Kinetic studies with alternative steady-state kinetics

On the reducing side of PS II, the primary acceptor (Qa had been considered the primary acceptor until pheophytin was discovered to precede it) is reduced in less than 400 ps by Pheo . The reduction of Qa is conveniently monitored by the increase of PS II fluorescence from an initial value, Fq, to a maximal level, indicative of the steady-state level of Q /Qa- K reoxidation of Qa is prevented by the specific inhibitor DCMU (or other herbicides having the same effect), the fluorescence yield of PS II increases sharply, because Qa becomes fully reduced. The reduced form is an anion semiquinone (see the review by Cramer and Crofts [36]), and the absorption spectrum of this compound with a maximum at 326 nm serves for its identification [19] and offers an alternative method for kinetic studies of Qa redox reactions (see Ref. 37 for review). 

Initial rate measurements, especially with alternative substrates and with a product or substrate analog as inhibitor, and measurements of the rate of isotope exchange at equilibrium, can give a great deal of information about mechanism, and in some cases allow estimates of individual velocity constants and dissociation constants. The results of such studies, which require little enzyme, are an essential basis for the proper interpretation, in relation to the overall catalytic reaction, of pre-steady-state studies and kinetic and thermodynamic studies of enzyme-coenzyme reactions in isolation. 

The CSTR, operated at steady state, can be a useful tool for kinetic studies. Conditions in a batch or semi-batch reactor often change rapidly with time and slopes of experimental data must be used in model equations. Rqiid phenomena, such as particle nucleation, are difiicult to study. Also particle size distributions can be narrow and therefore batch experiments may not be sensitive enough to choose between alternate particle growth models. 

The relatively long equilibration time of the stagnant thin layer of the OTTLE or reflection cell does not allow one to study the kinetics of fast reactions. Alternatively, the forced convection regimes of the RDE and the channel-flow cell allow production of steady state currents, and both have been investigated with optical detection of electrogenerated species. Further details can be found in Chapter 2.4 in this volume. The rotating OTE was investigated early on and the theory has recently been expanded [204]. The electrode is used... 

The mechanism described by reactions Ci, C2, and C3 was proposed by Markham and Reed 59) for the A. vinelandii enzyme. The supporting evidence consists chiefly of steady-state and presteady-state kinetic studies of the reaction carried out with an alternate substrate for GTP, guanosine-5 -0-3-thiotriphosphate (GTPyS). 

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