CHAPTER 5 

 Plant Agglutinins (Lectins) I 



Specificity of Proteins Other Than Antibodies 



Antibodies are not the only large molecules with specific biological 

 activity. Enzymes (also proteins) and hormones, many of which are 

 proteins, also exhibit this phenomenon. Enzymes, outstanding ex- 

 amples in this respect, show various degrees of specificity. Some en- 

 zymes, such as barley maltase and succinic acid dehydrogenase, are 

 very specific, catalyzing one reaction and only one reaction. Other 

 enzymes are specific for a particular chemical grouping in their sub- 

 strate. Enzymes catalyzing reactions in which optically active sub- 

 stances such as sugars or amino acids are involved frequently act 

 primarily or exclusively on one of the enantiomorphs. Even enzymes 

 such as trypsin attack only certain linkages in their substrates. En- 

 zymes can be inhibited by an excess of one of the products of the 

 reaction they catalyze, a behavior reminiscent of the specific inhibition 

 of antibody-antigen reactions by haptens. 



Although enzymes resemble antibodies in many ways, there are 

 striking differences. An enzyme combines with its substrate and then 

 catalyzes a chemical reaction in which the substrate is involved. The 

 result is often complete destruction of the substrate. Antiliodies, on 

 the other hand, have no known catalytic activity and do not them- 

 selves cause chemical changes in the antigens with which they com- 

 bine. 



Other proteins exhibit specific comljining power. Serum albumin 

 has the power of binding certain dyes and a number of other natural 

 and synthetic substances (Klotz, Walker, and Pivan, 1946; Karush, 

 1950). 



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