Ordered single-displacement reaction

The general rate equation for an ordered, single-displacement reaction where A is the leading substrate is... 

Note that the ternary complex EAB can be formed in two different ways. If the formation of EAB can occur with either substrate binding first, the reaction is known as a random single-displacement reaction. Many reactions catalyzed by phosphotransferases are of this type. If a particular substrate must bind first with the enzyme before the second substrate can bind, the reaction is known as an ordered single-displacement reaction. Many reactions catalyzed by dehydrogenases are of this type. The values for Km and Vmax for each substrate can be obtained from experiments in which the concentration of one substance is held constant at saturating levels while the concentration of the second substrate is varied. Kinetic analyses can distinguish between these types of reactions. 

In single displacement reactions both substrates A and B simultaneously must be present on the active site of the enzyme to yield a ternary complex EAB in order that the reaction may proceed. Single displacement reactions take place in two forms, random and ordered, and they are distinguished by the way the two substrates bind to the enzyme. 

In ordered single displacements there is a compulsory sequence for the reaction which dictates that a specific substrate, the leading substrate, must be bound first, before the second, or following substrate, can be bound, as shown below ... 

In this case A is the leading substrate. Many dehydrogenase reactions which utilise NAD as a co-substrate are good examples of ordered single displacements, e.g. malate dehydrogenase (MDH) ... 

A single replacement reaction occurs when an element reacts with a compound in such a way that the element replaces an ion of a similar element from the compound. In order for the replacement to occur, the elemental reactant must be more reactive than the elemental product. The general format for a single displacement reaction, when the elemental reactant is a metal, is ... 

Displacing One Element by Another Activity Series As we said, displacement reactions have the same number of reactants as products. We mentioned doubledisplacement (metathesis) reactions in discussing precipitation and acid-base reactions. The other type, single-displacement reactions, are all oxidation-reduction processes. They occur when one atom displaces the ion of a different atom from solution. When the reaction involves metals, the atom reduces the ion when it involves nonmetals (specifically halogens), the atom oxidizes the ion. Chemists rank various elements into activity series—one for metals and one for halogens— in order of their ability to displace one another. 

H. insolans cellulase BOV Orotato 5 phospho- /3-Side The active-site structure of the complex with cellobiose shows an ordered 11,0 near to but not H-bended to AsplO (general base). The authors staled that an inactive ccllohcxaose complex has water suitably bound to participate in a single displacement reaction.8S 86... 

Some acids react with active metals and release hydrogen gas, H2. Recall that metals can he ordered in terms of an activity series. Metals above hydrogen in the series undergo single-displacement reactions with certain acids. Hydrogen gas is formed as a product, as shown by the reaction of barium with sulfuric acid. 

To predict which single substitution reactions will occur, we need to know a little about the relative reactivities of some of the important metals and nonmetals. Some metals and a few nonmetals are listed in Table 8.2 in order of decreasing reactivity. A more complete list is given in Section 17.2. Hydrogen is included in the list of metals because it can be displaced from aqueous acids by reactive metals (Figure 8.5) and can displace less active metals from their compounds ... 

The chemical reaction then takes place to form a product-containing ternary complex, from which the products dissociate in reverse order, that is, with the nucleotide product dissociating last. This pathway is clearly established by the results of steady-state kinetic analyses carried out in several laboratories. The reaction proceeds with inversion of configuration at P of the nucleotide substrate in both reaction directions, as shown in Eq. (5) (9, 10). Therefore, the interconversion of ternary complexes proceeds in an uneven number of displacement steps. In the absence of any other evidence for the involvement of nucleophilic catalysis by the enzyme, it is almost certain that the UMP transfer occurs in a single displacement between bound substrates. 

Molecularity vs. Mechanism. Cyclization Reactions and Effective Molarity A useful illustration of the distinctions between mechanism, molecularity, and order arises in the analysis of intramolecular versions of typically intermolecular reactions. Consider a classic Sn2 reaction of an amine and an alkyl iodide. The reaction is second order (first order in both amine and alkyl iodide) and bimolecular (two molecules involved in the transition state that s what the "2" in "Sn2" Stands for). The mechanism involves the backside attack of the nucleophilic amine on the C, displacing the iodide in a single step. Now consider a long chain molecule i that terminates in an amine on one end and an alkyl iodide on the other. Now two types of Sn2 reactions are possible. If two different molecules react, we still have a second order, bimolecular, intermolecular reaction. The product would ultimately be a polymer, ii, and we will investigate this type of system further in Chapter 13. Alternatively, an intramolecular reaction could occur, in which the amine reacts with the iodide on the same molecule producing a cyclic product. Hi. This is still called an S 2 reaction, even though it will be first order and unimolecular. 

One alkyne will displace another from Pt(alkyne)(PR3)2 complexes, the order of alkyne displacement being C2H2 < alkylalkynes < arylalkynes < nitroalkynes. The reaction involves both a dissociative and an associative pathway. The reaction may not lead to a single replacement product in all cases. Thus cr-hydroxyalkynes lead to the formation of dialkynyl platinum(II) complexes.830... 

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