Enzyme short-time behavior

Deactivation of the enzyme catalyst often goes unnoticed, especially if the biocatalyst is fairly stable and/or the time of observation is short, such as during an initial rate measurement. If we analyze the short-time behavior of Eq. (17.23) we can replace the exponential by a linear term deviating from unity. At short times, exp (-kd obs t) 1 - kdobs t, and Eq. (17.24) is recovered. 

The enzyme - substrate complex concentration reaches its maximum value in a very short time, and decays very slowly afterwards. To explain this special behavior of the concentration [ES], write its kinetic equation in the form... 

Holwerda and Gray (97) proposed a mechanism for the reduction process involving a central role of Type 2 Cu2+ as the initijil point at which electrons enter and are subsequently distributed to the other electron acceptors. This interpretation would seem to be supported by the very recent observation of Branden and Reinhammar (98) that the Type 2 ion of Poljqjorus laccase is reduced and subsequently reoxidized in a very short time period. The authors also emphasize the parallel behavior of the T q)e 1 and Type 3 centers it is particularly striking that under a variety of conditions the rates of Type 1 and 3 reductions are very similar. Indeed, when Cr2+ was used as the reductant, a similar observation was made (99). Perhaps Cr2+ reduces the enzyme via a bridging ligand (H2O ) between it and Type 2 Cu2+. 

If the enzyme charged to a batch reactor is pristine, some time will be required before equihbrium is reached. This time is usually short compared with the batch reaction time and can be ignored. Furthermore, 5o Eq is usually true so that the depletion of substrate to establish the equilibrium is negligible. This means that Michaelis-Menten kinetics can be applied throughout the reaction cycle, and that the kinetic behavior of a batch reactor will be similar to that of a packed-bed PFR, as illustrated in Example 12.4. Simply replace t with thatch to obtain the approximate result for a batch reactor. 

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