Flexizyme

Flexizyme (a portmanteau of flexible and ribozyme) is a family of short (45 to 46 nucleotides), artificial aminoacyl-tRNA synthetase ribozymes evolved in vitro via SELEX by Hiroaki Suga and colleagues at the State University of New York at Buffalo and the University of Tokyo (first reported in 2001 and generalized in 2006). A flexizyme catalyzes the regioselective esterification (aminoacylation) of the 3'-terminal adenosine (A₇₆) 2'- or 3'-hydroxyl group of any tRNA molecule with a broad spectrum of natural L-α-amino acids, D-amino acids, β-amino acids, N-methylated amino acids, α-hydroxy acids, and fluorophore- or macrocycle-derivatized building blocks.

In evolutionary biology, the flexizyme provides direct experimental proof for a central step in the RNA World and Ribonucleoprotein World: prior to the emergence of large protein aminoacyl-tRNA synthetases (which require 300 to 1,000 amino acids and themselves depend on coded translation), a compact 45-nt catalytic RNA could charge primitive adaptor tRNAs with activated amino acids to launch coded peptide synthesis at the ribosomal Peptidyl Transferase Center. In modern biotechnology, flexizymes power the FIT (Flexible In-vitro Translation) and RaPID (Random non-standard Peptides Integrated Discovery) platforms used to discover protease-resistant macrocyclic peptide therapeutics.

Recognition of the Universal 3'-N₇₃CCA Tail and Leaving Group

Natural protein aminoacyl-tRNA synthetases (aaRSs) maintain translational fidelity by recognizing both the specific side chain of one cognate L-α-amino acid (activating it with adenosine triphosphate [ATP] into an aminoacyl-adenylate intermediate) and extended identity elements across the anticodon loop and acceptor stem of one cognate tRNA. Consequently, natural aaRS enzymes reject non-canonical backbones (such as D-amino acids or N-methyl amino acids) and cannot charge arbitrary tRNAs.

Hiroaki Suga bypassed both restrictions by evolving a ribozyme that decouples aminoacylation from the amino acid side chain and the tRNA body. The X-ray crystal structure of the flexizyme covalently linked to a miniature tRNA hairpin substrate, solved at 2.8 Å resolution in 2008 by Hui Xiao, Hiroshi Murakami, Hiroaki Suga, and Adrian R. Ferré-D'Amaré, revealed how a 45-nucleotide RNA achieves universal tRNA charging:

1. Base-Pairing to the Universal 3'-N₇₃CCA-3' Terminus: The flexizyme folds into a coaxially stacked P1–P2–J1a/3–P3 helical core terminating in a short single-stranded 3'-tail (G₄₃–G₄₄–U₄₅-3'). These three nucleotides form antiparallel Watson–Crick and wobble base pairs only with the universally conserved 5'-N₇₃–C₇₄–C₇₅–A₇₆-3' single-stranded acceptor overhang of any tRNA (where N₇₃ is the discriminator base A, G, or U). Because the flexizyme makes zero contacts with the tRNA TΨC arm, D-arm, or anticodon loop, a single flexizyme charges any tRNA regardless of its anticodon sequence.
2. Aromatic Pocket Recognition of the Activated Ester Leaving Group: Rather than binding the variable side chain (R-group) of the amino acid, the irregular J1a/3 internal bulge of the flexizyme forms an unstacked U-turn pocket (involving residues U23, A24, G25, and A40) that intercalates and stacks against a weakly activated aromatic or benzyl ester leaving group attached to the carbonyl carbon of the amino acid substrate.
3. Proximity-Driven Regioselective Transesterification: Once the flexizyme brings the carbonyl carbon of the pre-activated amino acid ester within bonding distance of the A₇₆ ribose ring (coordinated by hydrated Mg²⁺ ions), the 2'- or 3'-hydroxyl group of A₇₆ executes a nucleophilic attack on the ester carbonyl, expelling the leaving group and yielding the charged aminoacyl-tRNA. Because the amino acid side chain projects outward into bulk solvent away from the J1a/3 pocket, the ribozyme accepts hundreds of structurally diverse side chains and N-substituents.

In 2006, Hiroshi Murakami, Akimasa Ohta, Hideki Ashigai, and Hiroaki Suga (Nature Methods) evolved three complementary second-generation flexizymes (dFx, eFx, and aFx) that together charge nearly any carboxylic acid monomer onto any tRNA in aqueous buffer on ice:

• dFx (Dinitrobenzyl Flexizyme, 46 nt): Selected to recognize the electron-withdrawing 3,5-dinitrobenzyl ester (DBE) leaving group. Because the J1a/3 pocket of dFx binds the dinitrobenzyl ring of the leaving group rather than the amino acid side chain, dFx charges aliphatic, polar, charged, N-methylated, D-configured, and α-hydroxy acids that lack aromatic side chains.
• eFx (Enhanced Flexizyme, 45 nt): Evolved from the prototype aromatic flexizyme to recognize two leaving groups: cyanomethyl esters (CME) on substrates that already possess an aromatic side chain (such as phenylalanine, tyrosine, tryptophan, or non-canonical aryl-alanines), and 4-chlorobenzyl thioesters (CBT) on non-aromatic or sterically hindered substrates (such as D-cysteine, D-proline, β-amino acids, or chloroacetylated precursors).
• aFx (Amino-Derivatized Flexizyme, 47 nt): Engineered to recognize the 4-[(2-aminoethyl)carbamoyl]benzyl thioester (ABT) leaving group, whose positively charged primary ammonium tail solubilizes hydrophobic amino acids and lipidated monomers in aqueous Mg²⁺ buffer without organic co-solvents.

