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chapter 25
RNA and Protein Synthesis
role in hemostasis (Chapter 36), has a reactive arginine at
its active center. Since the oq-AT and antithrombin III
are structurally similar, the substitution of methionine of
a
j -AT for arginine at the active center alters its specificity.
This bleeding disorder is further complicated by the fact
that the concentration of normal antithrombin III does not
increase in response to inflammatory stress as does the
level of oq-AT. Therefore, with each hemolytic episode,
the abnormal
a
i -AT increases, creating a cycle resulting
in uncontrolled bleeding.
Inhibitors of Protein Synthesis and
Related Disorders
Many antibacterial agents (antibiotics) have been isolated
from fungi. Antibiotics are used both clinically and as
reagents for unraveling the details of protein, RNA, and
DNA synthesis.
Antibiotics
that have no effect on eukaryotic transla-
tion either fail to penetrate the cell membrane (which is
quite common) or do not bind to eukaryotic ribosomes.
The differences in effectiveness of antibiotics on the two
classes of cells
in vivo
is the basis of their usefulness as
therapeutic agents for bacterial infections (Figure 25-18).
Some antibiotics are active against both bacterial and
mammalian cells. One example is chloramphenicol, which
inhibits peptidyltransferase in both bacterial and mito-
chondrial ribosomes, although eukaryotic cytoplasmic ri-
bosomes are unaffected. Such a drug may be clinically
useful if a concentration range can be maintained in the
patient in which the antibacterial action is substantial but
toxic effects on host cells are minimal. However, because
of the potential for toxicity, such antibiotics are used only
in serious infections when other drugs fail.
Three antibiotic inhibitors that act only on prokaryotes
are streptomycin, tetracycline, and erythromycin. Other
antibiotics act primarily on eukaryotic cells (Table 25-4).
Eukaryotic protein synthesis can also be inhibited by tox-
ins of bacterial origin. An example is the toxin produced
by
C oryn ebacteriu m diph th eriae
bacteria carrying a lyso-
genic bacteriophage. Uninfected bacteria do not elaborate
the toxin, and therefore acquisition of the phage is essential
for the toxic effect.
Diphtheria
is an acute infectious dis-
ease usually localized in the pharynx, larynx, and nostrils
and occasionally in the skin. Initially, the cytotoxic effects
are restricted to those tissues immediately adjacent to the
bacterial growth. With increased bacterial growth, produc-
tion of toxin increases and is disseminated to the blood,
lymphatics, and other organ systems (e.g., cardiovascular
and nervous systems), where it brings about destructive
changes.
The pathogenesis of diphtheria is due entirely to the
toxin’s inhibition of protein synthesis in the host cells.
Diphtheria toxin consists of a single polypeptide chain
(M.W. 63,000) with two intramolecular disulfide linkages.
Toxin binds to the outer surface of susceptible cells at
specific sites and enters by receptor-mediated endocytosis.
The toxin is cleaved proteolytically into a small fragment,
A (M.W. 21,000), and a larger fragment, B (M.W. 42,000),
during its internalization. Fragment A is the biologically
active part of the molecule and presumably penetrates the
cell membrane with the aid of fragment B.
Upon entry into the cytoplasm, fragment A catalyzes the
ADP ribosylation of the transfer factor, EF-2, leading to its
inactivation and the interruption of protein synthesis. The
ADP-ribose group is donated by NAD+. The ADP ribosy-
lation reaction, catalyzed by the toxin, is specific for EF-2
of eukaryotic cells; other proteins of eukaryotic and bac-
terial cells are not substrates. This specificity is due to an
unusual amino acid residue in EF-2, diphthamide, which
is the acceptor of the ADP ribosyl group. Diphthamide
derives from the posttranslational modification of histi-
dine. The acute symptoms are treated with antitoxin. The
bacteria, which are gram-positive, succumb to a variety of
antibiotics, including penicillin. Diphtheria is effectively
prevented by immunization with toxoid (inactivated toxin)
preparations.
ADP ribosylation is also involved in the action of
cholera
toxin and in certain pathogenic strains of
E. coli
(Chapter 12). Cholera toxin catalyzes the ADP ribo-
sylation of the guanidinium. group of a specific argi-
nine residue in the guanine nucleotide-binding protein of
the adenylate cyclase. This activates adenylate cyclase,
which catalyzes the formation of cAMP from ATP. The
cAMP formed stimulates secretion of water and elec-
trolytes from intestinal epithelial cells. Thus, patients
infected with
Vibrio cholerae
secrete enormous quan-
tities (up to 20 L/d) of water and electrolytes. With-
out adequate and prompt replacement, death can en-
sue owing to dehydration and electrolyte imbalance.
Interestingly, in the case of diphtheria toxin, ADP-
ribosylation leads to inactivation of a protein, whereas with
cholera toxin, ADP ribosylation causes activation of the
protein.
A group of plant
lectins,
such as abrin, ricin, and mod-
eccin, are highly toxic to eukaryotic cells. Their mode of
action consists of inhibition of protein synthesis by en-
zymatically inactivating the EF-2 binding region of the
60S ribosomal subunit, whereas the diphtheria toxin in-
activates the EF-2 protein itself. Ricin is isolated from
castor beans and has a molecular weight of 66,000. Like
most plant and bacterial toxic proteins, ricin contains two
