title: An investigation of the interactions of the allosteric modifiers of pyruvate kinase with the enzyme from Carcinus maenas hepatopancreas.
abstract: 1. Pyruvate kinase purified from the hepatopancrease of Carcinus maenas exhibited sigmoidal saturation kinetics with respect to the substrate phosphoenolpyruvate in the absence of the allosteric activator fructose 1,6-bisphosphate, but normal hyperbolic saturation was seen in the presence of this activator. The activation appears to be the result of a decrease in the s0.5 (phosphoenolpyruvate) and not to a change in Vmax. 2. In the presence of ADP and ATP at a constant nucleotide-pool size the results indicate that phosphoenolpyruvate co-operativity is lost on increasing the [ATP]/[ADP] ratio. 3. Paralleling this change is the observation that the fructose 1,6-bisphosphate activation became less at the [ATP]/[ATP] ratio was increased. This  was due to the enzyme exhibiting a near-maximal activity in the absence of activator. 4. L-Alanine inhibited the enzyme, but homotropic co-operative interactions were only seen with a cruder (1000000g supernatant) enzyme preparation. The inhibition by alanine could be overcome by increasing the concentration of either phosphoenolpyruvate or fructose 1,6-bisphosphate, although increasing the L-alanine concentration did not appear to be able to reverse the activation by fructose 1,6-bisphosphate. 5. In the presence of a low concentration of phosphoenolpyruvate, increasing the concentration of the product, ATP, caused an initial increase in enzyme activity, followed by an inhibitory phase. In the presence of either fructose 1,6-bisphosphate or L-alanine only inhibition was seen. 6. The inhibition by ATP could not be completely reversed by fructose 1,6-bisphosphate.
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000409:Alanine / Q000494:pharmacology; D000494:Allosteric Regulation; D000818:Animals; D003386:Brachyura / Q000201:enzymology*; D005636:Fructosephosphates / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D010179:Pancreas / Q000201:enzymology; D010728:Phosphoenolpyruvate / Q000494:pharmacology; D011770:Pyruvate Kinase / Q000037:antagonists & inhibitors / Q000378:metabolism
chemical_list: D005636:Fructosephosphates; D010728:Phosphoenolpyruvate; D000255:Adenosine Triphosphate; D011770:Pyruvate Kinase; D000409:Alanine