Allosteric enzymes sequential interaction model

Two major models for allosteric enzymes have been proposed. These are the sequential interaction model and the concerted-symmetry ... 

Figure 4-50 The sequential interaction model of allosteric enzymes. As each site is occupied, the subunit carrying the site undergoes a change from the A conformation to the B conformation. As a result, new interactions between subunits are established and the affinities of the vacant sites change. K represents a dissociation constant. Thus, if the affinities of vacant sites increase, a, b, and c (the interaction factors) are <1 and we observe positive cooperativity (a sigmoidal velocity curve). The sequential interaction model also provides for, negative cooperativity (a, b, and c are > ). (o ) Dimer model. The two ways of arranging S to form a singly-occupied species is shown, (fe) Tetramer model. For simplicity, only one arrangement of each occupied species is shown.

An allosteric dimer has an interaction factor, a, of 0.2 when analyzed according to the sequential interaction model (i.e., the binding of the first molecule of S increases the binding constant of the vacant site by a factor of 5—the dissociation constant of the vacant site decreases to 0.2 of the original value), (a) What is the relative distribution of enzyme species at [S] = 0.3 Ks (b) What is the specific velocity at [S] = 0.3 Ks (c) Will the calculated value of n,pp equal 2 ... 

The MWC concerted-symmetry and KNF sequential interaction models may be considered as extreme cases of the more general model shown in Fig. 19. A general model for a four-site allosteric enzyme involves the hybrid oligomers. The first and the fourth column in Fig. 19 represent the concerted-symmetry model. The diagonal represents the sequential interaction model. As shown, there are 25 different types of enzyme forms. If the potential nonequivalent complexes are included (such as, e.g., two different T3RS2), the number raises to 44 possible enzyme forms (Hammes Wu, 1971). 

Schematic diagram of conformational changes of. sequentially induced-fit model for a dimeric allosteric enzyme. The TT conformation is progressively converted to the RR conformation via the intermediate TR conformation through cooperative interaction in the presence of the positive modulator. In the presence of the negative modulator, the opposite conformational changes occur. In this model, the notion of. symmetry is discarded and the concept of induced fit is emphasized.

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