Ordered ternary complex mechanism

FlC. 7. A set of possible relations between volume changes in a two-substrate reaction with an ordered ternary-complex mechanism. The activation volumes are defined by Eq. (79). 

The mechanism of enzyme catalysis drawn, using (a)random ternary complex theory, (b)ordered ternary complex mechanism and (c) ping-pong bi-bi mechanism ... 

Figure 4.9 Basic compulsory-order ternary-complex mechanism. The basic ordered mechanism for the general reaction A + B P + Q, with a = [A], b = [B], p = [P], and q = [Q] is illustrated. The four states are unbound enzyme (E), enzyme-substrate A complex (E-A), enzyme-substrate A-substrate B complex (E-AB), and enzyme-product Q complex (E-Q).

Another mechanism that may apply to a two substrate reaction is the ordered ternary-complex mechanism. For example, the complex EAB can be formed from EA by addition of B, but not from EB by addition of A the substrates must become attached in a particular order. This mechanism is represented in Figure 10.5, the second representation being that of Cleland. This mechanism also leads to a rate equation of the form of (10.26), but the significance of the constants is different. 

The ordered ternary-complex mechanism. The ternary complex EAB can only be formed from EA. 

No.te that this is similar to the ordered ternary-complex mechanism shown in Figure iO.5, except that there is no ternary complex EAB. B reacts with EA to form EY -j- X in one stage.] Apply the steady-state treatment to this mechanism and obtain an expression for the rate of formation of X (equal to the rate of formation of Y) in terms of [E], [A], [B], and the rate constants. 

As stated previously, kinetic studies showed that the enzyme-catalyzed reaction follows an ordered ternary complex mechanism in which the binding of NAD occurs prior to that of lactate, and release of pyruvate precedes that of NADH. The use of rapid mixing methods allowed rates of many of the individual steps in the reaction sequence to be elucidated, including the hydride ion-transfer step in the catalytically active ternary complex. 

This nomenclature has been introduced by Cleland (1963), but other descriptions of bisubstrate mechanisms are also found in the biochemical literature. For example, a sequential addition in bisubstrate reactions, an Ordered Bi Bi mechanism is also called a compulsory-order ternary-complex mechanism whereas a Random Bi Bi mechanism is called a random-order ternary-complex... 

The products of the ER reaction, butyryl-CoA and NAD, were utihzed as inhibitors in an attempt to determine whether the reaction proceeds via a random or compulsory order ternary-complex mechanism using the rules of Cleland[5]. Assays were carried out in triplicate. The data was analyzed by linear regression and statistical t-tests were employed to determine if lines intersected at the same or significantly different point on the vertical axis. 

When crotonyl-Co was varied in the presence of butyryl-CoA the lines appeared to intersect to the left of the vertical axis. However, statistical analysis of the y-axis intercepts showed that they in fact intercept the vertical axis at points which are not significantly different from each other. That is, they intercept at a common point and therefore show a competitive pattern of inhibition. This combination of inhibition patterns is indicative of a random order ternary complex mechanism. 

Pettersson, G. (1976). The transient-state kinetics of two substate enzyme systems operating by an ordered ternary complex mechanism. European Journal of Biochemistry, 69,273-8. 

Figure 2.13 Reaction pathway for a bi-bi compulsory ordered ternary complex reaction mechanism.

A second ternary complex reaction mechanism is one in which there is a compulsory order to the substrate binding sequence. Reactions that conform to this mechanism are referred to as bi-bi compulsory ordered ternary complex reactions (Figure 2.13). In this type of mechanism, productive catalysis only occurs when the second substrate binds subsequent to the first substrate. In many cases, the second substrate has very low affinity for the free enzyme, and significantly greater affinity for the binary complex between the enzyme and the first substrate. Thus, for all practical purposes, the second substrate cannot bind to the enzyme unless the first substrate is already bound. In other cases, the second substrate can bind to the free enzyme, but this binding event leads to a nonproductive binary complex that does not participate in catalysis. The formation of such a nonproductive binary complex would deplete the population of free enzyme available to participate in catalysis, and would thus be inhibitory (one example of a phenomenon known as substrate inhibition see Copeland, 2000, for further details). When substrate-inhibition is not significant, the overall steady state velocity equation for a mechanism of this type, in which AX binds prior to B, is given by Equation (2.16) ... 

