Leeper Group

   

Research

Cancer Imaging. (in collaboration with Prof. Kevin Brindle, CRUK Cambridge Institute) Altered glycosylation patterns of cell surface proteins are a hallmark of tumours. Changes in glycosylation include both increased and decreased expression of glycans, as well as expression of glycans normally restricted to embryonic tissues. We are exploring a novel metabolic-labeling approach to imaging cell surface glycans in vivo. In this approach a sugar bearing a bio-orthogonal tag is administered to the cells or animal. This sugar gets incorporated into newly synthesised cell-surface glycans and is then reacted with a probe that attaches itself exclusively to the tag and has an attached fluorophore or other imaging moiety (e.g. a radioisotope for PET or SPECT imaging). An azido group is the most commonly used tag, which reacts with strained alkynes such as our TMDIBO, but we are also exploring other possibilities and have recently introduced the isonitrile tag. We are also developing methods that use the much faster bio-orthogonal reaction between trans-cyclo-octenes and tetrazines.

glycan imaging in mice

Coenzyme Chemistry. Thiamin diphosphate (TPP) 1 is a coenzyme used by many enzymes which make and break bonds adjacent to keto groups. Crystal structures of many of these enzymes are available. In order to help understand how these reactions occur, we have synthesised analogues of TPP that bind to the enzymes in place of TPP but are unreactive. 3-DeazaTPP (which has a carbon in the place of the nitrogen of the thiazolium ring) is a very close analogue of TPP but binds far tighter than it, with picomolar affinity. We now plan to make derivatives of 3-deazaTPP (e.g. 2 and 3) which have side-chains on C-2 that match intermediates in the various enzyme-catalysed reactions. In addition to obtaining crystal structures of the analogues bound to the enzymes, we will also study inhibition. This should reveal the hidden stereochemistry of the enzymic reaction and may lead to new compounds of pharmaceutical or agrochemical interest.

some analogues of TPP

a thiazolium salt Novel Catalysts. Thiazolium salts catalyse a variety of reactions involving making and breaking bonds to a carbonyl carbon (e.g. the benzoin condensation). We have made chiral thiazolium salts (e.g. 4) and have observed asymmetric induction in formation of benzoin. We intend to further develop these catalysts to obtain better levels of asymmetric induction, to probe the mechanism and to introduce binding cavities for increased rate and better selectivities.

Biosynthesis of Prodigiosin. (in collaboration with Prof G. Salmond, Biochemistry) Prodigiosin is a bright red pigment, produced by various bacteria, that has potent anti-cancer activity. The biosynthesis of prodigiosin (see below) is of interest both for its chemistry and for its biochemical regulation. We have found the cluster of genes that codes for its biosynthesis in Serratia marcescens and have elucidated the role of each one in the biosynthetic pathway. We now want to study the mechanisms of some of the enzymic reactions involved, particularly PigE, which appears to be a bifunctional enzyme.

biosynthetic pathway

structure of bottromycin Biosynthesis of Bottromycin. (in collaboration with Dr Andy Truman, John Innes Centre, Norwich) Bottromycin is an antibiotic produced by Steptomyces bottropensis that has potent activity against MRSA in vitro. Analogues of bottromycin that are more stable in vivo might be clinically valuable. We have discovered the gene cluster for its biosynthesis and shown that it originates from a ribosomal peptide. Gene knockout studies have shown that three separate radical SAM methylases are responsible for adding the methyl groups in the b-position of a proline, a phenylalanine, and two valine residues. We are now trying to determine the functions of the other enzymes and, in particular, trying to prove which enzymes are responsible for the two unusual cyclisations, to form the thiazole and the macrocyclic amidine.

Synthetic Methods for PET. Positron Emission Tomography (PET) is a technique used for scanning human brain to detect the distribution of compounds of diagnostic interest. It relies on compounds labelled with very short-lived radioisotopes (e.g. 11C or 18F). The whole synthesis of such compounds must not take longer than ~30 mins. The aim of this project is to develop new synthetic methods, probably involving polymer-supported reagents, that will allow such rapid synthesis and isolation of labelled compounds of neuropharmacological importance.

Selected Publications

Cancer Imaging, Org. Biomol. Chem., 2013, 11 (42), 7297-7300; Bioconjugate Chem., 2013, 24 (6), 934941.

Coenzyme Chemistry, Org. Biomolec. Chem., 2008, 6, 3561-3572; Biochemistry, 2010, 49 (8), 1727-1736.

Thiazolium Salts as Catalysts, J. Chem. Soc., Perkin Trans. 1, 1998, 1891-1893; Tetrahedron Lett., 1997, 3611 and 3615.

Prodigiosin Biosynthesis, Chem. Commun., 2008, 18621864. Molec. Microbiology, 2005, 56 (4), 971-989; Nature Rev. Microbiol., 2006, 4, 887-899.

Bottromycin Biosynthesis, Chem. Sci., 2012, 3 (12), 3516-3521.

   

Department of Chemistry
University of Cambridge