How enzymes harness highly unfavorable proton transfer reactions

Abstract

Acid-base reactions that are exceedingly unfavorable under standard conditions can be catalytically important at enzyme active sites. For example, in triose phosphate isomerase, a glutamate side chain (nominal pK_a ≈ 4 in solution) can in fact deprotonate a CH group that is vicinal to a carbonyl (pK_a ≈ 18 in solution). This is true because of three distinct interactions: (a) ground state pK_a shifts due to environment polarity and electrostatics; (b) dramatic increases in effective molarity due to optimization of proximity and orientation; and (c) transition state pK_a shifts due to binding interactions and the formation of strong low barrier hydrogen bonds. In this report, we review the literature showing that the sum of these three effects supplies more than enough free energy to push forward proton transfer reactions that under standard conditions are exceedingly nonspontaneous and slow.

Keywords: acid-base chemistry; activation energy; enzyme catalysis.

FIGURE 1

Menger's “Split‐Site Model” of enzyme catalysis. The uncatalyzed reaction (S ➔ S^‡) is the purple curve. E' (black) is a hypothetical enzyme with K _m = 1 M (i.e., ∆G _b(E'·S) = 0, so ∆G _E'S(R) = −∆G _E'S(B)). E (red) is a typical enzyme with K _m ≈ 1 μM (i.e., ∆G _b(E·S) = −8 kcal/mol); ∆G _d(E·S) = ∆G _ES(B) + ∆G _ES(R) = x _B + x _R