Enzymatic and green synthesis of short oligopeptides is possible through five enzyme families: non-ribosomal peptide synthetases, ATP-grasp enzymes, α-amino acid ester acyltransferases, β-lactam acylases, and cyanophycinases. This review maps their activation chemistry, applications, and…
Five enzyme families form peptide bonds outside the ribosome, and each offers a different trade-off between substrate scope, specificity, and how well it is understood. Non-ribosomal peptide synthetases NRPSs assemble complex products on modular scaffolds. ATP-grasp enzymes activate carboxylates as acylphosphates. α-Amino acid ester acyltransferases AETs transfer acyl groups from amino acid esters to amines. β-Lactam acylases hydrolyze and rebuild amide bonds around penicillin-type cores. Cyanophycinases degrade a cyanobacterial storage polymer into a single defined dipeptide. Together they are the available biocatalytic routes to short oligopeptides, and they divide cleanly into ATP-dependent and ATP-independent activation chemistry. In the ATP-dependent branch, substrates are activated as aminoacyl-adenosine monophosphate, as in tRNA-dependent ligases, or as acylphosphates, as in ATP-grasp enzymes. In the ATP-independent branch, transacylases and related enzymes work through aminoacyl phosphate, reactive esters, or direct polymer hydrolysis.
The green case for these enzymes is straightforward. They operate in water at moderate temperatures, they are regioselective and stereospecific, and they remove the protection and deprotection cycles that dominate chemical peptide coupling. The ATP-dependent routes pay a cofactor cost: activating an amino acid consumes ATP, and preparative work usually requires a regeneration system. ATP-independent routes avoid that cost. The table below maps the five families.
| Enzyme family | Activation chemistry | Representative products or uses | Structures cited in the 2019 review |
|---|---|---|---|
| NRPS | ATP-dependent, aminoacyl-AMP | Penicillin, bleomycin, cyclosporine | PDB 2VSQ A, C, PCP, Te domains |
| ATP-grasp | ATP-dependent, acylphosphate | Purine biosynthesis, dipeptide ligation | PDB 2IP4 three conserved domains, Mg2+ |
| α-Amino acid ester acyltransferases | ATP-independent, acyl transfer from esters | Oligopeptides from unprotected amino acids | None reported |
| β-Lactam acylases | ATP-independent, amide acyl transfer | 6-APA, semi-synthetic β-lactam antibiotics | None cited |
| Cyanophycinases | ATP-independent, polymer hydrolysis | β-Asp-Arg dipeptide from cyanophycin | None cited |
A 2019 review by Wang and colleagues in Biomolecules 9 11 : 733 surveys these families, documents the activation chemistry, and records the field's structural gaps: two of the families still have no reported three-dimensional structure or mechanism. Some widely circulated summaries of the topic circulate bundled with commercial advertising, so individual claims should be checked against the primary literature.
NRPSs occur in bacteria and fungi and produce some of the most therapeutically important peptides in clinical use. The enzymes are organized into repeating domains: an adenylation A domain selects and activates an amino acid as its aminoacyl-adenosine monophosphate; a peptidyl carrier protein PCP carries the activated intermediate and the growing chain on a phosphopantetheine arm; a condensation C domain catalyzes amide bond formation; and a thioesterase Te domain releases the finished chain, often by cyclization. The structure of a representative NRPS, PDB ID 2VSQ, shows the A, C, Te, and PCP domains arranged along the assembly line.
The products named in the review include penicillin, bleomycin, and cyclosporine. Cyclosporine illustrates both the clinical weight and the clinical limits of NRPS products. A veterinary review concluded that cyclosporine had established therapeutic guidelines, while tacrolimus proved too toxic for most use in dogs PMID 9283240 , and a dermatology review summarizes cyclosporine's pharmacology and applications in skin disease PMID 11155577 . Registered clinical work continues, with a cautionary note: a phase 2 trial of cyclosporine in large granular lymphocyte leukemia was terminated after enrolling five participants NCT00363779 . The drug is established; the evidence base in every niche indication is not.
The most concrete current application of NRPS machinery to controlled peptide synthesis is not the full assembly line but the offloading step. SurE, a penicillin-binding protein-type thioesterase, head-to-tail cyclizes octapeptidyl intermediates. It was identified as the offloading cyclase in surugamide B biosynthesis PMID 29808953 , and it has since been used in vitro to prepare cyclic peptides from resin-bound diol ester precursors, with protein engineering of homologous enzymes broadening the substrate scope PMID 36638272 . A 2023 protocol describes the chemo-enzymatic synthesis of non-ribosomal macrolactams with SurE as the cyclization catalyst PMID 37184702 . The approach works because the cyclase does the hard part, ring closure with defined stereochemistry, while the linear precursor can be assembled by chemical means.
Full NRPS systems are less tractable. They are large, modular, and tuned to their native substrates, so reprogramming them to make new peptides is slow. The SurE approach sidesteps the problem by using only one domain. For short oligopeptides, the practical lesson is that the cyclization chemistry is mature; the assembly chemistry is not.
