Page 579 - Carrahers_Polymer_Chemistry,_Eighth_Edition

542 Carraher’s Polymer Chemistry

Reactant

Enzyme

Tight; inactive Relaxed; active

(a) (b) (c) (d)
FIGURE 16.7 Concerted model for allosteric enzymes. The major steps are (a) and (b). An equilibrium exists
between the tight (a) and relaxed (b) forms of the allosteric enzyme. The reactant molecule(s) approaches the
reactive site of one of the enzyme sites present in the relaxed form (c). Binding occurs, shifting the equilibrium
to the relaxed form(s). The second site is bound (d).

Reactant

Enzyme

Tight; inactive Relaxed; active

(a) (b) (c) (d)

FIGURE 16.8 Sequential model for allosteric enzymes. The individual steps are as follows: the reactive
molecule(s) approach the reactive site (a), which typically has a cavity similar to, but not the same as, the reac-
tant molecule(s); a conformational change is effected so that the reactive molecule(s) can be bound (b); the
bound portion of the enzyme changes shape; this shape change is transmitted to the other unit(s) containing
active sites (c); the remaining enzyme portion containing active sites undergoes a conformational change that
makes binding easier.

9/14/2010 3:43:14 PM
K10478.indb 542 9/14/2010 3:43:14 PM
K10478.indb 542