Structural enzymology of the biosynthesis of polyketide antibiotics

Abstract

Anthracyclines are an important group of aromatic antibiotics that exhibit antitumour activity, which makes them useful in treatment of various cancers. They are synthesised in the polyketide biosynthetic pathway as secondary metabolites by different Streptomyces species. An increasing number of anthracyclines have however been shown to exhibit cardiotoxic side-effects. The genetics and enzymology of this pathway has recently attracted considerable interest, not at least with the possible prospect for the production of novel antibiotics.

In this thesis some of the enzymes involved in biosynthesis of anthracyclines have been studied by protein crystallography and biochemical methods. The structure of SnoaL, a stereospecific cyclase was determined to a resolution of 1.35 A as a complex with a product analogue. SnoaL belongs to a hitherto uncharacterised family of enzymes with alpha+beta barrel like fold and catalyses a novel form of intramolecular aldolcondensation. The structure of the methylesterase RdmC in complex with product analogue shows the common alpha/beta hydrolase fold and contains a catalytic SerHis-Asp triad. RdmB is a hydroxylase built up by a Rossman-like fold common to methyltransferases. The enzyme utilizes the SAM moiety in a novel way as a cofactor in the hydroxylation reaction. DnrK is a methyltransferase with a structure very similar to that of RdmB.

RdmB and DnrK are thus two enzymes sharing the same fold but catalysing different reactions. They are illustrative examples of two enzymes evolved through divergent evolution. A common feature to all the enzymes studied in the thesis is that they bind their anthracycline substrates mainly through hydrophobic interactions with the involvement of only a few hydrogen bonds. Many of the enzymes have a very broad substrate specificity which might be due to these features.