PEP carboxytransphosphorylase

Reaction 3.1, the key reaction of propionic acid fermentation, is catalyzed by pyruvate carboxytransphosphorylase, a unique biotin-dependent transcarboxylase (see below). There are other reactions of carboxyl group transfer catalyzed by phosphoenolpyruvate (PEP) carboxytransphosphorylase and phosphoenolpyruvate carboxykinase, but these (i) do not require biotin and (ii) use CO2 as the source of carboxyl groups. The actual species involved may be HCO3" (or H2CO3) rather than free CO2 (Cooper et al., 1968), since free CO2 is not evolved in the PEP carboxytransphosphorylase reaction (Swick and Wood, 1960). Propionic acid bacteria are able to decarboxylate succinate, producing CO2 in a biotin-dependent reaction (Delwiche,1948 Lichstein, 1958). If succinate is accumulated as the end product, then the cycle (see Fig. 3.1) is broken, and oxaloacetic acid is not supplied by reaction 3.1, but is formed primarily by CO2 fixation onto PEP catalyzed by PEP carboxytransphosphorylase (PEP-CTP). 

PEP carboxytransphosphorylase. The enzyme catalyzes the first reaction in the C02-fixation sequence, the major C02-fixing mechanism in propionibacteria ... 

It is assumed that both the reactions are catalyzed by the complex of PEP-Pi-enzyme. CO2 competes for the complex, driving the reaction towards oxaloacetate and thus decreasing the rate of pyruvate production. Reaction 3.11 is irreversible under experimental conditions, but reaction 3.10 is reversible and interesting in that PPi can be used to form PEP from pyruvate (Davis and Wood, 1966). Therefore, the PPi derived from ATP can be reutilized, thus acting as a control mechanism for PEP preservation. And since PPi strongly inhibits the PEP carboxytransphosphorylase reaction, PEP can be diverted to the Krebs cycle (Frings and Schlegel, 1970). 

The reaction of CO2 fixation onto phosphoenolpyruvic acid by PEP carboxytransphosphorylase is considered (O Brien and Wood, 1974) as a control mechanism of propionic acid fermentation. They observed a conversion of the enzymatically active tetrameric form of PEP carboxytransphosphorylase isolated from P. shermanii into a less active dimeric form induced by oxalate, malate and fumarate. Therefore, the loss of activity by enzyme dissociation, accompanied by increased proteolysis, is an effective means of controlling the level of intermediates in propionic acid fermentation. Differential abilities of propionibacteria to fix CO2 could be associated (Wood and Leaver, 1953) with their abilities to carry out the reaction C02 Ci and to form sulfhydryl complexes with Ci. 

The uniqueness of propionic acid fermentation is due to the participation of PEP carboxytransphosphorylase, the enzyme not found in the other organisms that synthesize propionate. Due to the presence of this enzyme the propionic acid fermentation functions as a cyclic process (for the significance of cycling, see above). Another peculiarity of this fermentation is related to the way propionate is formed, which is coupled with the... 

---