Isotopic exchange ping pong mechanism

HALF-LIFE HALF-REACTIONS ISOTOPE EXCHANGE SUBSTRATE SYNERGISM PING-PONG MECHANISM COVALENT INTERMEDIATES... 

MULTISUBSTRATE MECHANISM Random Bi Bi ternary complex mechanism, ISOTOPE EXCHANGE AT EQUILIBRIUM RANDOM Bl UNI MECHANISM RANDOM Bl UNI UNI Bl PING PONG MECHANISM... 

Hexokinase does not yield parallel reciprocal plots, so the Ping Pong mechanism can be discarded. However, initial velocity studies alone will noi discriminate between the rapid equilibrium random and steady-state ordered mechanisms. Both yield ihe same velocity equation and families of intersecting reciprocal plots. Other diagnostic procedures must be used (e.g., product inhibition, dead-end inhibition, equilibrium substrate binding, and isotope exchange studies). These procedures are described in detail in the author s Enzyme Kinetics behavior and Analysis of Rapid Equilibrium and Steady-State Enzyme Systems, Wiley-Interscience (1975),... 

The isomerization of E-Zn-OH2 (the form that reacts with bicarbonate) back to E-Zn-OH" (the form that reacts with CO2) is rate limiting for V, as shown by the D2O solvent isotope effect on V but not on VIK or the rate of exchange between CO2 and bicarbonate (47). Carbonic anhydrase has, in fact, a Ping-Pong mechanism in which reaction of buffer with E-Zn-OHi to give E-Zn-OH" and protonated buffer is the second half-reaction. Thus, small... 

Equations for the initial rate of isotope exchange in Ping Pong mechanisms may be derived by one of the methods described in Chapter 4 the usual procedure will be to apply the King-Altman method or the net rate constant method (Cleland, 1975) (Table 4). 

Table 4. Isotope exchange-rate expressions in Ping Pong mechanisms...

The last equation in Table 4 for the Bi Bi Uni Uni Ping Pong mechanism (in the absence of C and R) is relatively long and complex. The right-hand portion of the denominator in this equation represents the reciprocal concentration of free enzyme in chemical equilibrium multiplied by [EJ. If the products C and R are present, the distribution equation for [E]/[Eo] would be more complex (Eq. (12.73)) consequently, the resulting full rate equation for the A - Q isotope exchange becomes rather cumbersome. 

One of the most useful appKcations of isotope exchange studies comes in determining the order of addition of substrates in trisubstrate Ping Pong mechanisms. The best way to understand this problem is in relation to a specific example. Consider, for example, the reaction catalyzed by a large number of synthetases (Walsh, 1998) ... 

Acetyl-CoA carboxylase is an en2yme having a ping-pong mechanism, and it is possible to measure isotope exchange between substrates and products for each half-reaction in the absence of the substrates for the other... 

Another important characteristic of ping pong reaction is the presence of exchange reactions in the absence of other substrates. See Isotope Exchange at Equilibrium Multisite Ping Pong Bi Bi Mechanism Half-Reaction Substrate Synergism... 

PING PONG Bl Bl MECHANISM ISOTOPE EXCHANGE MULTISITE PING PONG Bl Bl MECHANISM... 

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