Enhanced specificity of mint geranyl pyrophosphate synthase by modifying the R-loop interactions

Area of Science:

• Biochemistry
• Enzymology
• Molecular Biology

Background:

• Isoprenoids are vital biologically active compounds synthesized by prenyltransferases (PTSs).
• Mint geranyl pyrophosphate synthase (GPPS) is a heterotetrameric enzyme producing C(10)-GPP and C(20)-GGPP.
• The SSU subunit of mint GPPS contains a regulatory R-loop, unlike the catalytic LSU subunit.

Purpose of the Study:

• To investigate the role of the SSU's R-loop in mint GPPS activity and product specificity.
• To elucidate the mechanism by which the R-loop regulates isoprenoid chain length determination.

Main Methods:

• Site-directed mutagenesis (deletion and point mutations) of the R-loop in mint GPPS.
• In vitro enzymatic assays to determine product profiles (C(10)-GPP and C(20)-GGPP).
• X-ray crystallography to determine the structure of the R-loop deletion mutant.

Main Results:

• Disruption of the R-loop by mutation or deletion abolished C(20)-GGPP production, yielding only C(10)-GPP.
• Structural analysis revealed an intact heterotetrameric structure in the deletion mutant with a slightly more open active site.
• The R-loop regulates product release from the LSU active site, complementing the molecular ruler mechanism for chain-length control.

Conclusions:

• The R-loop in mint GPPS is crucial for conserving enzyme activity and ensuring precise control over isoprenoid chain length.
• The R-loop's product-retaining function provides stringent regulation, preventing premature release of intermediates.
• This regulatory mechanism contributes to the selective production of C(10)-GPP and potentially influences C(20)-GGPP formation in mint.