Rewriting the Codon Table In Vitro

When combined with a reconstituted, purified Escherichia coli cell-free translation system (PURE system, developed by Takuya Ueda) from which specific natural amino acids and their cognate protein aminoacyl-tRNA synthetases have been omitted, flexizymes create the FIT (Flexible In-vitro Translation) system. By charging synthetic orthogonal suppressor or elongator tRNAs with non-canonical monomers via dFx or eFx and adding those pre-charged tRNAs to the depleted translation reaction, researchers reassign vacant sense or nonsense codons in the genetic code.

For example, vacating 10 to 20 sense codon boxes simultaneously allows the ribosome to polymerize sequence-defined polyesters (from α-hydroxy acids), peptoids, D-peptide segments, and N-methylated backbones. Initiating translation with a flexizyme-charged N-chloroacetyl-L- or D-tyrosine on initiator tRNA^fMet causes the N-terminal chloroacetyl group to react spontaneously after translation with a downstream cysteine sulfhydryl side chain, cyclizing the nascent peptide into a head-to-side-chain thioether macrocycle with >99% efficiency.

The RaPID mRNA Display Platform for Macrocyclic Drugs

By coupling the FIT system to mRNA display (where each newly translated macrocyclic peptide is covalently linked via a puromycin linker to the 3'-end of its own encoding mRNA across libraries of 10¹² to 10¹⁴ sequences), Suga established the RaPID (Random non-standard Peptides Integrated Discovery) platform. Over 4 to 6 rounds of affinity selection against disease proteins, RaPID isolates protease-resistant, cell-permeable macrocyclic peptides (molecular mass 1.0 to 2.5 kDa) containing multiple N-methylated amide bonds and D-amino acids that inhibit protein–protein interactions with picomolar-to-nanomolar dissociation constants (K_d)—including clinical candidates such as the oral PCSK9 inhibitor enlicitide decanoate (MK-0616) and complement C5 inhibitor zilucoplan.

How can a 45-nucleotide flexizyme charge any tRNA with almost any amino acid?

Natural protein aminoacyl-tRNA synthetases inspect both the specific side chain of one amino acid and the anticodon/stem identity elements of one tRNA. A flexizyme bypasses both checks. First, its 3'-G₄₃G₄₄U₄₅ tail base-pairs only with the universal single-stranded 3'-N₇₃CCA tail found at the acceptor end of every tRNA, ignoring the rest of the tRNA body and anticodon. Second, its catalytic pocket binds an aromatic ester leaving group (such as a 3,5-dinitrobenzyl ester or 4-chlorobenzyl thioester) attached to the amino acid rather than the amino acid side chain, allowing the side chain to point out into solution while the ribozyme transfers the acyl group onto adenosine-76.

Why is the flexizyme important for the RNA World hypothesis?

A longstanding puzzle of the RNA World is how the first coded peptides could have been synthesized before protein aminoacyl-tRNA synthetases existed, since modern synthetases are large proteins that require an already-functioning translation system to be produced. The discovery of the flexizyme proved experimentally that a short, 45-nucleotide RNA catalyst can regioselectively attach activated amino acids onto the 3'-CCA terminus of tRNAs using only Mg²⁺ ions, demonstrating a plausible ribozyme bridge from the RNA World to coded ribosomal translation.

How does the RaPID platform use flexizymes to discover macrocyclic peptide drugs?

Conventional linear peptides make poor oral drugs because stomach and blood proteases cleave their amide bonds and their polar backbones cannot cross cell membranes. Using flexizymes in the FIT/RaPID system, researchers charge tRNAs with D-amino acids, N-methylated amino acids, and an N-terminal N-chloroacetyl-amino acid that spontaneously forms a non-reducible thioether ring with a downstream cysteine. Coupling this reprogrammed translation to mRNA display screens 10¹² to 10¹⁴ cyclic peptides simultaneously to isolate protease-resistant, antibody-like macrocycles.

1. Saito, H., Kourouklis, D., & Suga, H. (2001). "An in vitro evolved precursor tRNA with aminoacylation activity." The EMBO Journal, 20(7), 1797–1806. https://doi.org/10.1093/emboj/20.7.1797
2. Murakami, H., Ohta, A., Ashigai, H., & Suga, H. (2006). "A flexible tRNA acylation method for non-natural polypeptide synthesis." Nature Methods, 3(5), 357–359. https://doi.org/10.1038/nmeth877
3. Xiao, H., Murakami, H., Suga, H., & Ferré-D'Amaré, A. R. (2008). "Structural basis of specific tRNA aminoacylation by a small in vitro selected ribozyme." Nature, 454(7202), 358–361. https://doi.org/10.1038/nature07033
4. Passioura, T., & Suga, H. (2014). "Reprogramming the genetic code in vitro." Trends in Biochemical Sciences, 39(9), 400–408. https://doi.org/10.1016/j.tibs.2014.07.005