Theorell-Chance mechanism

Theorell-Chance mechanism is a simplified version of an Ordered Bi Bi mechanism where the steady-state level of central complexes is very low. TheoieU and Chance (1951) have proposed an Ordered Bi Bi mechanism without the central ternary complexes for alcohol dehydrogenase from equine liver. The hit-and-mn reaction sequence can be written  

The general rate equation for this mechanism, in the presence of both substrates and both products of reaction, in terms of rate constants, is 

Compared to the Ordered Bi Bi, the equation for the Theorell-Chance mechanism lacks A5P and BPQ terms in the denominator. In terms of kinetic constants, the rate equation becomes 

Definition of rate constants in terms of kinetic constants 

Definition of kinetic constants in terms of rate constants KiA = 

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Different abortives may be formed with alternative products or substrates. Such procedures can be useful in helping to distinguish Theorell-Chance mechanisms from ordered systems with abortive complexes . In the case of lactate dehydrogenase, the E-pyruvate-NAD+ and E-lactate-NADH abortive complexes may play a regulatory roles in aerobic versus anaerobic metabolism. 

Rate experiments that are typically carried out in the presence of different concentrations of an alternative product (or product analog) while using the normal substrates . This approach can be particularly useful when the normal product cannot be used because it is unstable, insoluble, or ineffective (the latter indicated by a very high Ki value). Moreover, the normal product may be consumed as an essential substrate in a coupled assay system for the primary enzyme. Fromm and Zewe used the alternative product inhibition approach in their study of hexokinase. Wratten and Cleland later applied this procedure to exclude the Theorell-Chance mechanism for liver alcohol dehydrogenase. See Abortive Complexes... 

Haldane is also valid for the ordered Bi Bi Theorell-Chance mechanism and the rapid equilibrium random Bi Bi mechanism. The reverse reaction of the yeast enzyme is easily studied an observation not true for the brain enzyme, even though both enzymes catalyze the exact same reaction. A crucial difference between the two enzymes is the dissociation constant (i iq) for Q (in this case, glucose 6-phosphate). For the yeast enzyme, this value is about 5 mM whereas for the brain enzyme the value is 1 tM. Hence, in order for Keq to remain constant (and assuming Kp, and are all approximately the same for both enzymes) the Hmax,f/f max,r ratio for the brain enzyme must be considerably larger than the corresponding ratio for the yeast enzyme. In fact, the differences between the two ratios is more than a thousandfold. Hence, the Haldane relationship helps to explain how one enzyme appears to be more kmeticaUy reversible than another catalyzing the same reaction. 

See also Random Bi Bi Mechanism Ordered Bi Bi Theorell-Chance Mechanism Multisubstrate Mechanisms... 

ORDERED ON-RANDOM OFF Bl Bl DUAL-THEORELL-CHANCE MECHANISM... 

The Theorell-Chance mechanism is an ordered mechanism in which the ternary complex does not accumulate under the reaction conditions, as is found for horse liver alcohol dehydrogenase ... 

T4 lysozyme 33,497 helix stability of 528, 529 hydrophobic core stability of 533, 544 Tanford j8 value 544, 555, 578, 582-Temperature jump 137, 138, 541 protein folding 593 Terminal transferase 408,410 Ternary complex 120 Tertiary structure 22 Theorell-Chance mechanism 120 Thermodynamic cycles 125-131 acid denaturation 516,517 alchemical steps 129 double mutant cycles 129-131, 594 mutant cycles 129 specificity 381, 383 Thermolysin 22, 30,483-486 Thiamine pyrophosphate 62, 83 - 84 Thionesters 478 Thiol proteases 473,482 TNfn3 domain O-value analysis 594 folding kinetics 552 Torsion angle 16-18 Tbs-L-phenylalanine chloromethyl ketone (TPCK) 278, 475 Transaldolase 79 Tyransducin-o 315-317 Transit time 123-125 Transition state 47-49 definition 55... 

The Theorell-Chance mechanism describes the predominating pathway for the conversion of a wide range of primary and also some secondary alcohols by HL-ADH. The same kind of mechanism is also valid for the reaction of DADH with secondary alcohols. As mentioned in the previous section, ternary complexes are kinetically insignificant for the compulsory ordered Theorell-Chance mechanism. 

Cleland (160), steady-state kinetics of a Theorell-Chance mechanism can generally apply also to a rapid-equilibrium random mechanism with two dead-end complexes. However, in view of the data obtained with site-specific inhibitors this latter mechanism is unlikely in the case of the transhydrogenase (70, 71). The proposed mechanism is also consistent with the observation of Fisher and Kaplan (118) that the breakage of the C-H bonds of the reduced nicotinamide nucleotides is not a rate-limiting step in the mitochondrial transhydrogenase reaction. 

