title: The atypical velocity response by  pyruvate carboxylase to increasing concentrations of acetyl-coenzyme A.
abstract: An investigation was made of the interaction of pyruvate carboxylase with its allosteric effector, acetyl-CoA, and the velocity profile of the deacylation of acetyl-CoA as a function of acetyl-CoA concentration indicated that this ligand does not bind to this enzyme in a positive homotropic co-operative manner. An examination was therefore made of the factors that contribute to the sigmoidicity of the rate curves obtained for pyruvate carboxylation with various concentrations of acetyl-CoA. Hill coefficients for acetyl-CoA obtained with both sheep and chicken liver pyruvate carboxylases were found to be dependent on the fixed pyruvate concentration used in the assay solution. Thus, by varying the acetyl-CoA concentration, the degree of saturation of the enzyme by pyruvate was also changed. A further consequence of non-saturating concentrations of pyruvate was that the non-productive hydrolysis of the enzyme- carboxybiotin complex increased, resulting in an under-estimate of the reaction velocity measured by oxaloacetate formation. Another factor contributing to the sigmoidicity is that, at non-saturating concentrations of acetyl-CoA, the enzyme undergoes inactivation upon dilution to low protein concentrations, again resulting in an under-estimate of the reaction velocity. Under conditions where none of the above factors was operating and the only effect of varying acetyl-CoA concentrations was to alter the proportion of the enzyme catalysing the carboxylation reaction at acetyl-CoA-dependent and -independent rates, the sigmoidicity of the acetyl-CoA velocity profile was completely eliminated.
mesh_terms: D000105:Acetyl Coenzyme A / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001710:Biotin / Q000378:metabolism; D002645:Chickens; D006868:Hydrolysis; D066298:In Vitro Techniques; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D011766:Pyruvate Carboxylase / Q000378:metabolism*; D011773:Pyruvates / Q000378:metabolism; D012756:Sheep
chemical_list: D011773:Pyruvates; D001710:Biotin; D000105:Acetyl Coenzyme A; D000255:Adenosine Triphosphate; D011766:Pyruvate Carboxylase; D008274:Magnesium; D011188:Potassium