Establishing a biocatalytic platform for small molecule c-alkylation

Student thesis: Doctoral Thesis

Abstract

The alkylation of aromatic small molecules is an essential synthetic process used throughout medicinal chemistry. Key limitations of synthetic methods such as the Friedel-Crafts reaction is its lack of chemoselectivity in complex organic scaffolds and stoichiometric amounts of metallic waste produced by this process. A potential solution to this issue could be the use of methyltransferase (MTase) enzymes as biocatalysts to carry out this process. In nature, Sadenosyl-L-methionine (SAM) dependent MTases carry out such reactions on a variety of substrates ranging from small molecules to DNA and proteins. SAM donates its methyl groupto the target substrate with S-adenosyl-L-homocysteine (SAH) generated as a stoichiometric byproduct.The MTase NovO regio specifically transfers alkyl groups from chemically synthesised SAM analogues to a coumarin scaffold which is a precursor to the naturally-occurring antibiotic novobiocin. Chemical synthesis of SAM analogues has the issue of producing a diastereomeric mixture with epimers generated at the sulfonium centre, and display limited stability. SAMcan be generated biosynthetically from the reaction of 5′-deoxy-5′-chloroadenosine (ClDA)with L-methionine (L-met), catalysed by an enzyme, SalL, from Salinospora tropica. Thismethod is particularly attractive as ClDA and L-met are cheap and produce only the biologically active (S,S) diastereomer.This thesis is a study of both SalL and NovO and their use in tandem SAM generation/alkylation (Scheme). The crystal structure of SalL has been solved to guide Site-Directed Mutagenesis (SDM). This method has then been used to better understand key residues in substrate binding and enzyme activity. Further to this, rational mutations were made in an attempt to increase space in the binding pocket to allow L-met analogues bearing groups larger than methyl to be accommodated. To this end, a range of L-met analogues were synthesised. The crystal structure of SalL revealed a channel which leaves the 2-position of ClDA solvent exposed. This raises the possibility of accommodation of 2-modified ClDA analogues as substrates of SalL. A suite of 2-modified ClDA analogues were synthesised to test this hypothesis.The current substrate scope of NovO is narrow so it was desired to investigate coumarins which have not yet been investigated with NovO. A range of coumarins were synthesised including a precursor to a compound which has shown activity as an inhibitor of Heat ShockProtein 90 (Hsp90) which has been implicated in a number of cancers. To explore the use of NovO as a method for late-stage functionalisation of drug molecules methylation of 7-hydroxywarfarin, a metabolite of clinically approved drug warfarin, has been carried out.
Date of Award13 Dec 2019
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde & EPSRC (Engineering and Physical Sciences Research Council)
SupervisorGlenn Burley (Supervisor) & Paul Hoskisson (Supervisor)

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