complex was anchored to streptavidin thanks to a specific and strong biotin-streptavidin interaction. [17] The formed ArM can catalyze the reduction of prochiral ketones. Taking advantages of protein evolvability, different mutants of streptavidin can achieve different stereoselectivity. Throughout the years, many streptavidin-based enzymes were developed, enabling catalysis of very complex transformations in water, under ambient conditions.
Besides biotin-streptavidin based ArMs, another important example of using supramolecular iassembly strategy is antigen-antibody recognition. First reported in 1989 by Lerner et al.., a monoclonal antibody-based ArM is raised to hydrolyze specific peptide. [18]
Another interesting scaffold used as a platform for supramolecularly assembled ArMs are multidrug resistance regulators (MDRs), particularly a PadR family of proteins without native catalytic activity, whose function in nature is the recognition of foreign agents and to activate subsequent cellular response. [19] Among them, Lactococcal multidrug resistance regulator (LmrR) was mainly used to create ArMs, using different strategies, including the supramolecular one. Namely, Roelfes et al. incorporated Cu(II) phenanthroline complex in the hydrophobic pocket of LmrR and performed Friedel-Crafts reaction enantioselectively; [20] [21] and Fe heme complex which catalyzed cyclopropanation enantioselectively. [22]
This strategy involves substitution of a native metal center in a metallocofactor, by another metal, that might or might not be already present in living systems. [23] In this way, electronic and steric properties of the catalytic active site are altered compared to the wild-type enzyme, and novel catalytic pathways are unlocked.
The dative anchoring strategy uses natural amino acid residue in the protein scaffold like His, Cys, Glu, Asp and Ser to coordinate to a metal center. Like the first example of Pd-fibroin, dative anchoring to natural amino acids is not commonly used nowadays and often resulted in a more ambiguous binding site for metal compared with previous three methods.
However, these challenges can be overcome by in vivo incorporating metal-chelating non-canonical amino acids (ncAAs) [24] in the protein scaffold. These genetically encoded ncAAs' side chains have chelating moieties, such as 2,2'-bipyridine (3-(2,2'-bipyridin-5-yl)-L-alanine) [25] and 8-hydroxyquinoline (2-amino-3-(8-hydroxyquinolin-3-yl)propanoic acid) [26] that can selectively coordinate different metals. Combining protein scaffolds featuring chelating ncAAs with different metals yields exceptionally selective artificial metalloenzymes with various application potentials. [5] ncAAs are usually incorporated through the means of Amber stop codon suppression, via the orthogonal translation system (OTS). [24]
In addition to anchoring artificial metal center in the protein scaffold, researchers like Frances Arnold and Yang Yang focused on changing the native environment of natural metallocofactors. Due to the large sequence space that can be evolved in natural metalloenzymes, they can be evolved to catalyse non-native transformations. This process is known as enzyme repurposing. Directed evolution is commonly used to tailor the catalytic capacity and repurpose the enzyme function. Mostly based on native porphyrin-metallocofactor, Arnold's lab has developed many ArMs catalysing regioselective and/or enantioselective transformations, such as Carbon-Boron bond formation, [27] carbene insertion, [28] and aminohydroxylation [29] by evolving the sequence context of the corresponding ArMs.
As the pioneers of metalloredox radical biocatalysis, Yang et al. repurposed cytochrome P450s to catalyze atom transfer radical cyclization (ATRC), [30] and Huang et al. repurposed non-heme Fe-dependent enzymes to catalyze an abiological radical-relay azidation [31] and radical fluorination. [32] [33]
So far, ArMs can catalyze planty of chemical reactions, such as: allylic alkylation, allylic amination, aldol reaction, alcohol oxidation, C-H activation, [34] click reaction, [35] catechol oxidation, CO2 reduction, cyclopropanation, [36] Diels-Alder reaction, [37] epoxidation, epoxide ring opening, Friedel-Crafts alkylation, [38] hydrogenation, hydroformylation, Heck reaction, Metathesis, [4] Michael addition, nitrite reduction, NO reduction, Suzuki reaction, [3] Si-H insertion, [39] polymerization (atom transfer radical polymerization), [40] atom transfer radical cyclization (ATRC) [30] and radical fluorination. [32] [33]
Enantioselective synthesis, also called asymmetric synthesis, is a form of chemical synthesis. It is defined by IUPAC as "a chemical reaction in which one or more new elements of chirality are formed in a substrate molecule and which produces the stereoisomeric products in unequal amounts."
