Chemistry Reference
In-Depth Information
this stop codon would abort translation, its presence explained why human PMNs
lacked
-defensin peptides. Later we found that the
DEFT
genes of chimpanzees
and gorillas carried an identical silencing mutation, but most orangutan
DEFT
genes were intact [2]. This time-line suggests that our ancestors had lost the ability
to make
θ
θ
-defensins at least 7.5 million years ago.
Figure 26.3
- Defensins. RC - 1 appears in pan-
els (a), (c) and (d), and RTD-1 in panel (b). The
four RTD-1 residues that differ from those in
RC-1 are marked with an asterisk. Structural
θ
hallmarks of
-defensins include a cyclic back-
bone, a tridisulfi de ladder that connects their
antiparallel
θ
-sheets, at least four arginines and
no anionic residues.
β
From the RTD studies, we learned which pro-peptide residues end up in the
mature peptide. Our human bone marrow cloning studies had exhumed an ances-
tral
-defensin mRNA [3] and we later found additional human
DEFT
pseudogenes
by database searching [2]. The retrieved sequence information allowed us to syn-
thesize
θ
-defensins perhaps last produced by an ancestral protohominid. We
named the new/old peptides retrocyclins (RCs) to commemorate their antiquity
(retro = backwards or behind) and their cyclic backbone. RC-1 and RTD-1 are
compared in Figure 26.3. The antimicrobial activity of RC-1 resembled that of
HNP-1-3. However, when we found that RC peptides protected human cells from
HIV-1 infection this became a focus of our work and led us to recognize their
lectin-like properties, which are illustrated below in an old-fashioned way.
θ
26.5
Hemagglutination
The historical survey in Chapter 15 reminds us of the important role of hemag-
glutination assays in the classical era of lectin research. Although we now have
many more ways to examine protein-carbohydrate interactions (please see Chapter
14), hemagglutination assays retain the virtue of simplicity. Accordingly, we tested
the ability of 22 different defensins to agglutinate human red blood cells. Mindful
of the century-old advice of Silas Weir Mitchell, a distinguished neurologist, sci-
