Reaction Pathway or Catalytic Cycle

A catalytic cycle is defined by a closed sequence of elementary steps, i.e., a sequence in which the active site is regenerated so that a cyclic reaction pattern is repeated and a large number of turnovers occurs on a single active site [5]. If the stoichiometric equation for each of the steps in the cycle is multiplied by its stoichiometry number, i.e., the number of times it occurs in the catalytic cycle, and this sequence of steps, the reaction pathway, is then added, the stoichiometric equation for the overall reaction is obtained. This equation must contain only reactants and products because all intermediate species must cancel out, and this overall reaction is represented by an equal sign  

It should be stressed that a eatalytic sequenee representing the reaction pathway may not contain a RDS. As an example, consider the catalytie gas-phase oxidation of carbon monoxide. On some metals the surface reaction between adsorbed CO moleeules and O atoms is the RDS, then, if is an active site, the sequence can be represented as  

Here 2 represents the appropriate stoichiometric number in the cycle and all the reaetive intermediates —, CO and O — cancel out to give the bottom 

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The function of enzymes is to accelerate the rates of reaction for specific chemical species. Enzyme catalysis can be understood by viewing the reaction pathway, or catalytic cycle, in terms of a sequential series of specific enzyme-ligand complexes (as illustrated in Figure 1.6), with formation of the enzyme-substrate transition state complex being of paramount importance for both the speed and reactant fidelity that typifies enzyme catalysis. 

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