Chemistry Reference
In-Depth Information
location of lectins is in the so- called protein bodies - membrane - lined cell organ-
elles which are derived from the vacuole. At this site, the lectins are associated
with storage proteins, also enzymes and phytin. The amounts present in seeds
and other storage organs vary considerably, ranging from more than 1 g (ConA,
PHA) to a few milligrams (lentil, gorse) per 100 g of plant material. Commonly,
the lectin contents in plants are thus much higher than for animal lectins, which
often require recombinant expression to enable structural studies. As noted above,
ConA with its exceptionally high amount in beans could even be purifi ed by crys-
tallization (please see Info Box in Chapter 16). Its biosynthesis encompasses a
pathway with a remarkable deviation from the common route (for details, please
see Info Box 2). As we have highlighted present gaps in our knowledge on lectin
occurrence in plants, with its current preference on cultivated plants, readers
might be motivated to enter this fi eld. Thus, we next provide the strategic informa-
tion on lectin isolation.
Figure 18.1
Distribution of lectins in the plant
kingdom. For each family, the number of lectin-
bearing species is given in parentheses.
Info Box 2
It is regarded as a foregone conclusion that alignment of sequences from
homologous proteins will be simple, yielding similarity from the fi rst amino
acids onward. This is not the case for ConA when compared to other legumi-
nous lectins such as SBA despite their conspicuous homology. Puzzling at fi rst,
these sequences reach a maximal sequence similarity only if residue 1 of ConA
is aligned to residue 112 in SBA - a phenomenon called circular permutation.
As a consequence, the
N
-terminal stretch of SBA and the sequence starting at
position 122 in ConA are homologous.
When this - at fi rst encounter - strange relationship was discovered [B.A.
Cunningham
et al.
Favin versus concanavalin A: circularly permuted amino
acid sequences.
Proc Natl Acad Sci USA
1979;
76
, 3218 - 3222; J.J. Hemperly
and B.A. Cunningham. Circular permutation of amino acid sequences among
