ABSTRACT

I. The Problem of Enzyme Catalysis .................................................................................. 766

A. Magnitudes of Catalytic Accelerations by Enzymes .............................................. 766

B. Transition-State Stabilization and Catalysis............................................................ 767

C. H-Bonds as a Means of Transition-State Stabilization ........................................... 768

D. Beyond the Transition-State Theory of Catalysis ................................................... 770

II. Structure and Strength of H-Bonds ................................................................................. 771

A. The Concept of H-Bond Strength............................................................................ 771

B. Categorization of H-Bonds ...................................................................................... 772

C. Some Probes of Hydrogen Bonds............................................................................ 772

1. NMR Approaches.............................................................................................. 773

2. Theoretical Studies of H-Bonds........................................................................ 773

3. Thermochemical, Spectroscopic, and Structural Approaches .......................... 774

III. Isotope Effects in Hydrogen Bonding ............................................................................. 775

A. Simple H-Bonds ....................................................................................................... 775

B. Unusual H-Bonds ..................................................................................................... 776

C. Primary Catalytic H-Bonds...................................................................................... 776

D. Secondary Catalytic H-Bonds.................................................................................. 777

IV. Issues in H-Bonding and Enzyme Catalysis ................................................................... 777

A. Cautionary Notes on Mutations at H-Bonding Sites in Enzymes .......................... 777

1. H-Bonds in the Orientation of Ligands for Optimal Catalysis ........................ 777

2. The Catalytic Triad of Serine Hydrolases ........................................................ 779

B. Primary Catalytic H-Bonds...................................................................................... 781

C. Secondary Catalytic H-Bonds.................................................................................. 781

1. The Catalytic Triad of Serine Hydrolases ........................................................ 781

2. The Oxyanion Hole of Serine Hydrolases........................................................ 785

D. Conformational Changes Signaled by Proton Inventories ...................................... 787

V. Summary .......................................................................................................................... 788

References..................................................................................................................................... 789

The kinetics of enzyme-catalyzed reactions (Lescovac;

Segel;

see also the chapter in this volume

by Cleland) are characterized for the most part by two kinds of rate constants, customarily given the

notation k

(a first-order rate constant, dimensions sec

, corresponding to the circumstance in

which the enzyme is fully saturated by substrates, reaction intermediates, or products as ligands)

and k

=K

(a second-order rate constant, dimensions M

sec

, corresponding to the

circumstance in which some form of the enzyme is reacting with a free substrate molecule). To

measure the magnitude of the catalytic acceleration produced by an enzyme, each of these rate

constants can be compared to the rate constant of a reaction not involving an enzyme, often called

the “uncatalyzed reaction” or sometimes the “standard reaction”.