Mimosinase degrades the non-protein amino acid mimosine and is thought to have evolved from cystathionine β-lyase (CBL) via gene duplication. However, no study has, to date, compared the molecular characteristics of mimosinase and CBL. We therefore cloned mimosinase and CBL from the Mimosoideae subfamily member Mimosa pudica (Mp) and explored the molecular relationship between mimosinase and CBL for the first time. The recombinant Mp mimosinase degraded both mimosine and cystathionine with a much higher turnover number (kcat) for mimosine compared with cystathionine, and Mp CBL utilized only cystathionine as a substrate. The critical residues implicated in the substrate binding of Arabidopsis thaliana CBL (Tyr-127, Arg-129, Tyr-181, and Arg-440) were highly conserved in both Mp mimosinase and CBL. However, homology modeling and molecular simulation of these enzymes predicted variations in the residues that interact with substrates. A mutation experiment on Mp mimosinase revealed that the disruption of a disulfide bond in the vicinity of the pyridoxal-5'-phosphate domain increased the enzyme's preference toward cystathionine. Treatment of Mp mimosinase with a disulfide-cleavage agent also decreased mimosinase activity. Furthermore, mutation near the conserved binding residue altered the substrate preference between mimosine and cystathionine. Molecular dynamics simulations of Mp mimosinase suggested a closer coordination of the residues that interact with mimosine at the active site compared with cystathionine, indicating a more compact pocket size for mimosine degradation. This study thus may provide new insights into the molecular diversification of CBL, a C-S lyase, into the C-N lyase mimosinase in the Mimosoideae subfamily.
Keywords: Cystathionine β-lyase; Homology modeling; Mimosinase; Mimosine; Molecular dynamics; Molecular evolution.