Michaelis-Menten kinetics adherence

The general property of all above mechanisms is their adherence to the Michaelis-Menten kinetics. In the absence of products, the double reciprocal plots for aU bisubstrate mechanisms represent a family of straight lines with a common intersection point, if one substrate is varied while the other substrate is held at different fixed concentrations. Similarly, the double reciprocal plots for all trisubstrate mechanisms represent a family of straight lines with a common intersection point, if one substrate is varied while the second substrate is held at different fixed concentrations, and the third substrate is held at a fixed concentration. This, however, is tme only if each substrate adds just once if one of them adds twice in sequential fashion, the reciprocal plots will be parabolic. 

This difference in the magnitude of the buffer effect in equilibrium studies versus initial rate studies could be accounted for by at least three possibilities. The first is that under equilibrium and initial rate conditions, BCA exhibits different mechanisms. This is an unattractive hypothesis, and is inconsistent with the well characterized adherence of BCA catalysis to Michaelis-Menten kinetics and to the Haldane relation under initial rate conditions. 

Ag+-catalysed isomerization of 4-substituted homocubanes (5) to norsnoutanes (6) also proceeds via pre-equilibrium complex formation and follows second-order kinetics. In the case R= Me adherence to Michaelis-Menten kinetics could also be demonstrated. C-4 Substituents capable of resonance interaction in the cationic transition state promote deviations in the rate of reaction relative to substituents which exhibit inductive effects only. With R=Bu bond-switching is reduced in rate, presumably because of steric inhibition of Ag+ attack on the homocubane to give an intermediate analogous to (4). Placement of deuterium or CDg at C-4 produces only a minor inverse kinetic deuterium isotope effect (kH/kD=0.97) which implies that a completely free carbonium ion intermediate is not involved and so argues in favour of a delocalized species analogous to (4). 

The cellular uptake process for taurocholate was found to be linear for at least the first four min for all substrate concentrations examined. The initial rate of uptake (Vq) was determined from linear regression analysis of the increase in taurocholate concentrations in the cell pellet with time (1-4 min). The regression correlation coefficient in all cases was greater than 0.95. Extrapolation of the Vq line to zero time yielded a positive intercept indicative of nonsaturable nonspecific binding such as adherence to the outer cell membrane. The derived values for Vq were combined within each age-group and substrate concentration. These values were then analyzed according to Michaelis-Menten kinetics using Lineweaver-Burk or Eadie-Hofstee plots to obtain and V gx (Dixon and Webb, 1964). 

Fia S. Type I (dotted line) and type II (solid line) difference spectra produced by addition of drug substrates to microsomal suspensions. The magnitude of the spectral change is related to the concentration of drug present and generally adheres to Michaelis-Menten kinetics. (Sasame and Gillette, MoL Pharmacol 5,... 

Many reactions catalyzed by enzymes obey kinetics described by the Michaelis-Menten rate equation however, this adherence does not guarantee that a simple mechanism occurs, such as that represented by steps 9.1 and 9.2 to give the overall reaction 9.3. More complicated reaction sequences can result in exactly the same kinetic behavior. For example, there is considerable evidence that the following mechanism describes a number of enzyme... 

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