A number of differences in the kinetic and physical properties of the @entity526 @entity1110 ( @entity1110 +) dependent malic enzyme have been found, depending upon whether @entity1025 or @entity3281 served to fulfill the divalent cation requirement. The velocity- @entity1110 + and velocity-cation saturation curves exhibit a simple hyperbolic response in the presence of either metal cofactor, but the affinity for @entity1110 + (and @entity5910 ) as well as the Vmax is increased in the presence of @entity3281 . The high affinity of the enzyme for @entity3281 coupled with the increased affinity for substrates indicates that @entity3281 is the preferred cofactor in vitro. With either @entity1025 or @entity3281 as cation, the velocity-malate saturation curves in the absence of effectors are complex at pH 7.45, indicating varying combinations of apparent positive and negative cooperative behavior. Greater initial positive cooperative behavior between malate binding sites is observed with @entity1025 as cation. The enzyme appears to be equally sensitive to inhibition by the allosteric inhibitors reduced @entity2125 ( @entity2125 ) and @entity5911 ( @entity44818 ) in the presence of either cation, but the interaction between malate binding sites, in the presence of effectors, varies significantly with the choice of metal cofactor. The inhibitor @entity2125 increases the interaction between malate binding sites in the presence of @entity3281 but has little effect on subunit interaction in the presence of @entity1025 . The inhibitor @entity44818 increases the interaction between malate binding sites in the presence of both cations, with increased positive cooperativity observed with @entity3281 but increased negative cooperativity with @entity1025 . The kinetic data can be explained by a model involving sequential ligand-induced conformational changes of the enzyme, resulting in a mixture of apparent positive and negative cooperative behavior. Alternative explanations involving different classes of noninteracting binding sites or different enzyme forms are also considered. The metal cofactors, @entity1025 and @entity3281 , appear to stabilize two distinct conformational states of the enzyme which differ in response to varying substrate and effector concentrations. Altered conformational states of the enzyme in the presence of the two cations are further substantiated by proteolytic digestion studies with the homogeneous enzyme. The results are strikingly similar to previous results reported on the @entity2125 ( @entity2125 ) dependent malic enzyme and the @entity2125 +-dependent @entity9275 dehydrogenase, supporting the suggestion that metal cofactors function as regulatory entities.

Role of metal cofactors in enzyme regulation. Differences in the regulatory properties of the XXXX @entity1110 specific malic enzyme, depending on whether magnesium ion or manganese ion serves as divalent cation.

@entity2125 :: ('MESH:D009243', 'Chemical') :: ['nicotinamide adenine dinucleotide', 'NAD+', 'NAD', 'NADH'] @entity1025 :: ('CHEBI:18420', 'Chemical') :: ['Mg2+'] @entity3281 :: ('CHEBI:29035', 'Chemical') :: ['Mn2+'] @entity5910 :: ('MESH:C030298', 'Chemical') :: ['malate'] @entity1110 :: ('MESH:D009249', 'Chemical') :: ['nicotinamide adenine dinucleotide phosphate', 'NADP'] @entity9275 :: ('MESH:C034219', 'Chemical') :: ['isocitrate'] @entity5911 :: ('MESH:D062907', 'Chemical') :: ['oxaloacetic acid'] @entity44818 :: ('CHEBI:30744', 'Chemical') :: ['OAA'] @entity526 :: ('562', 'Species') :: ['Escherichia coli']
@entity526 :: (562,Species) :: ['Escherichia coli']
