Quasi-catalytic biochemical reactions

Since the catalytic cycle operates with relatively rapid kinetics, E and ES will obtain a steady state governed by Equations (4.2) and (4.3) and the quasi-steady state concentrations of enzyme and complex will change rapidly in response to relatively slow changes in [S]. Thus the quasi-steady approximation is justified based on a difference in timescales between the catalytic cycle kinetics and the overall rate of change of biochemical reactions. 

Appropriate expressions for the fluxes of each of the reactions in the system must be determined. Typically, biochemical reactions proceed through multiple-step catalytic mechanisms, as described in Chapter 4, and simulations are based on the quasi-steady state approximations for the fluxes through enzyme-catalyzed reactions. (See Section 3.1.3.2 and Chapter 4 for treatments on the kinetics of enzyme catalyzed reactions.)... 

Other biochemical reactions are quasi-catalytic they involve biomacromolecules that meet the criteria (a) and (b) but not (c), i.e. only one turnover. For example, in the case of self-splicing, the intramolecular catalysis that occurs at a specific site within a biomacromolecule is accelerated by the folded structure of the molecule but the macromolecule is physically changed in the reaction. In this text, biocatalysts or catalytic biomacromolecules refer to ... 

The vast majority of biochemical reactions are catalyzed by enzymes (Bagg, 2004 Copeland, 2000), which are proteins fulfilling all the three criteria of being biocatalysts efficiency (a), specificity (b) and multiple turnover (c). The term enzymes is reserved for the natural catalytic proteins in this text. Although few quasi-catalytic enzymes are known such as type I restriction endonuclease, poly(ADP-ribose) synthetase and transmethylase for 0 -methylguanine mediate specific reactions with large acceleration but are inactivated in the reaction. 

---