Gluconeogenesis Malic enzyme

Both dehydrogenases of the pentose phosphate pathway can be classified as adaptive enzymes, since they increase in activity in the well-fed animal and when insulin is given to a diabetic animal. Activity is low in diabetes or starvation. Malic enzyme and ATP-citrate lyase behave similarly, indicating that these two enzymes are involved in lipogenesis rather than gluconeogenesis (Chapter 21). 

Answer Oxaloacetate might be withdrawn for aspartate synthesis or for gluconeogenesis. Oxaloacetate is replenished by the anaplerotic reactions catalyzed by PEP carboxykinase, PEP carboxylase, malic enzyme, or pyruvate carboxylase (see Fig. 16-15, p. 632). 

Keto-6-phosphogluconate is a probable intermediate. The reaction is similar to those catalyzed by malic enzyme (in gluconeogenesis) and by isocitrate dehydrogenase (in the TCA cycle). 

Phenylalanine is converted to tyrosine by a hydroxylation reaction. Tyrosine, pro-dnced from phenylalanine or obtained from the diet, is oxidized, nltimately forming acetoacetate and fnmarate. The oxidative steps required to reach this point are, surprisingly, not energy-generating. The conversion of fnmarate to malate, followed by the action of malic enzyme, allows the carbons to be used for gluconeogenesis. The conversion of phenylalanine to tyrosine and the production of acetoacetate are considered further in section IV of this chapter. 

Insulin Liver Increased glucokinase increased glucose uptake Increased glycogen synthase activity glycogen deposition Inhibited gluconeogenesis Increased malic enzyme, acetyl-CoA carboxylase, fatty acid synthetase and stearoylCoA desaturase increased lipogenesis Release of VLDL from hepatocytes... 

Cocoa bean fermentation is a mixed-culture process, consisting initially of fermentations by yeast and lactic acid bacteria followed by oxidation of the fermentation products ethanol and lactic acid into acetic acid and acetoin by several Acetohacter strains, of which /I. pasteurianus is the prominent one (Moens et al. 2014). A C-based carbon flux analysis of Acetohacter during cocoa pulp fermentation-simulating conditions revealed a functionally separated metabolism during co-consumption of ethanol and lactate. Acetate was almost exclusively derived from ethanol, whereas lactate served for formation of acetoin and biomass building blocks. This switch was attributed to the lack of phosphoenolpyruvate carboxykinase and malic enzyme activities, which prevents conversion of oxalo-acetate and malate formed by acetate metabolism in the TCA cycle to PEP and pyruvate and subsequently to acetoin (Adler et al. 2014). Lactate, on the other hand, can be converted to pyruvate, which is then used for acetoin formation or, after conversion to PEP by pymvate phosphate dikinase, for gluconeogenesis. The inability of conversion of TCA cycle intermediates to PEP resembles the situation in G. oxydans, where in addition no enzyme for conversion of pyruvate to PEP is present. 

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