In organic chemistry, the Ugi reaction is a multi-component reaction involving a ketone or aldehyde, an amine, an isocyanide and a carboxylic acid to form a bis-amide. The reaction is named after Ivar Karl Ugi, who first reported this reaction in 1959.
Deoxyribozymes, also called DNA enzymes, DNAzymes, or catalytic DNA, are DNA oligonucleotides that are capable of performing a specific chemical reaction, often but not always catalytic. This is similar to the action of other biological enzymes, such as proteins or ribozymes . However, in contrast to the abundance of protein enzymes in biological systems and the discovery of biological ribozymes in the 1980s, there is only little evidence for naturally occurring deoxyribozymes. Deoxyribozymes should not be confused with DNA aptamers which are oligonucleotides that selectively bind a target ligand, but do not catalyze a subsequent chemical reaction.
The Henry reaction is a classic carbon–carbon bond formation reaction in organic chemistry. Discovered in 1895 by the Belgian chemist Louis Henry (1834–1913), it is the combination of a nitroalkane and an aldehyde or ketone in the presence of a base to form β-nitro alcohols. This type of reaction is also referred to as a nitroaldol reaction. It is nearly analogous to the aldol reaction that had been discovered 23 years prior that couples two carbonyl compounds to form β-hydroxy carbonyl compounds known as "aldols". The Henry reaction is a useful technique in the area of organic chemistry due to the synthetic utility of its corresponding products, as they can be easily converted to other useful synthetic intermediates. These conversions include subsequent dehydration to yield nitroalkenes, oxidation of the secondary alcohol to yield α-nitro ketones, or reduction of the nitro group to yield β-amino alcohols.
A catalytic triad is a set of three coordinated amino acids that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes. An acid-base-nucleophile triad is a common motif for generating a nucleophilic residue for covalent catalysis. The residues form a charge-relay network to polarise and activate the nucleophile, which attacks the substrate, forming a covalent intermediate which is then hydrolysed to release the product and regenerate free enzyme. The nucleophile is most commonly a serine or cysteine amino acid, but occasionally threonine or even selenocysteine. The 3D structure of the enzyme brings together the triad residues in a precise orientation, even though they may be far apart in the sequence.
Catechol oxidase is a copper oxidase that contains a type 3 di-copper cofactor and catalyzes the oxidation of ortho-diphenols into ortho-quinones coupled with the reduction of molecular oxygen to water. It is present in a variety of species of plants and fungi including Ipomoea batatas and Camellia sinensis. Metalloenzymes with type 3 copper centers are characterized by their ability to reversibly bind dioxygen at ambient conditions. In plants, catechol oxidase plays a key role in enzymatic browning by catalyzing the oxidation of catechol to o-quinone in the presence of oxygen, which can rapidly polymerize to form the melanin that grants damaged fruits their dark brown coloration.
Enzyme catalysis is the increase in the rate of a process by an "enzyme", a biological molecule. Most enzymes are proteins, and most such processes are chemical reactions. Within the enzyme, generally catalysis occurs at a localized site, called the active site.
In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic compounds. Because of their similarity in composition and description, they are often mistaken as a misnomer for enzymes due to their comparable effects on reaction rates and forms of catalysis involved.
The Hajos–Parrish–Eder–Sauer–Wiechert and Barbas-List reactions in organic chemistry are a family of proline-catalysed asymmetric aldol reactions.
Bioconjugation is a chemical strategy to form a stable covalent link between two molecules, at least one of which is a biomolecule. Methods to conjugate biomolecules are applied in various field, including medicine, diagnostics, biocatalysis and materials. Synthetically modified biomolecules can have diverse functionalities, such as tracking cellular events, revealing enzyme function, determining protein biodistribution, imaging specific biomarkers, and delivering drugs to targeted cells.
In organic chemistry, Lewis acid catalysis is the use of metal-based Lewis acids as catalysts for organic reactions. The acids act as an electron pair acceptor to increase the reactivity of a substrate. Common Lewis acid catalysts are based on main group metals such as aluminum, boron, silicon, and tin, as well as many early and late d-block metals. The metal atom forms an adduct with a lone-pair bearing electronegative atom in the substrate, such as oxygen, nitrogen, sulfur, and halogens. The complexation has partial charge-transfer character and makes the lone-pair donor effectively more electronegative, activating the substrate toward nucleophilic attack, heterolytic bond cleavage, or cycloaddition with 1,3-dienes and 1,3-dipoles.
