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QUESTION: What does a lectin look like? What is its composition? Molecular weight?
ANSWER: Since the 1880's, it has been known that extracts from certain plants could agglutinate red blood cells. In the 1940's, agglutinins were discovered which could "select" types of cells based on their blood group activities. Although "lectin" was originally coined to define agglutinins which could discriminate among types of red blood cells, today the term is used more generally and includes sugar-binding proteins from many sources regardless of their ability to agglutinate cells. Lectins have been found in plants, viruses, microorganisms and animals, but despite their ubiquity, their function in nature is unclear. Although lectins share the common property of binding to defined sugar structures, their roles in various organisms are not likely to be the same.
Lectins are structurally diverse; examination of the amino acid sequence, molecular size and other molecular properties show that lectins have little in common other than they are all proteins.
For example soybean agglutinin is a glycoproteln with no di-sulphide bond; Its molecular weight is 120,000, It consists of four subunits and has two binding sites.
Wheat germ agglutinin is not a glycoprotein and is rich in di-sulphide bonds with a molecular weight of 36,000, It has two identical subunits and four binding sites for sugars.
Thus we can see that lectins don't have much in common with each other (soy bean lectin is four times bigger than wheat germ lectins!) besides the ability to bind to specific glyco-conjugates, such as the ABO blood groups.