| Comments |
Section 8.4.2 5th paragraph (paraphrased): "How efficient can an enzyme be? this question can be approached by determining whether there are any physical limits on the value of kcat/KM.
Note that this ratio depends on k1, k^-1, and k2, as can be shown by substituting for KM. kcat/KM=kcat/(k^-1+kcat/k1)=
(kcat/(kcat + k^-1))×k1<k1. The kcat/KM ratios of the enzymes superoxide dismutase, acetylcholinesterase,
and triosephosphate isomerase are between 10^8 and 10^9 s^-1×M^-1. Enzymes such as these that have kcat/KM ratios at the upper limits have attained kinetic perfection. Their catalytic velocity is restricted only by
the rate at which they encounter substrate in the solution." Section 8.4.2 6th paragraph: "The kcat/KM ratios of the enzymes superoxide dismutase, acetylcholinesterase, and triose phosphate isomerase are between 10^8 and 10^9 S^-1 M^-1. Enzymes such as these that have kcat/KM ratios at the upper limits have attained kinetic perfection. Their catalytic velocity is restricted only by the rate at which they encounter substrate in the solution (Table 8.8). Any further gain in catalytic rate can come only by decreasing the time for diffusion. Remember that the active site is only a small part of the total enzyme structure. Yet, for catalytically perfect enzymes, every encounter between enzyme and substrate is productive. In these cases, there may be attractive electrostatic forces on the enzyme that entice the substrate to the active site. These forces are sometimes referred to poetically as Circe effects." See BNID 103916 |