PRECURSOR-DIRECTED BIOSYNTHESIS OF POLYKETIDE AND NONRIBOSOMAL PEPTIDE NATURAL PRODUCTS - Modern Tools for the Synthesis of Complex Bioactive Molecules - page 481

14.2. NATURAL PRODUCT BIOSYNTHESIS
Before any discussion on precursor-directed biosynthesis can be undertaken, an
understanding of the principles governing the biosynthesis of the molecules to be
obtained is necessary. As mentioned previously, throughout this chapter two primary
classes of molecules will be discussed, polyketides and nonribosomal peptides, as
well as several hybrids thereof.
These classes of molecules have been chosen for practical purposes, as they have
been predominant targets for precursor-directed biosynthesis. Polyketide synthases
(PKSs) and nonribosomal peptide synthetases (NRPSs) both work in an assembly line
fashion, making the application of precursor-directed approaches to these systems
particularly rational and intuitive. The biosynthesis of these compounds remains an
active area of research with each new discovery broadening the potential for the
engineered production of novel natural product analogues. While a complete under-
standing of the mechanistic principles underlying PKS and NRPS activities remains a
distant goal a broad overview of the chemical logic used by these systems for the
production of their clinically relevant secondary metabolic products is outlined next.
14.2.1. Polyketide Biosynthesis
Polyketides are a class of natural products that demonstrate a range of biological
activities, including immunosuppression as well as the treatment of various cancers,
viral infections, and perhaps most prominently, bacterial infections. PKSs have been
classified into three types, only two of whichwill be considered here. Themechanisms
for polyketide chain growth by both these types of PKSs are similar. First, an initiation
module recruits an acyl coenzyme A (CoA) thioester as the source of a primer
molecule. This primer molecule is then passed to an initiation acyl carrier protein
(ACP) through a transthioesterification reaction. ACPs are activated by attachment of
a phosphopantetheinyl arm and act as noncatalytic carrier proteins throughout the
biosynthetic process, shuttling intermediates from one catalytic domain to the next.
This initiation ACP transfers the priming molecule to the first catalytic domain, a
ketosynthase (KS). Once this transfer occurs, an acyltransferase (AT) domain recruits
an extender molecule. These extenders vary, but malonyl- and methylmalonyl-CoA
are the most commonly used sources of extender units. Ethylmalonyl- and methox-
ymalonyl-CoA are used less frequently. This extender molecule is then transferred
onto a second ACP. With the priming unit in place on the KS and the first extender
bound to an ACP, the first chain elongation reaction takes place. The KS catalyzes
decarboxylation of the malonate-derived extender generating a stabilized carbanion.
This nucleophilic carbanion then attacks the thioester binding the primingmolecule to
the KS. The net result is the extension in length of the nascent natural product chain by
two carbon atoms and its transfer to a downstream ACP. After the extension reaction
has occurred, there is the potential for a series of tailoring reactions to take place on the
ketone b to the thioester. The series of reactions goes as follows: reduction of the
ketone to the corresponding hydroxyl group by a ketoreductase (KR) domain,
dehydration by a dehydratase domain (DH) to yield an a , b -unsaturated ester, and
subsequent reduction of the double bond to yield the fully reduced, unsubstituted