ATP-grasp enzymes, also called ATP-dependent carboxylate-amine ligases, use a different activation chemistry. The carboxylate substrate attacks the γ-phosphate of ATP to form an acylphosphate intermediate, and the amine nucleophile then attacks the acylphosphate, displacing phosphate and forming the amide bond. The family includes biotin carboxylase, the D-alanine-D-alanine ligase Ddl, and glutathione synthetase, and some members participate in de novo purine biosynthesis. The acylphosphate route is chemically simple but energy-hungry: every bond costs one ATP, and the released phosphate must be handled or recycled.
The family shares a characteristic architecture: three conserved domains and a nonclassical ATP-binding fold that encloses the nucleotide. The structure of glycinamide ribonucleotide synthetase, PDB ID 2IP4, illustrates the fold, and most family members require a magnesium ion coordinated by ATP in the active site.
As peptide-forming enzymes, ATP-grasp ligases are specialists. They make specific metabolic bonds, such as the dipeptide D-alanyl-D-alanine for peptidoglycan or the addition of glycine to γ-glutamylcysteine in glutathione synthesis, rather than acting as general catalysts for arbitrary amino acid pairs. The 2019 review positions them as a second ATP-dependent route alongside NRPSs, but it reports no engineered ATP-grasp platform for making short oligopeptides on demand. Their significance is mechanistic: they demonstrate that acylphosphate activation is a viable, nature-tested strategy for amide bond formation that does not require the multimodular machinery of an NRPS.
The most direct enzymes for practical dipeptide and oligopeptide synthesis from simple starting materials are the α-amino acid ester acyltransferases AETs . Isao ABE's team first cloned and expressed AETs from Empedobacter brevis and from another bacterial strain. The two amino acid sequences are 35% and 36% identical to the α-amino acid ester hydrolase from Acetobacter pasteurianus, close enough to place them in the same enzyme family but distant enough that function cannot be predicted by homology alone.
AETs have two activities: they act as dipeptidyl peptidases and as transferases, moving an acyl group from an α-amino acid ester to a nucleophile. Their specificity is strict for both the acyl donor and the nucleophile, so each enzyme accepts a narrow set of amino acid pairings. The catalytic advantages are significant. The acyl donor is an ester, not an activated adenylate or acylphosphate, so no ATP is consumed. The starting amino acids are unprotected, so the synthesis bypasses the protection and deprotection steps that dominate chemical peptide coupling.
Before the cloning work, Kenzo and colleagues reported an efficient enzymatic method for producing oligopeptides from unprotected amino acids using Empedobacter brevis ATCC 14234, and named the catalyst carboxypeptidase Y. The report described high yields, but it provided no amino acid sequence, no coding gene sequence, and no three-dimensional crystal structure. Whether the enzyme is related to the well-known yeast carboxypeptidase Y, or to the AETs cloned later from the same organism, cannot be verified from the published record. The name is a placeholder, not an identification.
The structural gap extends to the cloned AETs. As of the 2019 review, no three-dimensional structure and no reaction mechanism had been reported for any AET. That matters for rational engineering: without a structure, substrate specificity can only be mapped empirically, and the strict donor and nucleophile requirements mean that a useful AET for a particular peptide bond must be found, not designed.
β-Lactam acylases are the only family in this group with a major established industrial role. The group includes penicillin acylase, glutaryl acylase, and β-amino acid ester hydrolase, and its members act on compounds containing the β-lactam ring. Penicillin acylases are categorized into two types based on their substrate specificity, and they are used at scale to hydrolyze penicillin to 6-aminopenicillanic acid 6-APA , the core scaffold for semi-synthetic β-lactam antibiotics. The same enzymes catalyze the reverse acyl transfer, attaching a new side chain to 6-APA to build amoxicillin and related drugs.
The industrial position rests on cost and sustainability. Enzymatic synthesis of β-lactam antibiotics is considered environmentally friendly and cost-effective, and 6-APA production at scale is the demonstration. The review also lists broader uses: peptide synthesis, racemic resolution, and the production of chiral and achiral pharmaceutical intermediates. The clinical relevance of the antibiotic class remains visible in current registered research; a completed phase 1/2 study examined the pharmacokinetics of amoxicillin and clavulanic acid in 36 obese adults NCT02571959 .
The limitation is the same specificity that makes these enzymes safe to run at scale. Each penicillin acylase type accepts a defined set of acyl donors, which is exactly what an industrial process wants but also what prevents a single enzyme from serving as a general peptide ligase. The family is best understood as a proven template: amide bond formation and hydrolysis on a reactive scaffold, optimized for one chemical neighborhood and operated continuously for decades.
Cyanophycin granule polypeptide CGP is an intracellular storage polymer synthesized by most cyanobacteria. Its composition is unusual: equimolar arginine and aspartic acid, with each arginine linked through its α-amino group to the β-carboxyl group of an aspartic acid residue. The polymer is built by cyanophycin synthetase, encoded by cphA, and it accumulates when nitrogen is abundant.
