The Low Barrier Hydrogen Bond in Enzymatic Catalysis

Abstract

low barrier hydrogen bond. The proposal that low barrier (i.e.short, very strong) hydrogen bonds (LBHBs)1 play a role in enzymatic catalysis was first put forth in 1993 and 1994 (1Gerlt J.A. Gassman P.G. Biochemistry. 1993; 32: 11934-11952Crossref PubMed Scopus (376) Google Scholar, 2Gerlt J.A. Gassman P.G. J. Am. Chem. Soc. 1993; 115: 11552-11568Crossref Scopus (386) Google Scholar, 3Cleland W.W. Kreevoy M.M. Science. 1994; 264: 1887-1890Crossref PubMed Scopus (1062) Google Scholar, 4Frey P.A. Whitt S. Tobin J. Science. 1994; 264: 1927-1930Crossref PubMed Scopus (739) Google Scholar). The proposal was accepted by some but rejected by others (5Warshel A. Papazyan A. Kollman P.A. Science. 1995; 269: 102-104Crossref PubMed Scopus (334) Google Scholar, 6Warshel A. Papazyan A. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 13665-13670Crossref PubMed Scopus (186) Google Scholar, 7Guthrie J.P. Chem. Biol. 1996; 3: 163-170Abstract Full Text PDF PubMed Google Scholar, 8Scheiner S. Kar T. J. Am. Chem. Soc. 1995; 117: 6970-6975Crossref Scopus (135) Google Scholar). Initial rejection on theoretical grounds has been followed by increasing experimental support, and recent improvements in theory have been able to account for the experimental observations of LBHBs in enzymes (9Gerlt J.A. Kreevoy M.M. Cleland W.W. Frey P.A. Chem. Biol. 1997; 4: 259-267Abstract Full Text PDF PubMed Scopus (335) Google Scholar, 10Pudzianowski A.T. Recent Res. Devel. Physical Chem. 1997; 1: 81-97Google Scholar, 11Pan Y. McAllister M.A. J. Am. Chem. Soc. 1997; 119: 7561-7566Crossref Scopus (85) Google Scholar, 12Pan Y. McAllister M.A. J. Org. Chem. 1997; 62: 8171-8176Crossref PubMed Scopus (40) Google Scholar, 13Pan Y. McAllister M.A. J. Am. Chem. Soc. 1998; 120: 166-169Crossref Scopus (83) Google Scholar, 14Smallwood C.J. McAllister M.A. J. Am. Chem. Soc. 1997; 119: 11277-11281Crossref Scopus (72) Google Scholar, 15Kumar G.A. McAllister M.A. J. Am. Chem. Soc. 1998; 120: 3159-3165Crossref Scopus (62) Google Scholar). In this minireview we will explain the original proposal, summarize the experimental data from the past few years, and argue that LBHBs do play important roles in enzymatic reactions. The strength of a hydrogen bond depends on its length and linearity, the nature of its microenvironment, and the degree to which the pK values of the conjugate acids of the heavy atoms sharing the proton are matched. In water, the hydrogen-bonded oxygens are separated by ∼2.8 Å, and the ΔH of formation is ∼5 kcal mol−1. The hydrogen bonds in water are, however, weak because of the poor pK match between the participating oxygen atoms. Because the pKs of H3O+ and H2O are −1.7 and 15.7, respectively, the proton in the structure H2O···H–OH is tightly associated with the OH− group as a water molecule. In the gas phase, where the dielectric constant is low, hydrogen bonds between heteroatoms with matched pKs can be very short and strong, and experimental as well as calculated values of ΔH of formation can approach 25 or 30 kcal mol−1 (16Garcia-Viloca M. Gonzalez-Lafont A. Lluch J.M. J. Am. Chem. Soc. 1997; 119: 1081-1086Crossref Scopus (144) Google Scholar, 17McAllister M.A. Can. J. Chem. 1997; 75: 1195-1202Crossref Scopus (65) Google Scholar). Likewise, in crystals hydrogen bonds can be very strong. The O–O distance in the ion [H–O···H···O–H]– in a crystal of a chromium complex is only 2.29 Å (18Abu-Dari K. Raymond K.N. Freyberg D.P. J. Am. Chem. Soc. 1979; 101: 3688-3689Crossref Scopus (92) Google Scholar, 19Abu-Dari K. Freyberg D.P. Raymond K.N. Inorg. Chem. 1979; 18: 2427-2433Crossref Scopus (29) Google Scholar). In organic solvents, strong hydrogen bonds can also form, although the ΔH of formation probably never exceeds 20 kcal mol−1. Recent calculations suggest that once the dielectric constant is at least 6 the strength of a strong hydrogen bond levels off at a level about half that in the gas phase (13Pan Y. McAllister M.A. J. Am. Chem. Soc. 1998; 120: 166-169Crossref Scopus (83) Google Scholar, 17McAllister M.A. Can. J. Chem. 1997; 75: 1195-1202Crossref Scopus (65) Google Scholar). Because the active site of an enzyme is no longer aqueous once it has closed around a substrate, the properties of hydrogen bonds in organic solvents are highly pertinent to enzymatic catalysis. What happens energetically as hydrogen bonds become shortened can be seen in Fig. 1. Structure Arepresents the situation in water, where the hydrogen is firmly attached to either the left-hand or right-hand oxygen and is more loosely bonded to the other one, with an O–O distance of ≥2.8 Å. There is an energy barrier between the two possible positions of the hydrogen, with the zero point energy levels shown in Fig. 1. Such a hydrogen bond is essentially electrostatic, and the covalent O–H bond is the usual 0.9–1.0 Å in length. As the overall O–O distance is shortened, the energy barrier drops until it reaches the zero point energy level at an O–O distance of ∼2.5 Å (Fig. 1 B); this is