REACTIONS OF CARBON DIOXIDE 95 



tained a cr^^stalline preparation of an oxalacetate decarboxy- 

 lase from the bacterium Micrococcus lysodeikticus. Attempts 

 have been made to couple an oxalacetic decarboxylase with 

 a malic dehydrogenase system in the presence of reduced 

 coenzyme I, so that by starting with pyruvate, CO2 and 

 DPNH any oxalacetate formed would be removed by reduc- 

 tion to malate. The failure to produce any reaction with 

 carbon dioxide served to show that if any oxalacetate was 

 formed the concentration was too small to allow any reaction 

 with the DPN-dependent malic system. Thus the decarb- 

 oxylase was to be regarded as virtually irreversible. This 

 also serves to emphasize the distinct nature of the reversible 

 'malic enzyme' system of Ochoa, that it is not a separable 

 combination simply of a decarboxylase and a dehydrogenase. 

 Phosphoryl-enolpyruvate carboxylase. Bandurski and Grei- 

 ner (1953) have described an enzyme obtained from spinach 

 leaves which catalyses the reaction: 



COOH COOH 



H2O + C— O— PO3H2 + CO2 = CO + H3PO4 (6.5) 



Inorganic 

 phosphate 



CH2 CH2 Inorganic 



Phosphoryl- COOH 



enolpyruvate 



Oxalacetate 



The phosphoryl-enolpyruvate can be formed from pyruvate 

 and ATP in presence of a specific phosphokinase. 



HOOC— CO— CH3 + ATP 



= HOOC— C(— OP03H2):=CH2 + ADP (6.6) 



Hence by combining equations (6.5) and (6.6) the reversal 

 of equation (6.4) results, together with the hydrolysis of one 

 phosphate group from ATP giving inorganic phosphate and 

 ADP. The incorporation of CO2 into pyruvate may thus be 

 accomplished by coupling with either of two processes: (i) 

 by hydrogen transfer via the formation of malate, and (2) by 

 phosphate group transfer via the production of phosphoryl- 

 enolpyruvate. The significance of coupled reactions, of 

 which this last mentioned system is an example, is discussed 



