ENZYMES 161 



(c) Transpeptidases 

 The general reaction catalysed by transpeptidases is the following : 



RCO— HNCHCO— NHR" + NH.X ^ 



R' 



RCOHNCHCO— NHX + NH2R" 



Certain peptidases catalyse these transpeptidations e.g. papain and 

 tryspin. 



(d) Transmethylases 



In general, in the biosphere, methyl groups can be transferred from a 

 given donor to a given acceptor in the following way: 



R— CH3 + R'— H ^ RH + R'— CH3 



An example of a catalyst for transmethylation is nicotine-methyl- 

 transferase which catalyses the reaction : 



active methionine + nicotinamide ■i=i- N-methylnicotinamide 



{e) Transacylases 



These are enzymes catalysing the transfer of acyl residues. Certain 

 enzymes catalyse the transfer of the acyl residue from its combination with 

 CoA (cofactor for transacylations) to an acceptor. 



Example : choline acetylase, catalyses the reaction 



acteyl-CoA + choline i? acetylcholine + CoA 



(/) Transadetiylases and Transuridylases 



These are enzymes which catalyse the transfer from a given donor to a 

 given acceptor of a purine or pyrimidine nucleotide residue. An example 

 is that ATP -^ nicotinamide — mononucleotide — transadenylase, catalysing 

 (in presence of Mg++) the reaction: 



ATP + nicotinamide mononucleotide ^ DPN + pyrophosphate 

 or UTP + glucose- 1 -phosphate ^ UDP-glucose + pyrophosphate 



[g) Transketolases {Glycolaldehyde- Transferases) 



They catalyse the transfer of a glycolaldehyde residue, for example in 

 the reaction : 



ribulose-5 -phosphate + ribose-5 -phosphate 



^ 3-phosphoglyceraldehyde + sedoheptulose-7-phosphate 



M 



