Biomedical Engineering Reference
In-Depth Information
INTRODUCTION
Acremonium chrysogenum
is an ascomycete fungus isolated by Giuseppe
Brotzu from sewage-contaminated marine waters. It is industrially used
for the production of cephalosporins, a class of medically important
β-lactams for the treatment of several bacterial infections. Due to this
importance, the genes encoding the cephalosporin biosynthetic enzymes
have received considerable attention. The cephalosporin biosynthetic
genes are arranged in two clusters located in different chromosomes.
The “early” cluster contains the genes
pcbAB
and
pcbC
, which encode the
enzymes involved in the biosynthesis of the intermediate isopenicillin N
(IPN),
cefD1
and
cefD2
, which encode a two-protein system involved in the
epimerization of IPN to penicillin N (PenN) and the membrane transporter
encoding genes
cefT
,
cefM
and
cefP
. This cluster is located in chromosome
VII, the largest chromosome resolved from the
A. chrysogenum
genome.
A second cluster, named the “late” gene cluster located in chromosome
I, encodes the two last steps of cephalosporin biosynthesis;
cefEF
and
cefG
. The enzymes encoded by these two genes catalyze the conversion
of PenN to deacetylcephalosporin C (DAC) and fi nally, to cephalosporin
C. Most of the enzymes encoded by these genes have been characterized
from the biochemical point of view and three of them (the IPN synthase,
the deacetoxycephalosporin C (DAOC) synthase/hydroxylase and the
DAC acetyltransferase) have been crystallized. The organization of the
cephalosporin genes in two clusters requires that expression of the genes in
the two separate clusters is coordinated for optimal yield of cephalosporin.
In strains in which there is not a good coordination, considerable amounts
of the IPN or PenN intermediates (compounds having weak antibiotic
activity) are secreted to the culture medium.
In this chapter we review the state of the art on the genes and proteins
involved in the biosynthesis of cephalosporin C, transport of intermediates
through subcellular compartments and regulatory proteins that control the
expression of the cephalosporin biosynthetic genes and differentiation.
Finally, some biotechnological applications for the biosynthesis of several
cephalosporin derivatives are also described.
THE CEPHALOSPORIN BIOSYNTHETIC PATHWAY:
AN OVERVIEW
Cephalosporins and penicillins (particularly semisynthetic derivatives) are
widely used in treatment of several infections diseases. The cephalosporin
biosynthetic pathway (
Fig. 1)
has the two fi rst steps in common with that of
penicillin (Aharonowitz et al. 1992). The pathways to synthesize penicillins
