Enzymatic Detoxication, Conformational Selection, and the Role of Molten Globule Active Sites

Matthew T Honaker,Mauro Acchione,Wei Zhang,Bengt Mannervik,William M Atkins

doi:10.1074/jbc.m112.445767

Matthew T Honaker, Mauro Acchione + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m112.445767

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Abstract

The role of conformational ensembles in enzymatic reactions remains unclear. Discussion concerning "induced fit" versus "conformational selection" has, however, ignored detoxication enzymes, which exhibit catalytic promiscuity. These enzymes dominate drug metabolism and determine drug-drug interactions. The detoxication enzyme glutathione transferase A1-1 (GSTA1-1), exploits a molten globule-like active site to achieve remarkable catalytic promiscuity wherein the substrate-free conformational ensemble is broad with barrierless transitions between states. A quantitative index of catalytic promiscuity is used to compare engineered variants of GSTA1-1 and the catalytic promiscuity correlates strongly with characteristics of the thermodynamic partition function, for the substrate-free enzymes. Access to chemically disparate transition states is encoded by the substrate-free conformational ensemble. Pre-steady state catalytic data confirm an extension of the conformational selection model, wherein different substrates select different starting conformations. The kinetic liability of the conformational breadth is minimized by a smooth landscape. We propose that "local" molten globule behavior optimizes detoxication enzymes.

Highlights

It is unknown whether enzyme promiscuity is achieved by induced fit or conformational selection
The detoxication enzyme glutathione transferase A1–1 (GSTA1–1), exploits a molten globule-like active site to achieve remarkable catalytic promiscuity wherein the substrate-free conformational ensemble is broad with barrierless transitions between states
The results of the current work, in contrast, demonstrate that nature exploits molten globule behavior to optimize the function of a detoxication enzyme, to the extent that the low temperature DSC transitions in wild-type GSTA1–1 and promiscuous GST mutants correspond to local conformational heterogeneity and rapidly fluctuating structures

Summary

Background

It is unknown whether enzyme promiscuity is achieved by induced fit or conformational selection. The detoxication enzyme glutathione transferase A1–1 (GSTA1–1), exploits a molten globule-like active site to achieve remarkable catalytic promiscuity wherein the substrate-free conformational ensemble is broad with barrierless transitions between states. The specific advantages and disadvantages of conformational selection and induced fit are not defined well This uncertainty becomes more interesting for detoxication enzymes that must access chemically disparate transition states for different substrates. The results reveal how catalytic networks are advantageous to functionally promiscuous enzymes, and they provide an example wherein nature has exploited a localized fluid molten globule at an enzyme active site to minimize the kinetic liability of conformational diversity. These structural variants provide an ideal model system to explore the costs and benefits of conformational ensembles in detoxication catalysis in a thermodynamic context

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