The Protein Phosphatases Involved in Cellular Regulation. 3. Fatty Acid Synthesis, Cholesterol Synthesis and Glycolysis/Gluconeogenesis

Abstract

The nature of the protein phosphatases involved in the regulation of glycolysis/gluconeogenesis, fatty acid synthesis and cholesterol synthesis in rat liver has been investigated using L-pyruvate kinase, ATP-citrate lyase, acetyl-CoA carboxylase and hydroxymethylglutaryl-CoA reductase as substrates. The results show that protein phosphatases-1, 2A and 2C are the only significant protein phosphatases in rat liver acting on these four substrates. The relationship of these three enzymes to other protein phosphatases described in the literature is discussed.Glycolysis, gluconeogenesis and the synthesis of lipids (fatty acid and cholesterol) are under hormonal control; several key regulatory enzymes in these metabolic pathways are known to be controlled by phosphorylation-dephosphorylation reactions. The conversion of phosphoenolpyruvate to pyruvic acid by L-pyruvate kinase is an important control point in both glycolysis and gluconeogenesis in mammalian liver. L-pyruvate kinase is phosphorylated on a single serine residue by cyclic-AMP-dependent protein kinase, increasing its K , for phosphoenolpyruvate and decreasing its Ki for ATP and alanine.

Glucose Controls Phosphoregulation of Hydroxymethylglutaryl Coenzyme A Reductase through the Protein Phosphatase 2A-related Phosphatase Protein, Ppe1, and Insig in Fission Yeast

“…This is consistent with Sds23 acting as a negative regulator of Ppe1 (12). Mammalian HMGR is also regulated by PP2A (26). Although Hmg1 regulation can occur in the absence of AMPK (Fig.…”

Insulin stimulates the dephosphorylation and activation of acetyl-CoA carboxylase.

“…The alterations in citrateindependent activity, maximal enzyme velocity (at maximal stimulating citrate concentrations), and Ka for citrate observed in the present study in response to insulin and the factor are entirely consistent with the demonstration of enzyme dephosphorylation, based on analogy to the protein phosphatase experiments. Each of these kinetic parameters has been shown to be influenced in the same way by in vitro dephosphorylation of AcCoACase (7,17,(20)(21)(22)(23)(24).…”

“…Therefore, dephosphorylation of these sites would be expected to increase AcCoACase activity. This same phosphorylation site(s) also appears to be phosphorylated in response to glucagon and epinephrine in rat cells coincident with enzyme inactivation (1,3) Dephosphorylation of AcCoACase in vitro by various protein phosphatases has long been recognized to lead to enzyme activation (4,7,11,17,(20)(21)(22)(23). This mechanism also has been demonstrated directly to occur in vivo in rat mammary tissue with alterations in nutrition (23) and has been postulated to occur in vivo in response to an acute glucose challenge (17,24).…”

The protein phosphatases involved in cellular regulation. Glycolysis, gluconeogenesis and aromatic amino acid breakdown in rat liver

“…The protein phosphatase 2C/2A1 region from DEAE-cellulose It has previously been established that the phosphorylase phosphatase activity in this region is entirely accounted for by protein phosphatase-2A1 [26], and that the PK phosphatase activity is also largely attributed to this enzyme, with a minor contribution from protein phosphatase-2C [27]. To determine which protein phosphatases accounted for the PFlK phosphatase, PF2K/F2,6Pase phosphatase, F1,6Pase phosphatase and PAH phosphatase activities observed in these fractions, aliquots were subjected to gel filtration on TSK G3000SW HPLC columns and assayed in the presence of 1.0 mM MnClZ.…”

“…In our previous work, PK was the only phosphorylated protein of the glycolytic/gluconeogenic pathway that was investigated as a substrate for protein phosphatases [27]. It therefore seemed important to extend these studies to include the other enzymes whose phosphorylation may be important in the control of this metabolic process.…”