Characterisation and exploitation of a promiscuous non-ribosomal peptide cyclase

The majority of clinically used antibiotics are derived from natural products produced by Streptomyces species and other closely related soil bacteria.

These drugs were primarily discovered and introduced into the clinic during a 'golden era' of antibiotic discovery that spanned 1940-1960. The utility of these agents has been eroded over the last half-century due to misuse. As a consequence, there is now an urgent need to discover new antibiotics to treat drug resistant bacterial infections.

Growing concerns about resistance to antibacterial agents combined with the failure to find new leads from the screening of large libraries of synthetic compounds has led to a renewed interest in natural products discovery.

Unfortunately, the overwhelming majority of microbes have yet to be cultured, and for those that have, only a small fraction of their natural products are produced in the laboratory.

Conventional approaches to overcome this problem, typically rely upon time consuming genetic modification of the producing organism or the use of poorly understood 'elicitor' compounds to switch on production.

The bottleneck with this approach is the fact that a large amount time can be spent on one biosynthetic pathway whose product could never be produced or is not an antibiotic.

Moreover, even if success in activating an antibiotic pathway is achieved, discovery of a lead compound is only the first stage of drug discovery. Many antibiotics are derived from a class of microbial natural products called non-ribosomal peptides. Once an exciting prospect is identified, future development is ultimately dictated by its accessibility.

Microbial fermentation rarely provides sufficient compound to move forward and therefore chemical synthesis is typically used to produce the quantity, and importantly, the chemical diversity of analogues necessary for testing the clinical potential of a discovery lead.

The problem here is that the vast majority of non-ribosomal peptides are cyclic and cyclisation reactions are typically very problematic and produce a low yield of the final compound. In nature, the cyclisation reaction is carried out by a part of the biosynthetic pathway called a thioesterase domain.

We recently identified a novel cyclase enzyme, which is promiscuous with respect to the peptide substrates it cyclises. This is exciting and we want to understand how this enzyme works so we can harness its potential for biotechnology. For example, to improve chemical synthesis of antibiotics.

We believe this could ultimately help more medicines reach the clinic, possibly making them less expensive and more widely available.