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. Author manuscript; available in PMC: 2022 Oct 26.
Published in final edited form as: Trends Chem. 2021 Oct 5;4(1):91–92. doi: 10.1016/j.trechm.2021.09.007

MIO-containing aminomutases for α- to β-amino acids

Edward Kalkreuter 1, Ben Shen 1,2,3,*
PMCID: PMC9600766  NIHMSID: NIHMS1844521  PMID: 36304426

Graphical Abstract

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ORIGIN

The family of 2,3-aminomutases catalyzes the migration of an amine group from the α-carbon to the β-carbon of l-arylalanines. The first 4-methylideneimidazole-5-one (MIO)-containing aminomutase was discovered in 2003, catalyzing (S)-β-tyrosine formation in the biosynthesis of the enediyne C-1027 and sharing homology with the well-studied ammonia lyases. Since then, several MIO-containing aminomutases have been characterized, catalyzing the formation of varying β-arylalanines in the biosynthesis of Taxol, kedarcidin, and other natural products.

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REACTION MECHANISM

Before the discovery of the MIO-containing l-tyrosine aminomutase from Streptomyces globisporus (SgTAM), all known aminomutases proceeded through radical mechanisms using a pyridoxal phosphate (PLP) cofactor in combination with either adenosylcobalamin (AdoCbl) or S-adenosyl methionine (SAM) with a [4Fe-4S] cluster. However, the family of MIO-containing aminomutases utilizes MIO as an electron sink and general acid–base chemistry to catalyze elimination of the α-amino group and its subsequent Michael addition at the β-carbon. The MIO cofactor is derived autocatalytically from Ala-Ser-Gly within the active site via two dehydration steps, requiring no exogenous cofactors, and is more electrophilic than the similar dehydroalanine due to its gain of aromaticity upon nucleophilic addition. The mechanism for TAM and l-phenylalanine aminomutase (PAM) is initiated by a nucleophilic attack of MIO by the neutral amine (A). The MIO-tethered ammonia is then eliminated through an E2 mechanism, yielding an α,β-unsaturated acid (B). The differentiation between aminomutases and ammonia lyases is the retention and rebound of the eliminated ammonia at the β-carbon (C) before its release via an E1cB mechanism (D). Like most enzymatic reactions, the process is reversible; however, while SgTAM shows poor enantioselectivity due to β-tyrosine racemase activity, PAM yields only the (R)-β-phenylalanine enantiomer. This mechanism is convincingly demonstrated by structures of the SgTAM active site with MIO covalently bound to the β-amine of (R)-α,α-difluoro-β-tyrosine (a mechanism-based inhibitor) (E). Additionally, the enzymes overcome the low acidity of the β-hydrogen (pKa > 40) through orientation of the positive termini of α-helices. Finally, conversion of cinnamate to (R)-β-phenylalanine by PAM supports it as an intermediate (B).

IMPORTANCE

The study of these enzymes and their mechanisms highlights an example of convergent and divergent evolution (MIO versus other cofactors and mutases versus lyases, respectively). These enzymes also have considerable value for the chemoenzymatic synthesis of enantiomerically pure α- and β-amino acids. Finally, the family of MIO-containing aminomutases highlights once again how natural product biosynthesis serves as inspiration for novel enzyme discovery.

Footnotes

Declaration of interests

No interests are declared.

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