Pyruvate carboxylase, function

Jitrapakdee, S. Wallace, J.C. (1999) Structure, function, and regulation of pyruvate carboxylase. Biochem. J. 340, 1-16. 

A bound divalent metal ion, usually Mn2+, is required in the transcarboxylation step. A possible function is to assist in enolization of the carboxyl acceptor. However, measurement of the effect of the bound Mn2+ on 13C relaxation times in the substrate for pyruvate carboxylase indicated a distance of 0.7 ran between the carbonyl carbon and the Mn2+, too great for direct coordination of the metal to the carbonyl oxygen.68 Another possibility is that the metal binds to the carbonyl of biotin as indicated in Eq. 14-11. Pyruvate carboxylase utilizes two divalent metal ions and at least one monovalent cation.683... 

In bacteria and green plants PEP carboxylase (Eq. 13-53), a highly regulated enzyme, is responsible for synthesizing oxaloacetate. In animal tissues pyruvate carboxylase (Eq. 14-3) plays the same role. The latter enzyme is almost inactive in the absence of the allosteric effector acetyl-CoA. For this reason, it went undetected for many years. In the presence of high concentrations of acetyl-CoA the enzyme is fully activated and provides for synthesis of a high enough concentration of oxaloacetate to permit the cycle to function. Even so, the oxaloacetate concentration in mitochondria is low, only 0.1 to 0.4 x 10-6 M (10-40 molecules per mitochondrion), and is relatively constant.65 79... 

Latzko, E. Kelly, G.J. (1983). The many-faceted function of phosphoeno/pyruvate carboxylase in C3 plants. Physiologie Vegetale 21, 805-15. 

Oxaloacetate, the product of the pyruvate carboxylase reaction, functions both as an important citric acid cycle intermediate in the oxidation of acetyl CoA and as a precursor for gluconeogenesis. The activity of pyruvate carboxylase depends on the presence of acetyl CoA so that more oxaloacetate is made when acetyl CoA levels rise. 

Thus pyruvate carboxylase generates oxaloacetate for gluconeogenesis but also must maintain oxaloacetate levels for citric acid cycle function. For the latter reason, the activity of pyruvate carboxylase depends absolutely on the presence of acetyl CoA the biotin prosthetic group of the enzyme cannot be carboxy-lated unless acetyl CoA is bound to the enzyme. This allosteric activation by acetyl CoA ensures that more oxaloacetate is made when excess acetyl CoA is present. In this role of maintaining the level of citric acid cycle intermediates, the pyruvate carboxylase reaction is said to be anaplerotic, that is filling up. ... 

Pyruvate carboxylase is another enzyme which is not a part of the citric acid cycle per se but which functions in close association with it. The function of this enzyme is described in Chap. 11. but it is useful to consider its action, and that of pyruvate dehydrogenase, in relation to the citric acid cycle. 

Enzymes which contain Mn include pyruvate carboxylase, argi-nase, and superoxide dismutase. This paper will focus on the role of Mn as a component of Mn superoxide dismutase (MnSOD) and the functional significance of alterations in the activity of this enzyme. 

The important function of biotin is its role as coenzyme for carboxylase, which catalyses carbon dioxide fixation or carboxylation reaction. The epsilon amino group of lysine in carboxylase enzymes combines with the carboxyl group of biotin to form covalently linked biotinyl carboxyl carrier protein (BCCP or biocytin) (Figure 6.8). This serves as an intermediate carrier of carbon dioxide. The carboxylation of acetyl CoA to malonyl CoA in presence of acetyl CoA carboxylase requires biotin as coenzyme. Propionyl carboxylase and pyruvate carboxylase are also associated with biotin. 

In order for pyruvate carboxylase to be ready to function, it requires biotin. Mg", and Mn" ". It is allosterically activated by acetyl CoA. The biotin is not carboxylated until acetyl CoA binds the enzyme. By this means, high levels of acetyl CoA signal the need for more oxaloacetate. When ATP levels are high, the oxaloacetate is consumed in gluconeogenesis. When ATP levels are low, the oxaloacetate enters the citric acid cycle. Gluconeogenesis only occurs in the liver and kidneys. 

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