Molecular Mechanisms: From Enzymes to Evolution - Molecular Theory of the Living Cell - page 204

Fig. 7.2 The Franck-Condon mechanism of selective ligand uniport (i.e., the movement of one
ligand across the membrane)
Stein et al. [1974]). The mechanism of action of all molecular motors and
membrane pumps appear to depend on both BFC ( transformation ) and BPC
( translocation ) as depicted in Fig. 7.2 .
One interesting consequence of the Principle of Slow and Fast Processes
(PSFP), or the generalized Franck-Condon principle (GFCP) (see Sect. 2.2.3 ), as
applied to enzymic catalysis, is that enzymes must undergo conformational changes
before substrates can bind to their active sites to initiate catalysis. This conclusion is
diametrically opposed to the induced-fit hypothesis of Koshland (1958) widely
discussed in biochemistry textbooks [e.g., see Fig. 8.10 on p. 200 in (Berg et al.
2002)]. It may be convenient to refer to the PSFP-based mechanism of the
substrate-enzyme interactions as the “pre-fit hypothesis” to contrast it with the
induced-fit hypothesis of Koshland, “pre-fit” because enzymes are postulated to
have been selected by evolution (and hence carry genetic information) on the basis
of their ability to assume certain conformational states capable of “capturing” or
binding their elusive substrates as they bump into them randomly due to Brownian
motions or thermal fluctuations. These two contrasting mechanisms of binding are
schematically represented in Processes 7.8 and 7.9 , where L stands for a ligand,