Ping-pong catalytic pathway

Km and Umax have different meanings for different enzymes. The limiting rate of an enzyme-catalyzed reaction at saturation is described by the constant kcat, the turnover number. The ratio kcat/Km provides a good measure of catalytic efficiency. The Michaelis-Menten equation is also applicable to bisubstrate reactions, which occur by ternary-complex or Ping-Pong (double-displacement) pathways. 

Enzymes often require multiple substrates to complete their catalytic cycle. This may involve combining two compounds into one molecule or transferring atoms or electrons from one substrate to another. The substrates may both bind to an enzyme and react collectively, or each substrate might bind, react, and release sequentially. With two substrates, if both bind to the enzyme, a ternary complex (ES S2) will form (Scheme 4.8). The order of substrate addition may be important (ordered) or not (random order). Cases in which the two substrates react sequentially follow a double-displacement, or ping-pong, mechanism (Scheme 4.9). Enzymes requiring more than two substrates have more complicated complexation pathways. 

Despite the fact that reaction kinetics cannot be Ping-Pong, the enzyme is phosphorylated by substrates and the phosphoenzyme reacts with ADP to form ATP. Is this process a part of the mechanism of reaction (16) The stereochemical analysis of phospho transfer argues strongly against the importance of the phosphoenzyme in the reaction mechanism. The phospho transfer proceeds with inversion of configuration at phosphorus, which is consistent with a single displacement at phosphorus and inconsistent with a double-displacement mechanism (64). Therefore, the phosphoenzyme does not appear to be on the main catalytic pathway. 

Three possible kinetic mechanisms for a two-substrate/two-product reaction depicted in Cleland notation. Substrates are designated as A and B , products as C and D while E represents the enzyme and E is a covalently modified enzyme that forms transiently as part of the normal catalytic pathway in the ping-pong mechanism. 

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