Hydrogen-bond catalysis is a type of organocatalysis that relies on use of hydrogen bonding interactions to accelerate and control organic reactions. In biological systems, hydrogen bonding plays a key role in many enzymatic reactions, both in orienting the substrate molecules and lowering barriers to reaction. The field is relatively undeveloped compared to research in Lewis acid catalysis.
Synergistic catalysis is a specialized approach to catalysis whereby at least two different catalysts act on two different substrates simultaneously to allow reaction between the two activated materials. While a catalyst works to lower the energy of reaction overall, a reaction using synergistic catalysts work together to increase the energy level of HOMO of one of the molecules and lower the LUMO of another. While this concept has come to be important in developing synthetic pathways, this strategy is commonly found in biological systems as well.
Proline organocatalysis is the use of proline as an organocatalyst in organic chemistry. This theme is often considered the starting point for the area of organocatalysis, even though early discoveries went unappreciated. Modifications, such as MacMillan’s catalyst and Jorgensen's catalysts, proceed with excellent stereocontrol.
Supramolecular catalysis refers to an application of supramolecular chemistry, especially molecular recognition and guest binding, toward catalysis. This field was originally inspired by enzymatic system which, unlike classical organic chemistry reactions, utilizes non-covalent interactions such as hydrogen bonding, cation-pi interaction, and hydrophobic forces to dramatically accelerate rate of reaction and/or allow highly selective reactions to occur. Because enzymes are structurally complex and difficult to modify, supramolecular catalysts offer a simpler model for studying factors involved in catalytic efficiency of the enzyme. Another goal that motivates this field is the development of efficient and practical catalysts that may or may not have an enzyme equivalent in nature.
This article covers protein engineering of cytochrome (CYP) P450 enzymes. P450s are involved in a range of biochemical catabolic and anabolic process. Natural P450s can perform several different types of chemical reactions including hydroxylations, N,O,S-dealkylations, epoxidations, sulfoxidations, aryl-aryl couplings, ring contractions and expansions, oxidative cyclizations, alcohol/aldehyde oxidations, desaturations, nitrogen oxidations, decarboxylations, nitrations, as well as oxidative and reductive dehalogenations. Engineering efforts often strive for 1) improved stability 2) improved activity 3) improved substrate scope 4) enabled ability to catalyze unnatural reactions. P450 engineering is an emerging field in the areas of chemical biology and synthetic organic chemistry (chemoenzymatic).
Jieping Zhu is an organic chemist specializing in natural product total synthesis and organometallics. He is a professor of chemistry at EPFL and the head of the Laboratory of Synthesis and Natural Products.
The ketimine Mannich reaction is an asymmetric synthetic technique using differences in starting material to push a Mannich reaction to create an enantiomeric product with steric and electronic effects, through the creation of a ketimine group. Typically, this is done with a reaction with proline or another nitrogen-containing heterocycle, which control chirality with that of the catalyst. This has been theorized to be caused by the restriction of undesired (E)-isomer by preventing the ketone from accessing non-reactive tautomers. Generally, a Mannich reaction is the combination of an amine, a ketone with a β-acidic proton and aldehyde to create a condensed product in a β-addition to the ketone. This occurs through an attack on the ketone with a suitable catalytic-amine unto its electron-starved carbon, from which an imine is created. This then undergoes electrophilic addition with a compound containing an acidic proton. It is theoretically possible for either of the carbonyl-containing molecules to create diastereomers, but with the addition of catalysts which restrict addition as of the enamine creation, it is possible to extract a single product with limited purification steps and in some cases as reported by List et al.; practical one-pot syntheses are possible. The process of selecting a carbonyl-group gives the reaction a direct versus indirect distinction, wherein the latter case represents pre-formed products restricting the reaction's pathway and the other does not. Ketimines selects a reaction group, and circumvent a requirement for indirect pathways.
René Peters is a German chemist and since 2008 Professor of Organic Chemistry at the University of Stuttgart.
Metallopeptides are peptides that contain one or more metal ions in their structure. This specific type of peptide are, just like metalloproteins, metallofoldamers. And very similar to metalloproteins, metallopeptide's functionality is attibuted through the contained metal ion cofactor. These short structured peptides are often employed to develop mimics of metalloproteins and systems similar to artificial metalloenzymes.
{{cite journal}}
: Cite journal requires |journal=
(help)