An Activator-Blocker Pair Provides a Controllable On-Off Switch for a Ketosteroid Isomerase Active Site Mutant

Chemical Modification for the "off-/on" Regulation of Enzyme Activity

Enzymes with excellent catalytic performance play important roles in living organisms. Advances in strategies for enzyme chemical modification have enabled powerful strategies for exploring and manipulating enzyme functions and activities. Based on the development of chemical enzyme modifications, incorporating external stimuli-responsive features-for example, responsivity to light, voltage, magnetic force, pH, temperature, redox activity, and small molecules-into a target enzyme to turn "on" and "off" its activity has attracted much attention. The ability to precisely control enzyme activity using different approaches would greatly expand the chemical biology toolbox for clarification and detection of signal transduction and in vivo enzyme function and significantly promote enzyme-based disease therapy. This review summarizes the methods available for chemical enzyme modification mainly for the off-/on control of enzyme activity and particularly highlights the recent progress regarding the applications of this strategy. This article is protected by copyright. All rights reserved.

The Organization of Active Site Side Chains of Glycerol-3-phosphate Dehydrogenase Promotes Efficient Enzyme Catalysis and Rescue of Variant Enzymes

A comparison of the values of k_cat/K_m for reduction of dihydroxyacetone phosphate (DHAP) by NADH catalyzed by wild type and K120A/R269A variant glycerol-3-phosphate dehydrogenase from human liver (hlGPDH) shows that the transition state for enzyme-catalyzed hydride transfer is stabilized by 12.0 kcal/mol by interactions with the cationic K120 and R269 side chains. The transition state for the K120A/R269A variant-catalyzed reduction of DHAP is stabilized by 1.0 and 3.8 kcal/mol for reactions in the presence of 1.0 M EtNH₃⁺ and guanidinium cation (Gua⁺), respectively, and by 7.5 kcal/mol for reactions in the presence of a mixture of each cation at 1.0 M, so that the transition state stabilization by the ternary E·EtNH₃⁺·Gua⁺ complex is 2.8 kcal/mol greater than the sum of stabilization by the respective binary complexes. This shows that there is cooperativity between the paired activators in transition state stabilization. The effective molarities (EMs) of ∼50 M determined for the K120A and R269A side chains are ≪10⁶ M, the EM for entropically controlled reactions. The unusually efficient rescue of the activity of hlGPDH-catalyzed reactions by the HP_i/Gua⁺ pair and by the Gua⁺/EtNH₃⁺ activator pair is due to stabilizing interactions between the protein and the activator pieces that organize the K120 and R269 side chains at the active site. This “preorganization” of side chains promotes effective catalysis by hlGPDH and many other enzymes. The role of the highly conserved network of side chains, which include Q295, R269, N270, N205, T264, K204, D260, and K120, in catalysis is discussed.

Modulating Antibody Structure and Function through Directed Mutations and Chemical Rescue

Monoclonal antibody therapeutics have revolutionized the treatment of diseases such as cancer and autoimmune disorders, and also serve as research reagents for diverse and unparalleled applications. To extend their utility in both contexts, we have begun development of tunable antibodies, whose activity can be controlled by addition of a small molecule. Conceptually, we envision that incorporating cavity-forming mutations into an antibody can disrupt its structure, thereby reducing its affinity for antigen; addition of a small molecule may then restore the active structure, and thus rescue antigen binding. As a first proof of concept toward implementing this strategy, we have incorporated individual tryptophan to glycine mutations into FITC-E2, an anti-fluorescein single-chain variable fragment (scFv). We find that these can disrupt the protein structure and diminish antigen binding, and further that both structure and function can be rescued by addition of indole to complement the deleted side chain. While the magnitude of the affinity difference triggered by indole is modest in this first model system, it nonetheless provides a framework for future mutation/ligand pairs that may induce more dramatic responses. Disrupting and subsequently rescuing antibody activity, as exemplified by this first example, may represent a new approach to "design in" fine-tuned control of antibody activity for a variety of future applications.