Random Bi Bi Ternary Complex Mechanism The random or noncompulsory ordered mechanism is noticeably symmetrical, with two different paths for producing the EAB complex from free enzyme E and its substrates A and B, as well as two different pathways for producing the EPQ complex from free enzyme E and its products P and Q ... 

These qualitative conclusions can be justified in terms of rate equations derived for each exchange catalyzed by ordered and random ternary complex mechanisms . ... 

Steady-state Dependency of a Family of Deuterium Isotope Effects Figure 6. The steady-state dependence of various isotope effects for a hypothetical enzyme catalyzing a reaction described as an ordered ternary complex kinetic mechanism. 

Ternary-complex mechanism refers to order bi bi mechanism. 

Scheme 5.5 Ordered equilibrium ternary complex mechanism.

One mechanism which may apply to a two-substfate reaction is the so-called random ternary-complex mechanism. In this mechanism the enzyme E can form binary complexes EA and EB with the two substrates A and B. It can also form the ternary complex EAB, with no restriction on the order in which A and B are attached. In Figure 10.4 this mechanism is represented in two different ways. The lower one is a shorthand notation introduced by the American biophysical chemist... 

The reaction proceeds via a ternary complex mechanism (CAT Ac-CoA Cm CAT CoA Ac-Cm) with a random order of addition of substrates (18). The progression from the binary complex to the ternary complex involves subtle changes in the structure of the enzyme and/or the conformations of bound substrates and accompanies a threefold decrease in affinity the fCj of Cm and Ac-CoA in respective binary complexes are 4 and 30 fxM, whereas the corresponding values of the ternary complex are 12 and 90 (iM, respectively (20). The RDS is thought to involve both product release and ternary complex interconversion. No evidence exists for the formation of an acetyl-enzyme intermediate. 

The first possibility envisages essentially the same mechanism as for the second-order process, but with Bt2 replacing solvent in the rate-determining conversion to an ion pair. The second mechanism pictures Bt2 attack on a reversibly formed ion-pair intermediate. The third mechanism postulates collide of a ternary complex tiiat is structurally similar to the initial charge-transfer complex but has 2 1 bromine alkene stoichiometry. There are very striking similarities between the second-order and third-order processes in terms of magnitude of p values and product distribution. In feet, there is a quantitative correlation between the rates of the two processes over a broad series of alkenes, which can be expressed as... 

A steady-state kinetics study for Hod was pursued to establish the substrate binding pattern and product release, using lH-3-hydroxy-4-oxoquinoline as aromatic substrate. The reaction proceeds via a ternary complex, by an ordered-bi-bi-mechanism, in which the first to bind is the aromatic substrate then the 02 molecule, and the first to leave the enzyme-product complex is CO [359], Another related finding concerns that substrate anaerobically bound to the enzyme Qdo can easily be washed off by ultra-filtration [360] and so, the formation of a covalent acyl-enzyme intermediate seems unlikely in the... 

AF values for cyanide attack at [Fe(phen)3] +, [Fe(bipy)3] + and [Fe(4,4 -Me2bipy)3] " in water suggest a similar mechanism to base hydrolysis, with solvation effects dominant in both cases. Cyanide attack at [Fe(ttpz)2] , where ttpz is the terdentate ligand 2,3,5,6-tetrakis(2-pyridyl)pyr-azine, follows a simple second-order rate law activation parameters are comparable with those for other iron(II)-diimine plus cyanide reactions. Interferences by cyanide or edta in spectro-photometric determination of iron(II) by tptz may be due to formation of stable ternary complexes such as [Fe(2,4,6-tptz)(CN)3] (2,4,6-tptz= (66)). ... 

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