Loganic Acid Methyltransferase: Insights into the Specificity of Methylation on an Iridoid Glycoside

Abstract

Loganin is an iridoid glycoside of interest as both an intermediate in the biosynthesis of indole alkaloids in plants and as a bioactive compound itself. Loganic acid methyltransferase catalyzes the methylation of a monoterpenoid glycoside precursor to produce loganin and demonstrates stereospecificity for the (6S,7R) substrate. We have biochemically characterized this biocatalyst and elucidated the basis for its strict substrate specificity. These studies could help facilitate the design of new classes of monoterpenoid indole alkaloids of pharmaceutical interest.

Carboxyl Methyltransferases: Natural Functions and Potential Applications in Industrial Biotechnology

“…The enzyme is well characterised, including a crystal structure which shows a homodimeric structure with a distinct dimerisation interface that differs significantly from other plant natural product MTs as it involves only a small percentage of the enzyme surface area, a common feature of other SABATH MTs [22,32] . Crystal structures for two other SABATH CMTs have also been solved (Table 1); LAMT from Catharanthus roseus and IAMT from Arabidopsis thaliana which also both exist as homodimers [28,33] . In homodimeric plant small molecule O ‐MTs that are not part of the SABATH family, dimerisation is critical to enzyme activity as the dimer interface contributes to the substrate binding site [34] .…”

“…Vinblastine/vincristine is formed from two molecules of loganin, through a pathway that diverges at stemmadenine and then ultimately re‐converges through the combination of catharanthine and vindoline to form 3′,4′‐anhydrovinblastine [51] . Extensive studies of LAMT have been carried out including a crystal structure of the C. roseus enzyme (Table 1) and LAMT demonstrates high substrate specificity and strict stereospecificity [33,40] . As well as being part of the biosynthetic pathway of MIAs, loganin is also bioactive including the lowering of blood glucose levels and neuroprotective effects [52] .…”

“…As well as being part of the biosynthetic pathway of MIAs, loganin is also bioactive including the lowering of blood glucose levels and neuroprotective effects [52] . As such, LAMT and related enzymes have potential for application in the design and synthesis of novel classes of MIAs for pharmaceutical purposes [33] …”

“…as homodimers. [28,33] In homodimeric plant small molecule O-MTs that are not part of the SABATH family, dimerisation is critical to enzyme activity as the dimer interface contributes to the substrate binding site. [34] In contrast, dimerisation of SABATH MTs is not required for enzyme activity as the residues from one monomer do not contribute to the catalytic site of the related monomer.…”

“…[51] Extensive studies of LAMT have been carried out including a crystal structure of the C. roseus enzyme (Table 1) and LAMT demonstrates high substrate specificity and strict stereospecificity. [33,40] As well as being part of the biosynthetic pathway of MIAs, loganin is also bioactive including the lowering of blood glucose levels and neuroprotective effects. [52] As such, LAMT and related enzymes have potential for application in the design and synthesis of novel classes of MIAs for pharmaceutical purposes.…”