The theoretical analysis of kinetic behaviour of “hysteretic” allosteric enzymes. II. The dissociating and associating enzymic systems in which the rate of installation of equilibrium between the oligomeric forms is small in comparison with that of enzymatic reaction

Investigating the Reaction Mechanism of F420-Dependent Glucose-6-phosphate Dehydrogenase from Mycobacterium tuberculosis: Kinetic Analysis of the Wild-Type and Mutant Enzymes

“…This lack of thermodynamic cooperativity in the FGD E109Q F 420 binding curves strongly suggests that the pH-dependent sigmoidal F 420 kinetic curves are a result of kinetic cooperativity. , Random addition of substrates and enzyme hysteresis are two mechanisms that can be used to explain kinetic cooperativity. ,, For a two-substrate oligomeric enzyme that follows a random substrate addition mechanism, the positive kinetic cooperativity curves due to one substrate will depend on the concentration of the other substrate, and saturating concentrations of the other substrate will result in substrate inhibition. , FGD E109Q G6P kinetic curves did not display substrate inhibition at saturating G6P concentrations (SI Figure 4; data not shown for pH 8.0 and 9.0), and we can therefore eliminate the random substrate addition mechanism as an explanation for the observed kinetic cooperativity. It is plausible that the pH-dependent kinetic cooperativity for FGD E109Q is due to enzyme hysteresis as a result of F 420 -induced isomerization from a less active to a more active form of the enzyme. , The pH profile studies presented here have established that the Glu109 residue protonates the F 420 cofactor at position N1. Therefore, it is likely that in the absence of the Glu109 residue, the FGD E109Q variant will undergo conformational changes with increasing concentrations of F 420 before it is able to attain its most active form as reflected in the k cat values in SI Figure 7.…”

“…It is plausible that the pH-dependent kinetic cooperativity for FGD E109Q is due to enzyme hysteresis as a result of F 420 -induced isomerization from a less active to a more active form of the enzyme. 38,41 The pH profile studies presented here have established that the Glu109 residue protonates the F 420 cofactor at position N1. Therefore, it is likely that in the absence of the Glu109 residue, the FGD E109Q variant will undergo conformational changes with increasing concentrations of F 420 before it is able to attain its most active form as reflected in the k cat values in SI Figure 7.…”

Allostery and Hysteresis Are Coupled in Human UDP-Glucose Dehydrogenase

“…It is possible for enzyme association to result in hysteresis 40, 41 , but this is not the case for hUGDH. Instead, hysteresis in hUGDH is caused by the slow isomerization from the inactive E* state to the active E state upon binding of NAD + and substrate 19, 22 .…”

“…The specific activity of hUGDH shows a hyperbolic dependency on protein concentration that can be modeled as three low-activity dimers associating to form a higher-activity hexamer . It is possible for enzyme association to result in hysteresis, , but this is not the case for hUGDH. Instead, hysteresis in hUGDH is caused by the slow isomerization from the inactive E* state to the active E state upon binding of NAD + and substrate. , While the increased stability of the hUGDH A136M hexamer does not explain the absence of a lag in progress curves, it does focus attention on the conformational flexibility of the allosteric switch in the E* state (Figures and A).…”

“…The specific activity of hUGDH shows a hyperbolic dependency on protein concentration that can be modeled as three low-activity dimers associating to form a higher-activity hexamer. 16 It is possible for enzyme association to result in hysteresis, 40,41 but this is not the case for hUGDH. Instead, hysteresis in hUGDH is caused by the slow isomerization from the inactive E* state to the active E state upon binding of NAD + and substrate.…”