10.3. Molecular implementation of mechanical principles[[Student version, January 17, 2003]] 375
−∆G unchangedtotal free energyG‡
E+SESES‡EPE+Pb
free energyfree energyunassisted S→Preaction coordinatedeformation of ESS‡Pa
interactionG‡−∆Greducedreaction coordinateFigure 10.17:(Sketch graphs.) (a)Imagined free energy landscapes corresponding to the story line in Figure 10.16.
Topcurve:The substrate S can spontaneously convert to P only by surmounting a large activation barrierG‡,the
free energy of the transition state S‡relative to S.Middle curve:the interaction free energy between substrate and
product includes a large binding free energy (dip), as well as the entropic cost of aligning the substrate properly
relative to the enzyme (slight bumps on either side of the dip).Lower curve:The binding free energy may be partly
offset by a deformation of the enzyme upon binding, but still the net effect of the enzyme is to reduce the barrierG‡.
All three curves have been shifted by arbitrary constants in order to show them on a single set of axes. (b)Imagined
net free energy landscape obtained by summing the three curves in (a). The enzyme has reducedG‡,but it cannot
change ∆G.
machine,progressively processing a large batch of S molecules. When many molecules of S are
available, then the net driving force for the reaction includes an entropic term of the formkBTlncS,
wherecSis the concentration. (See Equation 8.3 on page 261 and Equation 8.13 on page 267.)
The effect of a high concentration of S, then, is to pull the left end of the free-energy landscape
(Figure 10.17b) upward, reducing or eliminating any activation barrier to the formation of the
complex ES and thus speeding up the reaction. Similarly, an increase incPpulls up the right end