In vitro studies of the enzymes involved in fluorometabolite biosynthesis in Streptomyces cattleya

Abstract

Enzymatic fluorination of natural products is extremely rare. Of the 4000 halogenated natural products identified, only 13 possess a fluorine atom. The C-F bond forming enzyme from the soil bacterium, Streptomyces cattleya, remains the only native enzyme to be identified that is capable of such biochemistry. It generates 5’-fluoro-5-deoxyadenosine (5‘-FDA) from S-adenosyl-L-methionine (SAM) and F-. The “fluorinase” is the first committed step toward the biosynthesis of the two fluorometabolites, 4-fluorothreonine and fluoroacetate, via the common intermediate, fluoroacetaldehyde (FAld). The enzymatic steps responsible for the conversion of 5’-FDA to the fluorometabolites remained to be fully characterised when this project began.

Previously, a purine nucleoside phosphorylase was identified that was capable of generating 5-fluorodeoxyribose-1-phosphate (5-FDRP) from 5’-FDA. 5-FDRP is subsequently isomerised to 5-fluorodeoxyribulose-1-phosphate (5-FDRulP) by an aldose-ketose isomerase enzyme.

Chapter 2 describes the identification of the isomerase gene from the genomic DNA of S. cattleya and the corresponding protein product was capable of generating 5-FDRulP from 5-FDRP.

The next intermediate, FAld, is generated from 5-FDRulP by a fuculose aldolase. Attempts to identify the aldolase gene from S. cattleya were unsuccessful, however a putative fuculose aldolase from Streptomyces coelicolor was isolated that could generate FAld from 5-FDRulP, which is described in Chapter 3.

Following the identification and over expression of a PLP-dependant transaldolase, which generates 4-fluorothreonine (4-FT) from FAld and L-threonine in S. cattleya, Chapter 4 details the successful in vitro reconstitution of fluorometabolite biosynthesis using five over- expressed enzymes.

In Chapter 5, attempts to develop a novel assay for fluorinase activity was explored. The colorimetric detection of L-methionine produced by the fluorinase in a coupled L-amino acid oxidase and horseradish peroxidase assay, leading to the oxidation of a dye substance. This was carried out with interest in developing a high-throughput assay for fluorinase mutants, generated by random mutagenesis, in order to identify those with increased activity. In the event, it proved unsuccessful.