Enzymes phosphoenol pyruvate carboxylase

An intriguing stress-induced alteration in gene expression occurs in a succulent plant, Mesembryanthemum crystallinum, which switches its primary photosynthetic CO2 fixation pathway from C3 type to CAM (Crassulacean acid metabolism) type upon salt or drought stress (Winter, 1974 Chapter 8). Ostrem et al. (1987) have shown that the pathway switching involves an increase in the level of mRNA encoding phosphoenol-pyruvate carboxylase, a key enzyme in CAM photosynthesis. 

Figure 10 A hypothetical model of carbon acquisition in the marine diatom Thalassiosira weissflogii. Solid circles represent transporters. Catalyzing enzymes CA, carbonic anhydrase PEPC, phosphoenol pyruvate carboxylase PEPCK phospoenol pyruvate carboxykinase RUBISCO, Ribulose 1-5 bisphosphate Carboxylase Oxygenase (after...

Figure 5 Model of phosphorus (P) deficiency-induced physiological changes associated with the release of P-mobilizing root exudates in cluster roots of white lupin. Solid lines indicate stimulation and dotted lines inhibition of biochemical reaction sequences or mclaholic pathways in response to P deliciency. For a detailed description see Sec. 4.1. Abbreviations SS = sucrose synthase FK = fructokinase PGM = phosphoglueomutase PEP = phosphoenol pyruvate PE PC = PEP-carboxylase MDH = malate dehydrogenase ME = malic enzyme CS = citrate synthase PDC = pyruvate decarboxylase ALDH — alcohol dehydrogenase E-4-P = erythrosc-4-phosphate DAMP = dihydraxyaceConephos-phate APase = acid phosphatase.

Hydroxycyclopropanecarboxylic acid phosphate HCP 34 is an analogue of phosphoenolpyruvate (PEP) 35 which is metabolized by various enzymes. HCP 34 is a potent competitive inhibitor of enzymes utilizing PEP 35, such as PEP carboxylase, enolase, pyruvate kinase, and probably other enzymes. It is a substantially better inhibitor than phospholactate 36 or phosphoglycolate 37, presumably because of the similarity of its geometric and electronic structures with phosphoenol pyruvate,Eq. 12 [28]. 

From all the hypotheses mentioned above, the low flux of pyruvate into the TCA cycle is better explained by a decreased activity of several enzymes pyruvate carboxylase, pyruvate phosphoenol carboxykinase, pyruvate dehydrogenase, and malic enzyme II (Figure 4.3). Then, the pyruvate accumulated is converted to lactate by the enhanced catalytic action of the lactate dehydrogenase, as an alternative pathway to generate energy for cellular processes. 

Today the metabolic network of the central metabolism of C. glutamicum involving glycolysis, pentose phosphate pathway (PPP), TCA cycle as well as anaplerotic and gluconeogenetic reactions is well known (Fig. 1). Different enzymes are involved in the interconversion of carbon between TCA cycle (malate/oxaloacetate) and glycolysis (pyruvate/phosphoenolpyruvate). For anaplerotic replenishment of the TCA cycle, C. glutamicum exhibits pyruvate carboxylase [20] and phosphoenol-pyruvate (PEP) carboxylase as carboxylating enzymes. Malic enzyme [21] and PEP carboxykinase [22,23] catalyze decarboxylation reactions from the TCA cycle... 

In the C4 pathway, CO2 in the form of HCO3- reacts with phosphoenol-pyruvate (PEP) via the enzyme PEP carboxylase located in the cytosol of the mesophyll cells (Fig. 8-15b).8 The initial product is oxaloacetate, which is rapidly converted to malate and aspartate. For all chloroplasts in photo-respiring (C3) plants, and for the chloroplasts in the bundle sheath cells of C4... 

Pyruvate Carboxylase. This enzyme has manganese firmly in its structure and acts together with phosphoenol pyruvate (PEP) carboxykinase, an enzyme that is activated by manganese ions. These enzymes are required to catalyze the formation of PEP from pyruvate, a key reaction in the hepatic synthesis of glucose. 

Pyruvate carboxylase is a mitochondrial enzyme, whereas the other enzymes of gluconeogenesis are present primarily in the cytoplasm. Oxaloacetate, the product of the pyruvate carboxylase reaction, must thus be transported to the cytoplasm to complete the pathway. Oxaloacetate is transported from a mitochondrion in the form of malate oxaloacetate is reduced to malate inside the mitochondrion by an NADH-linked malate dehydrogenase. After malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD -linked malate dehydrogenase in the cytoplasm (Figure 16.26). The formation of oxaloacetate from malate also provides NADH for use in subsequent steps in gluconeogenesis. Finally, oxaloacetate is simultaneously decarboxylated and phospho-ry lated by phosphoenolpyruvate carboxy kinase to generate phosphoenol pyruvate. The phosphoryl donor is GTP. The GO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. 

PEPC catalyses carboxylation of phosphoenol pyruvate (PEP), employing bicarbonate as carboxyl donor (Cooper and Wood, 1971). As in all such enzymes employing bicarbonate in place of CO2, the enzyme must activate bicarbonate towards carboxyl transfer. In this respect, as in others, PEPC resembles biotin-dependent carboxylases. Two distinct mechanisms are postulated for PEPC one involving the intermediacy of carboxyphosphate, the other a cyclic transition state and pseudorotation at phosphorus. 

Several enzymes are common to both the glycolytic and gluconeogenic pathways, but four enzymes catalyse steps that only occur in gluconeogenesis pyruvate carboxylase (reaction 3.6), phosphoenol-... 

Phosphoenol pyruvate carboxykinase and pyruvate carboxylase are the major anaple-rotic enzymes supplying the precursor for the aspartate family of amino acids. Engineering of these two enzymes resulted in redirecting the anaplerotic flux toward the amino... 

Phosphoenol acetylphosphonate (48) and its dimethyl ester (49) were examined as potential inhibitors of three phosphoenopyruvate (PEP) enzymes ". Both compounds inhibited enolase and PEP carboxylase but they did not bind to pyruvate kinase. Neither compounds was a substrate for any of the enzymes. 

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