Biomedical Engineering Reference
In-Depth Information
thioester by an enoyl reductase (ER). This process of sequential reduction can be
stopped at any point and examples of all the intermediates are abundant in the
polyketide world: ketones, hydroxyls, olefins, and fully reduced alkanes [1]. The final
domain in the PKS assembly line is most often a thioesterase (TE) responsible for the
cleavage of the fully extended molecule from the last ACP and, in many cases,
cyclizing themolecule to yield amacrocycle. After the formation of a polyketide core,
a series of post-PKS enzymes modify this scaffold to produce the final natural product.
Several of these tailoring enzymes will be discussed in-depth in the examples later in
the chapter.
Throughout this chapter, examples will be discussed of natural products
biosynthesized, in whole or in part, by two types of PKSs referred to as type I and
type II. Examples of type I PKSs are those that produce large macrolactones such as
rapamycin (Section 14.4.1.2) and erythromycin (Section 14.4.2.2), whereas type II
PKSs cyclize their products in a very different way, producing polyaromatics such as
tetracycline (Section 14.4.1.3). The general mechanism by which the priming,
extension, and termination reactions take place is outlined in Figure 14.1.
The substantial differences in the structures of the products are reflected in
marked differences in the organization of the systems themselves. Type I PKSs are
organized as multimodular systems that resemble a large assembly line. Each
module consists minimally of three active sites—a KS, an AT, and an ACP. Each of
the extension reactions described above takes place between a KS and an ACP of a
given module after receipt of the nascent chain by the KS from the upstream ACP.
This organization differs from a type II system in which a single KS carries out
each of the extension reactions. The chain length is not determined by the number
of modules, but rather by a protein associated with the KS called the chain length
factor (CLF). Once the KS-CLF has performed the requisite number of extension
reactions, the chain is hydrolyzed from the ACP, and the polyaromatic scaffold
takes
form through a series of
spontaneous and enzymatically catalyzed
cyclizations [2,3].
Studies toward the better understanding of the ways in which PKSs catalyze
the formation of these important natural products will open up numerous engi-
neering opportunities for the production of novel, clinically important natural
products. In the remainder of this chapter, it will become apparent that the stepwise
synthetic strategy of these systems situates them well for exploitation in precursor-
directed biosynthesis.
14.2.2. Nonribosomal Peptide Biosynthesis
The biosynthesis of many small peptide natural products with antibiotic and other
activities is not catalyzed by the ribosome. These nonribosomal peptides are produced
by multimodular enzymatic assemblies called NRPSs. The general organization of an
NRPS is shown in Figure 14.2. NRPSs share an evolutionary heritage with the type I
polyketide synthase systems mentioned previously, with both being traceable back to
fatty acid biosynthesis.
NRPSs, similar to PKSs, possess an initiation module consisting of two
domains. In a fashion similar to a PKS AT domain, the NRPS adenylation domain
Search WWH ::
Custom Search