Biology Reference
In-Depth Information
TABLE 12.1
Specificity For Some Common Lectins.
Abbreviation
Name
Carbohydrate bound
Con A
Concanavalin A
Mannose, Glucose
GNA
Snowdrop Lectin
Mannose
RCA
Ricinus communis
Galactose, N-Acetyl Glucosamine
Agglutin
PNA
Peanut Agglutin
Galactose, N-Acetyl Galactosamine
WGA
Wheat Germ
Sialic Acid, N-Acetyl Glucosamine
Agglutin
UFA
Aleuria europaeus
Fucose
Agglutin
bind to a specific simple sugar to many that bind to di-, tri-, and even polysaccharides.
Table 12.1
lists a few common lectins with their carbohydrate specificity. This table is
overly simplified as several of the lectins actually bind to a di- or tri-polysaccharide con-
taining the listed sugars and not just the simple sugar itself. Lectins are bound to carbohy-
drates by many weak reversible non-covalent bonds
[35,36]
, similar to binding of
a substrate to the active site of an enzyme. Most lectins actually share little structural simi-
larity as demonstrated by their amino acid sequence, molecular size, and other molecular
properties. It is hard to comprehend how these widely varying structures can all bind
carbohydrates.
Many free lectins are commercially available and several are even available conjugated to
inert matrices that can be packed into columns for use in lectin-affinity chromatography.
Conjugation to the inert matrix, often Sepharose or Agarose, does not affect the lectin's ability
to bind to specific sugars, and so is useful for membrane isolations. Sugar binding is also not
affected by detergents, compounds that are essential for membrane protein isolation (Chapter
13). Lectin-affinity chromatography consists of pouring the cell homogenate over a lectin
column. Membranes that do not have the requisite sugar selected by the lectin simply pass
through the column and are removed. The bound membranes are washed and then released
from the column by a solution containing a high concentration of the simple sugar that the
lectin recognizes. A single pass over an appropriate lectin-affinity column can greatly purify
a mixed membrane population in one step. However, this procedure only works for
membrane fractions that have sugars on their surfaces, mostly plasma membrane microsomes.
The fact that a simple sugar can block the attachment of a lectin to its normal polysaccha-
ride target can have important medical implications. For example, a urinary tract infection
can result from binding of
E. coli
to a resident urinary tract lectin
[37]
. It was noted that
the
E. coli
infection can be greatly reduced by administrating mannose which dislodges
the
E. coli
. This may explain why cranberry juice, rich in mannose, has historically been
used to prevent bladder infections.
