166 UNITY AND DIVERSITY IN BIOCHEMISTRY 



G. ISOMERASES AND RaCEMASES 



Racemases catalyse the conversion of an optically active substance into 

 its racemate. An example is alanine racemase 



D-alanine ^ L-alanine 

 glutamic racemase 



D-glutamic acid ^ L-glutamic acid 

 and mutarotase 



a-glucose ^ ^-glucose 

 Isomerases catalyse molecular rearrangements, either by modifying the 

 structure of part of the molecule, or by displacing a part of it. To the first 

 class belong the following enzymes : 



Reaction catalysed 

 Galactowaldenase Gal-l-P ^ G-l-P (cofactor : uridine- 



diphosphate-glucose) 

 Phosphoribose — isomerase Ribose-5-P ^ ribulose -5-P 



Triosephosphate^ — isomerase Dihydroxyacetone-P ^ 3-phospho- 



D -gly ceraldehy de 

 To the second class belong : 



Phosphoribumutase Ribose-1-P ^ ribose-5-P 



Phosphoglucomutase G-l-P ^ G-6-P (cofactor : G-1, 6-PP) 



Phosphoglyceromutase 2-phospho-D-glycerate ^ 3-phospho- 



D-glycerate 

 (2-phospoglyceric acid) (3-phospho- 



glyceric acid) 

 (cofactor : 2, 3-diphosphoglyceric acid) 



IV. KINETICS 



The velocity of an enzymatic reaction can be measured by following the 

 change in concentration (increase or decrease) of one of the substances 

 involved in the catalysed reaction, either a reactant or a product of the reaction. 



For example, consider the hydrolysis of the amide groups of urea or 

 carbamide CO(NH2)2, a hydrolysis catalysed by a specific enzyme (the 

 amidase named urease) : 



CO(NH2)2 + H2O -> CO2 + 2NH3 



Having fixed the conditions (pH, temperature, urea concentration, urease 

 concentration), we may remove samples of the solution, at definite intervals 

 of time, inactivate the enzyme and determine, say, the ammonia. 



Now if we plot these quantities of ammonia nitrogen as ordinates against 

 time intervals as abscissae, we shall obtain the graph shown in Fig. 31. 

 From the graph we can obtain the reaction velocity in moles/1. /sec. It is 

 the initial reaction velocity which is of interest, since as the reaction pro- 

 gresses the conditions change as a result of the disappearance of the 

 substrate and the accumulation of the reaction products, etc. 



