SUBSTRATE SPECIFICITY 25 



chemical catalysts in that they are thermolabile or destroyed 

 by heat. A number of enzymes have now been isolated in a 

 pure and even crystalline state and are found to be proteins 

 the structure of which is altered (or denatured) by heat. 



The substance whose chemical change is catalysed by an 

 enzyme is said to be the '' substrate " of that enzyme and 

 the majority of enzymes display a strict specificity towards 

 their substrate. This specificity may be such that the 

 enzyme can catalyse the alteration of one substance only; 

 thus succinic dehydrogenase is an enzyme which will catalyse 

 the removal of hydrogen from succinic acid to form fumaric 

 acid: 



CH2.COOH Succinic CH.COOH 



I - 2H > II 



CHo . COOH dehydrog. CH . COOH 



but the enzyme is specific towards succinic acid and will 

 dehydrogenate no other substance. Where the substrate 

 exists in two or more isomeric forms the enzyme is usually 

 specific for One isomer only; thus L-lysine decarboxylase will 

 decarboxylate L-lysine to cadaverine but is unable to attack 

 D-lysine or any other amino-acid: 



H2N.CH2.(CH2)3.CHNH2.COOH - CO2 > 



L-lysine 



H2N . CH2 . (CH2)3 . CH2 . NH2 



Cadaverine 



Such optical specificity has been used in the past for the 

 resolution of racemic mixtures since the enzyme will attack 

 one isomer and leave the other intact. The specificity may, 

 on the other hand, be less restricted so that an enzyme may 

 catalyse the alteration of any substance containing a certain 

 chemical grouping; thus some proteases may hydrolyse any 

 substance having the linkage — CO — NH — in its structure 

 and will hydrolyse proteins and peptides down to amino-acids. 

 Other proteases, however, will attack the peptide link only 

 if the residues R and S on either side of the link R — CO — 

 NH — S have specific structures. 



