Subcellular Systems - Molecular Theory of the Living Cell - page 323

Fig. 11.18 The distribution of the kinetic parameters (on-times) measured from COx molecules.
( Left ) The distribution of the on-times (i.e., the duration of the times that the FAD molecule
remains in its oxidized state and fluorescent). This phenomenon of varying on-times is known as
“dynamic disorder” or “dynamic heterogeneity” (Zwanzig 1990). Cholesterol concentration 0.2

mM. ( Right ) The distribution of the rate constants, k 2 , for the reduction of FAD to FADH 2
measured from 33 COx molecules. At any given time, the k 2 values measured from different
molecules of COx vary by a factor of up to 5 (Reproduced from http://www.nigms.nih.gov/News/
Reports/single_molecules.htm )

¼

the probability of the oxidized form of COx to be reduced by cholesterol (present at
0.2 mM) within less than 500 ms is about 80%. Since the off-times in Fig. 11.17
appear shorter than the on-times on average, the probability of reduced COx being
reoxidized by molecular oxygen is faster than the rate of the reduction of COx
by cholesterol. Also, since the concentration of cholesterol and oxygen molecules
were approximately the same (0.20 vs. 0.25 mM), it may be concluded that the rate
constant for the reduction of COx, i.e., k 1 in Fig. 11.17 , is smaller than the rate
constant, k 2 , for the reoxidation of COx.
Lu et al. (1998) proposed a simple mechanism, Scheme ( 11.16 ), to account for
the distribution of the off-times shown in Fig. 11.18 :

k 1
k - 1

k 2

S ! E-FAD

a

þ E-FAD S ! E-FADH c þ P (11.16)

b