Biology Reference
In-Depth Information
Carbohydrate Asymmetry
The primary function of membrane-bound carbohydrates is to interact with the
extra-cellular environment by providing the cell surface with a unique sugar identity
(Chapter 7). Function strongly implies, but does not prove, that carbohydrates should face
outside the cell. Fortunately, carbohydrates are relatively easy handles to access and label,
and so a variety of methodologies are available. The best and most employed methodology
involves lectins.
Lectins
Lectins are small, sugar-binding proteins that are highly specific for their sugar ligand [6] .
In fact, the term 'lectin' is derived from the Latin word legere meaning 'to select'. Lectins are
ubiquitous in living organisms and are often referred to as 'agglutins' due to their ability to
aggregate animal cells. Lectins are non-enzymic in action and non-immune in origin. Their
most common biological role is assisting in various kinds of cell recognition events. In addi-
tion, lectins can target mannose-6-phosphate-containing hydrolytic enzymes to the lysosome,
play important roles in the immune system by recognizing carbohydrates that are found
exclusively on target pathogens and clear certain glycoproteins from the circulatory system.
Lectins are also routinely used for blood typing.
The lectin Ricin, isolated from the seeds of castor bean, is currently employed as a highly
toxic biochemical warfare agent. A famous incident involving ricin occurred in 1978 when
the Bulgarian dissident writer Georgi Markov was stabbed in his calf by an umbrella while
waiting for a bus on the Waterloo Bridge in London. The umbrella, wielded by a member of
the Soviet KGB, injected a fatal dose of ricin into Markov's calf [7] .
In addition to their many important biological functions, lectins have been employed as
biochemical tools to study membranes. Lectins are routinely used in affinity chromatog-
raphy, affinity electrophoresis, and affinity immunoelectrophoresis to isolate glycoproteins
and plasma membrane vesicles. This aspect of using lectins as biochemical tools in
isolating plasma membranes is discussed in Chapter 12. There are now hundreds of
commercially available lectins, the most common being concanavalin A, or Con A. A brief
list of commonly used lectins and their sugar ligands can be found in Table 12.1. Unmod-
ified lectins are, of course, not detectable and so would be useless for membrane imaging
studies. Therefore a variety of tags have been successfully attached to lectins and many of
these are now commercially available. Included in the modified lectins are fluorescent-
labeled (visible light), radio-labeled (autoradiography) and heavy metal-labeled (electron
microscopy) lectins. All lectin-based imaging studies confirm that carbohydrates are
100% asymmetrically distributed across the membrane with sugars always facing the
outside.
Lipid Asymmetry
It probably surprised no one that proteins and carbohydrates were shown to be 100%
asymmetrically distributed across membranes. Their functions would strongly imply this.
However, it was not as evident what to expect of lipid asymmetry. The major function of
membrane lipids is to provide the fundamental bilayer that serves as the barrier controlling
leakiness as well as the environment to support membrane biochemical activity. There is no
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