Systematics of beta decays of odd-A nuclides with 160&lt;A&lt;180

Meijer, B.J.

doi:10.1007/bf01437750

Cited by 8 publications

(2 citation statements)

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“…But whether the metal activates them by complexing with the substrate PEP or by complexing with the enzyme is still a matter of debate. Several studies on PEPC from maize leaves concluded that the MgPEP complex is the true substrate of the enzyme [16,[26][27][28]. However, a more recent study claimed that PEPC binds the free species in an ordered fashion, with Mg# + binding before PEP [25].…”

Section: Discussionmentioning

confidence: 99%

“…No activation by free PEP ( f PEP) was detected in these studies. In contrast, from steady-state studies, also of the non-phosphorylated enzyme, in which the MgPEP complex was considered the variable substrate [16,[26][27][28], it was concluded that the binary MgPEP complex is the substrate of the reaction and that either free Mg# + ( f Mg# + ) [26] or f PEP [16] behaves as an activator. Thus the role that Mg# + , PEP or MgPEP has in the kinetics of the maize leaf PEPC-catalysed reaction is not clear at present.…”

Section: Introductionmentioning

confidence: 99%

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Re-examination of the roles of PEP and Mg2+ in the reaction catalysed by the phosphorylated and non-phosphorylated forms of phosphoenolpyruvate carboxylase from leaves of Zea mays

Tovar‐Méndez

Rodríguez‐Sotres

López-Valentín

et al. 1998

Biochemical Journal

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To study the effects of phosphoenolpyruvate (PEP) and Mg2+ on the activity of the non-phosphorylated and phosphorylated forms of phosphoenolpyruvate carboxylase (PEPC) from Zea mays leaves, steady-state measurements have been carried out with the free forms of PEP (fPEP) and Mg2+ (fMg2+), both in a near-physiological concentration range. At pH 7.3, in the absence of activators, the initial velocity data obtained with both forms of the enzyme are consistent with the exclusive binding of MgPEP to the active site and of fPEP to an activating allosteric site. At pH 8.3, and in the presence of saturating concentrations of glucose 6-phosphate (Glc6P) or Gly, the free species also combined with the active site in the free enzyme, but with dissociation constants at least 35-fold that estimated for MgPEP. The latter dissociation constant was lowered to the same extent by saturating Glc6P and Gly, to approx. one-tenth and one-sixteenth in the non-phosphorylated and phosphorylated enzymes respectively. When Glc6P is present, fPEP binds to the active site in the free enzyme better than fMg2+, whereas the metal ion binds better in the presence of Gly. Saturation of the enzyme with Glc6P abolished the activation by fPEP, consistent with a common binding site, whereas saturation with Gly increased the affinity of the allosteric site for fPEP. Under all the conditions tested, our results suggest that fPEP is not able to combine with the allosteric site in the free enzyme, i.e. it cannot combine until after MgPEP, fPEP or fMg2+ are bound at the active site. The physiological role of Mg2+ in the regulation of the enzyme is only that of a substrate, mainly as part of the MgPEP complex. The kinetic properties of maize leaf PEPC reported here are consistent with the enzyme being well below saturation under the physiological concentrations of fMg2+ and PEP, particularly during the dark period; it is therefore suggested that the basal PEPC activity in vivo is very low, but highly responsive to even small changes in the intracellular concentration of its substrate and effectors.

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