Phosphoenolpyruvate carboxykinase degradation

Most cestodes which have been investigated, however, conform to the second category, type 2, which is characterised by a C02-fixation step. Carbohydrate is degraded to the level of PEP by glycolysis, the steps involved being similar to those in mammalian tissue. At this point, the enzymes pyruvate kinase and phosphoenolpyruvate carboxykinase (PEPCK) compete for available substrate and a branch-point occurs (Fig. 5.4). The relative activities of these two enzymes determine the fate of the PEP and the subsequent types and amounts of end-products formed (see below). 

J. M. Gunn, F. J. Ballard, and R. W. Hanson, Influence of hormones and medium composition on the degradation of phosphoenolpyruvate carboxykinase (GTP) and total protein in Reuber H35 cells. J Biol Chem 251 3586-3593 (1976). 

PCK1 Phosphoenolpyruvate carboxykinase, key enzyme in gluconeogenesis, catalyzes early reaction in carbohydrate biosynthesis, glucose represses transcription and accelerates mRNA degradation, regulated by Mcmlp and Cat8p, located in the cytosol... 

Enzymes present in the liver cytosol with short half-lives include ornithine decarboxylase, thymidine kinase, tyrosine aminotransferase, tryptophan oxygenase, hydroxymethylglutaryl-CoA reductase, serine dehydratase, and phosphoenolpyruvate carboxykinase. All of these enzymes have degradation rate constants greater than 0.1/h—more than 10 times more rapid than the average ka for liver cytosol proteins (Schimke, 1970). Perhaps a scrutiny of the group can provide information on the enzyme properties as well as the nature of reactions catalyzed by enzymes with rapid turnover rates. 

Although insulin decreases protein breakdown in cultured cells, it has no effect on the degradation rate constants of two enzymes with short half-lives, phosphoenolpyruvate carboxykinase (Gunn et al., 1976) and tyrosine aminotransferase (Reel et al., 1970). This negative result is in keeping with the concept that insulin acts by reducing auto-phagy, a process that seems trivial in the breakdown of proteins with short half-lives (Knowles and Ballard, 1976). 

Figure 3. Appearance of phosphoenolpyruvate carboxykinase in developing rat liver, (a) The activity increase is expressed as units of enzyme per total liver, (b) Enzyme synthesis ( ) is expressed as the percent of radioactivity in the enzyme pool as compared to radioactivity in cytosol protein after injections of radioactive leucine. Degradation (o), in the same terms, is shown at various times after a leucine chase was given. The half-lives at each age are indicated. Details are given by Philippidis et al. (1972). Values are means SEM. T, term.

Attempts to measure intermediates in the in vivo degradation of proteins have been restricted by the technical limitations outlined in Section VI. One approach, the use of specific antibodies to search for breakdown products of phosphoenolpyruvate carboxykinase or ornithine aminotransferase, gave no indications of an accumulation of intermediates in vivo, even though the antibody preparations could be expected to react and precipitate fragments of the enzymes (Ballard et al., 1974 Kominami and Katunuma, 1976). However, the method is rather crude, since an intermediate would need to make up a few percent of the starting enzyme in order to be detected. More satisfactory results can be expected when pure labeled proteins are inserted into cells and the accumulation of all radioactive products is followed. 

In this malate dismutation pathway, carbohydrates are degraded to phosphoenolpyruvate (PEP) via the classical glycolytic pathway. This PEP is then carboxylated by PEP carboxykinase (PEPCK) to oxaloacetate, which is subsequently reduced to malate. This malate is transported into the mitochondria and is degraded in a split pathway. A portion of the malate is oxidized to acetate and another portion is reduced to succinate, which can then be further metabolized to propionate (Fig. 20.1). 

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