Enzymes are extraordinary catalysts that can accelerate chemical reactions by many orders of magnitude. Yet, how optimized are enzyme catalysts? This question is addressed in two remarkable pieces from 2011 and 2015 titled “the moderately efficient enzyme” by Dan Tawfik and co-workers. They show that, on average, only 1 in 10,000 encounters between substrate molecules and active sites were productive. During my tenure in Dan’s laboratory, we explored some aspects of this ‘moderate’ efficiency. Specifically, we investigated enzymatic temperature dependencies, and found that all enzymes, independent of their adaption to extreme temperature, exhibit similar temperature dependencies and can not track the rate increase of corresponding spontaneous rates. A possible reason for the enzymes ‘moderate’ efficiency is their relative ‘floppiness’, as they exist as an ensemble of conformations. This feature may also be the origin of promiscuous activity, an important mechanism in the emergence of new enzymatic functions. To illustrate this phenomenon, I will review the case of phosphotriesterases and their progenitor lactonases that provide mechanistic insights on how different reactions can occur within the same active site.<br/>These observations pose the question: how optimized proteins can be? We focused on the bacterial phosphate transporter, including representatives from organisms living in arsenate-rich environments. While phosphate is essential and arsenate is toxic, both anions are chemically nearly identical and their discrimination is challenging. We could show that the phosphate transporter discriminates between these two anions, and more so in organisms living in high arsenate conditions. The transporter achieves this prowess via a unique, extremely finely tuned, low-barrier hydrogen bond that allows the transporter and thereby microbes to prefer phosphate over arsenate by ~1,000-fold. Further characterizations of the system reveal conditions for these bonds to form, suggesting their potential existence and putative importance in other optimized systems.

Insights into enzymatic promiscuity and exquisite molecular specificity

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.