A ping-pong di Theorell-Chance mechanism has been deduced for tree laccase from steady-state kinetics (123). This mechanism is characterized by the sequential entry of the two substrates and the immediate... 

As pointed out previously in this review the steady-state kinetics of mitochondrial transhydrogenase, earlier interpreted to indicate a ternary Theorell-Chance mechanism on the basis of competitive relationships between NAD and NADH and between NADP and NADPH, and noncompetitive relationships between NAD" and NADP" and between NADH and NADPH, has been reinterpreted in the light of more recent developments in the interpretation of steady-state kinetic data. Thus, although the product inhibition patterns obtained in the earlier reports [75-77] using submitochondrial particles were close to identical to those obtained in a more recent report [90] using purified and reconstituted transhydrogenase, the reinterpretation favors a random mechanism with the two dead-end complexes NAD E NADP and NADH E NADPH. A random mechanism is also supported by the observation that the transhydrogenase binds to immobilized NAD as well as NADP [105] in the absence of the second substrate. 

LADHee and that the activity disappeared after carboxymethylation of a cysteine residue at the active site of LADH s [145]. In a recent study by Okuda and Okuda it was demonstrated that the -hydroxysteroid dehydrogenase activity in human liver was associated with a major isoenzyme of liver alcohol dehydrogenase (/82, 2) that the activity was inhibited by a chelating agent for Zn, which resides in the active site of the enzyme [146], Kinetic studies with the highly purified isoenzyme showed that neither a Theorell-Chance mechanism nor a simple ordered BiBi mechanism applied to the reaction. Evidence was obtained that the reaction was asymmetric in both directions. It has been established by Fukuba that the 4A-hydro-gen in NADH is involved [147]. 

In the presence of activator, pyruvate, the substrate saturation curves of the R. ruhrum ADP-Glc PPase are hyperbolic at low temperatures. Using kinetic studies its reaction mechanism was studied. The product inhibition patterns eliminated all known sequential mechanisms except the ordered BiBi or Theorell—Chance mechanisms. Small intercept effects suggested the existence of significant concentrations of central transis-tory complexes. Kinetic constants obtained in the study also favored the ordered BiBi mechanism. In addition studies using ATP-[ P]-pyrophosphate isotope exchange at equilibrium supported a sequential-ordered mechanism, which indicated that ATP is the first substrate to bind and that ADP-Glc is the last product to... 

The Haldane relation, Eq. (9), common to all the mechanisms considered in Section II,B, is reasonably well satisfied by data for very many dehydrogenases, in some cases over a range of pH, especially when the difficulty of estimating < pq is taken into account. For the Theorell-Chance mechanism there is a second Haldane relation (S), but this derives from the two limiting maximum rate relations which can be tested separately with greater precision. 

Detailed initial rate studies at pH 7.0 have also been reported with ethanol and the acetylpyridine (APAD) and hypoxanthine (NHD) analogs of NAD as coenzyme (69,70). As would be expected for an ordered mechanism, all the kinetic coefficients are changed. With APAD, the maximum rate relations are, however, again consistent with a Theorell-Chance mechanism, but the maximum rate is significantly smaller than both the rate of dissociation of E-APADH measured directly by the stopped-flow method and the rate of hydride transfer measured from burst experiments. A rate-limiting isomerization of the E-APADH complex has been suggested (71), and is discussed in Section IV. 

The initial rate equation is discussed in detail elsewhere (39,60), and a qualitative examination of the mechanism will suffice here to indicate its main features. If < A-i, fc-2, and k, substrate inhibition will occur at sufficiently large concentrations of B, and will be most pronounced when A is saturating. A large steady-state concentration of EP (k-i < k, k-2) will favor substrate inhibition. If k-t > fc i, substrate activation can occur but only if EP dissociation is the rate-limiting step. (Theorell-Chance mechanism). If k-e = 0, EPB is a dead-end complex, and only 00 in Eq. (1) will be affected by the inhibition which will therefore be uncompetitive with respect to A and complete at high concentration of... 

In the general case, the steady-state maximum rate Eoa (190) will therefore be approached by a biphasic exponential burst of total amplitude jSEo, but the degree of resolution of the two phases will depend upon the relative values of the rate constants in Eq. (21). For the Theorell-Chance mechanism, with k i <3C k-t and k, Eq. (22) simplifies to Eq. (23) ... 

This parameter tells where the rate-limiting step is in the direction with the slower maximum velocity. In an ordered mechanism, R can only vary from zero to one, with 0 corresponding to a Theorell-Chance mechanism where second... 

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