Pyruvate 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). 

Pyruvate carboxytransphosphorylase (PCTP). The enzyme, frequently referred to as the transcarboxylase, catalyzes the carboxyl transfer reaction 3.1 without an intermediary of CO2 or any significant expenditure of energy ... 

Pyruvate carboxytransphosphorylase contains about 1.5 pg of biotin per mg of protein. Under low ionic strength and alkaline conditions the enzyme dissociates spontaneously into the inactive subunits. There are two species of subunits, one contains biotin, and the other contains metals, Co and Zn (Wood et al., 1963 Northrop and Wood, 1969 Gerwin et al., 1969). The transcarboxylase activity with succinyl-CoA is 10-15 times lower than with methylmalonyl-CoA. Specificity to carboxyl donors is fairly broad, with the Km values for propionyl-CoA of 1.0 mM for acetyl-CoA, 0.5 mM for butyryl-CoA, 0.1 mM and for acetoacetyl-CoA, 0.025 mM. The broad specificity to the CoA esters enables the carboxyl group of oxaloacetate to enter the citric acid cycle or to be used in the synthesis of fatty acids. 

This enzyme [EC 4.1.1.38] (also known as phosphoenolpyruvate carboxytransphosphorylase, phosphopyruvate carboxylase, and phosphoenolpyruvate carboxylase) catalyzes the reaction of phosphoenolpyruvate with orthophosphate and carbon dioxide to produce oxaloacetate and pyrophosphate (or diphosphate). The enzyme also catalyzes the reaction of phosphoenolpyruvate with orthophosphate to produce pyruvate and pyrophosphate. 

Phosphoenolpyruvate.—Phosphoenolpyruvate carboxytransphosphorylase catalyses two separate conversions of phosphoenolpyruvate (6) (Scheme 3). In the absence of carbon dioxide pyruvate and inorganic pyrophosphate may be formed enzymic dephosphorylation of (6) to enolpyruvate is probably followed by the non-enzymic protonation of the latter giving rise to pyruvate. 

Cooper TG, Tchen TT, Wood HG, Benedict CR. (1968). The carboxylation of phospho-enolpyruvate and pyruvate. I. The active species of COj utilized by phosphoenolpyruvate carboxykinase, carboxytransphosphorylase, and pyruvate carboxylase. J Biol Chem, 243, 3857-3863. 

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). 

Davis JJ, Willard JM and Wood HG (1969) Phosphoenolpyruvate carboxytransphosphorylase. ni. Comparison of the fixation of carbon dioxide and the conversion of phosphoenolpyruvate and phosphate into pyruvate and pyrophosphate. Biochemistry 8 3127-3136... 

---