Enzyme concerted mechanism

Theoretical evidence [Hartree-Fock (RHF) calculations and density functional theory] has been obtained for a concerted mechanism of oxirane cleavage and A-ring formation in oxidosqualene cyclization. A common concerted mechanistic pathway has been demonstrated for the acid-catalysed cyclization of 5,6-unsaturated oxiranes in chemical and enzymic systems. For example, the conversion of (24) into (26) proceeds via (25) and not via a discrete carbocation (27). Kinetic studies and other evidence are presented for various systems. 

Such a species cannot be ruled out in reactions of iron-EDTA complexes with hydroperoxides recently described by Bruice and coworkers (27). On the other hand, a hydroperoxide complex that reacts with the substrate such that bond formation fiom O to substrate is concerted with 0-0 bond breaking, as proposed by Klinman for dopamine P-monooxygenase (18), could provide compensation for the cost of 0-0 bond cleavage in the transition state. In fact, it is interesting to speculate that for each of these enzymes, the mechanism by which the substrate is oxidized may be dependent on the reactivity of the substrate. One could envision certain substrates that would react with the metal-bound hydroperoxide ligand prior to or concerted with 0-0 bond cleavage. This possibility is difficult to assess because of our lack of information concerning the reactivity of HQ2" when complexed to different metal ions. 

Orotidine 5 -phosphate undergoes an unusual decarboxylation (Fig. 25-14, step/), which apparently is not assisted by any coenzyme or metal ion but is enhanced over the spontaneous decarboxylation rate 1017-fold. No covalent bond formation with the enzyme has been detected.268 It has been suggested that the enzyme stabilizes a dipolar ionic tautomer of the substrate. Decarboxylation to form an intermediate ylid would be assisted by the adjacent positive charge.269,270 Alternatively, a concerted mechanism may be assisted by a nearby lysine side chain.270a d Hereditary absence of this decarboxylase is one cause of orotic aciduria. Treatment with uridine is of some value.271... 

One of the important mechanistic uses of isotopic substitution is that it can selectively affect different steps in a stepwise mechanism and so help resolve mechanisms and pathways. For example, in reactions where there is hydride transfer coupled to C—C bond formation, the measurement of both 13C/12C and D/H kinetic isotope effects can distinguish whether the steps are concerted or stepwise. Consider, for example, the oxidative decarboxylation of malate catalyzed by the malic enzyme, which could occur either via an intermediate mechanism (2.81) or in a concerted mechanism (2.82).59... 

Corey also pointed out that 16 reflects the transition-state of an enzyme-substrate complex. Its formation was later supported by the observation of Michaelis-Menten-type kinetics in dihydroxylation reactions and in competitive inhibition studies [37], This kinetic behavior was held responsible for the non-linearity in the Eyring diagrams, which would otherwise be inconsistent with a concerted mechanism. Contrary, Sharpless stated that the observed Michaelis-Menten behavior in the catalytic AD would result from a step other than osmylation. Kinetic studies on the stoichiometric AD of styrene under conditions that replicate the organic phase of the catalytic AD had revealed that the rate expression was clearly first-order in substrate over a wide range of concentrations [38],... 

A fully concerted mechanism for reaction 299 has been eliminated as inconsistent with 14C and 15N KIEs and also with the observed inverse solvent D2O effect. The reaction path for the deamination of AMP has been formulated613 as a stepwise conversion involving the formation of tetrahedral intermediate 515 characterized by full-bonded hydroxyl and amino groups (equation 300). The TS for slow formation of 515, resulting from the attack of the hydroxyl from enzyme zinc-activated water at the C(6), is characterized by the C(6) OH bond order of 0.8 0.1 (late TS) and fully bonded NH2, that is by the nearly complete conversion to sp3 at C(6), and by nearly complete protonation of Nq), 516, The protonation of NH2 (in 515) and departure of NH3 (with TS 517) take place in the subsequent rapid steps as shown in equation 300, Zinc hydroxide is formed prior to attack514 at C(6). Enzymatic degradation of [6-14C]AMP has been carried out to prove the position of the radiolabel in 513 (equation 301). No radioactivity in the allantoin... 

When a proposed intermediate is so unstable that it cannot exist, i.e. it would have a lifetime less than that of a bond vibration ( 10 13 s), the reaction must proceed by a concerted mechanism (see reference [11], p. 5). The proton-transfer steps and other covalent bondforming and bond-breaking processes are concerted but with varying degrees of coupling between their motions. However, it is still not clear whether a concerted mechanism can occur when the intermediates, which would be formed in an alternative stepwise mechanism, have significant lifetimes. This is an important question for reactions catalysed by enzymes because the nature of the intermediate itself will control whether the enzyme- and non-enzyme-catalysed mechanisms are forced to be similar if the sole criterion for a concerted mechanism is the stability of the intermediate. 

As emphasised recently by Hegarty and Jencks (1975), the interpretation of the third-order terms, kAp HAi[HAj][Af], in the rate law for acetone enolisation (4) has been the subject of much controversy and has played an important role in the development of ideas on the mechanism of acid-base catalysis in solution and at the active sites of enzymes . The problem has been not only to interpret this term, which can be accounted for by a concerted mechanism (17), but also to examine whether this mechanism, with H20 acting as an acid or a base, is valid for interpreting the other terms fcHA,[HAj] and fcA,lAj ]. 

S-adenosyl-L methionine (ADO-Met) dependent DNA methyl transferase catalyzed the transfer of a methyl group from AdoMet to a specific nucleotide within the DNA helix (Cheng et al., 1993). In a concerted reaction in the enzyme active site (Fig X) with a simultaneous addition of methyl residue of AdoMet to the cytosine ring and with an elimination of the ring proton by a water molecule requires involving seven heavy nuclei (two ofCys 81, four of AdoMet and one of water. An estimation with aid of Eq. 2.44 leads to value of the reaction synchronization factor asyn 10 4, that does not rule out the concerted mechanism, if the activation energy is less than 10 kcal/mole Nevertheless, a... 

Figure 20 (a) An ultrafast radical clock substrate capable of distinguishing between radical and carbocation intermediates (b) The proposed two-oxidant pathway of C-H bond oxidation by P450 enzymes in which Compound 1 follows a concerted mechanism with no intermediate while the ferric-hydroperoxo attacks substrates by insertion of HO+ leading to carbocation intermediates which rearrange to give the alcohol product... 

R versus T. An allosteric enzyme that follows the concerted mechanism has a T/R ratio of 300 in the absence of substrate. Suppose that a mutation reversed the ratio. How would this mutation affect the relation between the rate of the reaction and substrate concentration ... 

Gerlt, J.A. and Gassman, P.G. (1993). An explanation for rapid enzyme-catalyzed proton abstraction from carbon acids importance of late transition states in concerted mechanisms. J. Am. Chem, Soc. 115, 11552-11568... 

Gerlt, j. a., Gassman, P. G., An Explanation for Rapid Enzyme-Catalyzed Proton Abstraction from Carbon Acids Importance of Late Transition States in Concerted Mechanisms, J. Am. Chem. Soc. 1993, 115, 11552-11568. 

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