# Molecular basis and biological relevance of bacterial and plant pinoresinol/lariciresinol reductase specificities

## TL;DR

This paper explores how bacterial and plant enzymes break down specific plant compounds, revealing differences in their molecular mechanisms and potential applications in biodegradation.

## Contribution

The study identifies bacterial PLR homologs with broad substrate versatility and provides a molecular explanation for stereochemical outcomes in plant and bacterial PLR enzymes.

## Key findings

- Bacterial NrPinZ, NaPinZ, and SlPinZ reduce racemic furanofuran lignans with similar catalytic efficiency.
- Plant PLR_Tp2 preferentially reduces (+)-pinoresinol to produce (−)-secoisolariciresinol due to its smaller active site pocket.
- Bacterial PLR homologs may play a key role in lignin and lignan biodegradation with evolutionary and engineering implications.

## Abstract

A bacterial pinoresinol/lariciresinol reductase (PLR) homolog named NrPinZ was obtained from a Novosphingobium rhizosphaerae sp. LY bacterial strain, with NrPinZ being part of its 5‐step biochemical system catabolizing pinoresinol into coniferyl aldehyde and vanillin. Recombinant NrPinZ reduces racemic 8–8′ furanofuran lignans [(±)‐pinoresinols, medioresinols, and syringaresinols] with similar overall catalytic efficiencies. In those reductions, only one of the two furan ring systems is reduced. Two other bacterial PLR homologs, NaPinZ and SlPinZ, from N. aromaticivorans F199 and Sphingobium lignivorans SYK‐6, respectively, had comparable substrate versatilities and catalytic efficacies. Plant PLR homologs, by comparison, are either enantiospecific, enantioselective, or variants thereof, being able to reduce either one or both furan rings. For example, a recombinant enantioselective PLR (PLR_Tp2) from western red cedar (Thuja plicata) preferentially reduces both (+)‐pinoresinol furan rings to afford (−)‐secoisolariciresinol. BoltZ‐2 modeling of NrPinZ and PLR_Tp2, together with substrate docking of (+)‐ and (−)‐pinoresinols, medioresinols, and syringaresinols, was very instructive. The NrPinZ active site P1/P2 sub‐pockets allow for both racemic forms to be catabolized. Conversely, the smaller P1 pocket in PLR_Tp2 preferentially positions (+)‐pinoresinol for downstream metabolism into (−)‐secoisolariciresinol, thereby providing a biochemical explanation for the different stereochemical outcomes. NrPinZ, NaPinZ, and SlPinZ, catalyzing substrate versatile catabolism of both racemic forms, may have important ramifications for gymnosperm and angiosperm lignin and lignan biodegradation, including its evolutionary significance and potential in enzyme engineering.