The degrading enzymes are the synthetic opportunity. Cyanophycinases CphB and CphE catalyze the intracellular and extracellular breakdown of CGP, respectively, and both release the dipeptide β-Asp-Arg. Because the polymer is a homopolymer of one repeating dipeptide, enzymatic digestion yields a single product rather than a mixture. That product has a defined use: β-Asp-Arg is a dipeptide form of arginine, and the review proposes it for feed or food applications that require arginine supplementation. Simultaneous production of CGP and its degrading enzyme allows efficient synthesis of the dipeptide in one system.
Production of CGP has been established in recombinant hosts, including Escherichia coli, Nicotiana tabacum, Pseudomonas putida, and Pseudomonas alcaligenes DIP1. More recently, co-expression of CGP and a cyanophycinase in N. tabacum was achieved, and a synthetic model has been proposed in which the polymer serves as a storage and transport form for arginine and β-Asp-Arg. The plant-based route is attractive for scale-up because the polymer accumulates in biomass. The open problem is industrial: the co-expression system works at the laboratory and proof-of-concept scale, and the path to tonnage quantities of β-Asp-Arg for feed or food use has not yet been reported.
The strongest evidence in this area concerns the products and the offloading enzymes, not the peptide-forming enzymes themselves. Cyclosporine has documented therapeutic value in dermatology PMID 11155577 , and a veterinary review supports its use in animals PMID 9283240 ; amoxicillin is the subject of ongoing pharmacokinetic research NCT02571959 . The SurE cyclization work is the most convincing modern demonstration that NRPS-derived enzymes can be applied in vitro: the offloading cyclase closes linear precursors head-to-tail, and engineered homologs broaden the range of acceptable substrates PMID 29808953; PMID 36638272; PMID 37184702 . These are real, reproducible results at the level of isolated enzymes and protocols.
The neighboring field of enzymatic synthesis of amino-acid-derived metabolites shows what the peptide field is aiming for. A one-pot enzymatic synthesis of S-adenosyl-L-homocysteine from racemic homocysteine thiolactone and adenosine reaches completion with near-stoichiometric reactants, and the same system makes nucleoside analogs PMID 39282651 . That standard, complete conversion with minimal excess of starting material, is the benchmark against which the peptide-forming enzymes can be judged. None of the five families described here has yet been shown to meet it for general dipeptide synthesis.
The weak points are specific and should be stated plainly. The Empedobacter brevis enzyme named carboxypeptidase Y was never characterized by sequence, gene, or structure, so the catalyst behind the reported high yields cannot be independently identified. The AETs, despite being cloned and expressed, have no reported structure or mechanism, and their 35% to 36% sequence identity to a related hydrolase leaves their detailed chemistry uncertain. The registered clinical evidence for cyclosporine in some indications is thin: the phase 2 trial in large granular lymphocyte leukemia was terminated with five participants enrolled NCT00363779 . And several headline claims in this area trace back to the single 2019 review rather than to independent structural or mechanistic papers. Treat the uncharacterized enzymes as early stage.
Match the enzyme family to the product. For head-to-tail cyclic peptides, SurE-type offloading cyclases are the demonstrated option, and protein engineering has already broadened their substrate scope PMID 36638272; PMID 37184702 . For 6-APA and semi-synthetic β-lactams, penicillin acylase is the industrial incumbent. For β-Asp-Arg, the CGP and CGPase co-expression route is the only described biocatalytic path, with E. coli as the lab-scale host, N. tabacum demonstrated for plant-based production, and P. putida and P. alcaligenes DIP1 also established. For a short oligopeptide from unprotected amino acids, an AET is the only family described as doing this directly, but its strict substrate specificity means the enzyme must match the specific donor and nucleophile pair, and no structure exists to guide that choice.
For researchers and buyers evaluating a claimed enzymatic peptide synthesis, the first request should be for the amino acid sequence, the gene, and a structure or mechanism. The two most direct routes in the 2019 review, the Empedobacter brevis carboxypeptidase Y-like enzyme and the AETs, supply none of these. A method that cannot be traced to a characterized enzyme cannot be reproduced, transferred to another host, or engineered.
The unresolved questions follow directly from the gaps. What is the sequence, gene, and three-dimensional structure of the oligopeptide-producing enzyme from Empedobacter brevis ATCC 14234? What is the structure and catalytic mechanism of the AETs? Can the CGP and CGPase co-expression system be scaled to industrial production of β-Asp-Arg? And how far can SurE-type cyclase engineering extend beyond its current substrate range? Answering the first two is a structural biology problem; the third is an engineering problem; the fourth is a protein engineering problem. Until the first two are answered, enzymatic synthesis of short oligopeptides will remain a set of promising demonstrations rather than a general, designable technology.
Peptides referenced: Glutathione.
Related reading: How Cyclic Peptides Cross Membranes: Mechanisms and Design Rules, Antitumor Peptides: Mechanisms, Production, and Applications, Neoantigen Peptide Synthesis Services and GMP Manufacturing, How Chameleon Cyclic Peptides Cross Membranes for Oral Drugs.