Rate constants cooperative transitions

Rate constants were determined as follows ki was determined from the rate of clearance of [ N]ammonia from rat blood (Freed, B. R., and Cooper, A. J. L., unpublished results), kg was deduced by determining the best fit necessary to yield a BUI of 24% given a CBF of 100 mL/100 g/min (42) and a brain transit time of 3 s (39). kg was assumed to be one-half kg based on the known pH difference between the brain and the blood (48). k and kg were assumed to equal kg (i.e., no pH difference between small and large pools was assumed), kg (and 7) were calculated from the known activity of whole brain glutamine synthetase... 

With the exception of helix/coil transitions of polypeptides and polynucleotides, the kinetics of conformational transitions has only been investigated to a slight extent. Relatively large rate constants of 10 -10 s" have been obtained for these helix/coil transitions. However, rate constants of 10" to 1 s have been obtained for denaturing processes where helix / coil transitions also play a role (see also Chapter 4.4.2). The high rates of helix / coil transitions are undoubtedly due to the cooperativity of the process. The low rates for denaturing must therefore result from nonhelical regions. 

Identical rate constants have been reported for saccharide-binding by concanavalin A when Mn +, Co +, or Zn + occupy the SI site and Ca + occupies the S2 site. The affinity of the protein for saccharides is dependent upon the metal occupancies of both sites and is sensitive to the identity of the metal ion in site S2. Titrations of metal employing e.s.r. spectroscopy and equilibrium dialysis have revealed that the binding of Mn + to concanavalin A is cooperative in the presence of Ca + and non-co-operative in its absence. The degree of co-operativity also increases with increasing pH. At pH 7.2 only Ca + and not a transition-metal ion is required for saccharide binding to concanavalin A, whereas at pH 5, at which most other studies have been carried out, both a transition metal and Ca + are necessary for binding. ... 

Phosphofructokinase was one of the first enzymes to which Monod and his colleagues applied the symmetry model of allosteric transitions. It contains four identical subunits, each of which has both an active site and an allosteric site. The cooperativity of the kinetics suggests that the enzyme can adopt two different conformations (T and R) that have similar affinities for ATP but differ in their affinity for fructose-6-phosphate. The binding for fructose-6-phosphate is calculated to be about 2,000 times tighter in the R conformation than in T. When fructose-6-phosphate binds to any one of the subunits, it appears to cause all four subunits to flip from the T conformation to the R conformation, just as the symmetry model specifies. The allosteric effectors ADP, GDP, and phosphoenolpyruvate do not alter the maximum rate of the reaction but change the dependence of the rate on the fructose-6-phosphate concentration in a manner suggesting that they change the equilibrium constant (L) between the T and R conformations. 

Modulation of the z component of the local field cannot infiuence the z component of the spin magnetization and thus cannot induce spin flips. However, the X and y components of the magnetization that stem from coherent superposition of spin states with lAm l = 1 are influenced, as the modulation of the z component can induce a cooperative flip-flop of two spins. The energy change during such a flip-flop is close to zero, as the two transition frequencies are almost equal. Such transverse or spin-spin relaxation with time constant 7] is thus related to spectral density /(O) at zero frequency. This spectral density 7(0) increases with increasing Tc, i.e., with a slowdown of the process. The total rate of transverse relaxation includes a contribution of spin flips, so that T2- =(T2 ) +(271)- . 

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