The role of protein dynamics in the evolution of new enzyme function

An optimal distance cutoff for contact-based Protein Structure Networks using side-chain centers of mass

Proteins are highly dynamic entities attaining a myriad of different conformations. Protein side chains change their states during dynamics, causing clashes that are propagated at distal sites. A convenient formalism to analyze protein dynamics is based on network theory using Protein Structure Networks (PSNs). Despite their broad applicability, few efforts have been devoted to benchmarking PSN methods and to provide the community with best practices. In many applications, it is convenient to use the centers of mass of the side chains as nodes. It becomes thus critical to evaluate the minimal distance cutoff between the centers of mass which will provide stable network properties. Moreover, when the PSN is derived from a structural ensemble collected by molecular dynamics (MD), the impact of the MD force field has to be evaluated. We selected a dataset of proteins with different fold and size and assessed the two fundamental properties of the PSN, i.e. hubs and connected components. We identified an optimal cutoff of 5 Å that is robust to changes in the force field and the proteins. Our study builds solid foundations for the harmonization and standardization of the PSN approach.

Specificity Effects of Amino Acid Substitutions in Promiscuous Hydrolases: Context-Dependence of Catalytic Residue Contributions to Local Fitness Landscapes in Nearby Sequence Space

Catalytic promiscuity can facilitate evolution of enzyme functions-a multifunctional catalyst may act as a springboard for efficient functional adaptation. We test the effect of single mutations on multiple activities in two groups of promiscuous AP superfamily members to probe this hypothesis. We quantify the effect of site-saturating mutagenesis of an analogous, nucleophile-flanking residue in two superfamily members: an arylsulfatase (AS) and a phosphonate monoester hydrolase (PMH). Statistical analysis suggests that no one physicochemical characteristic alone explains the mutational effects. Instead, these effects appear to be dominated by their structural context. Likewise, the effect of changing the catalytic nucleophile itself is not reaction-type-specific. Mapping of "fitness landscapes" of four activities onto the possible variation of a chosen sequence position revealed tremendous potential for respecialization of AP superfamily members through single-point mutations, highlighting catalytic promiscuity as a powerful predictor of adaptive potential.

Directed evolution to improve the catalytic efficiency of urate oxidase from Bacillus subtilis

Urate oxidase is a key enzyme in purine metabolism and catalyzes the oxidation of uric acid to allantoin. It is used to treat hyperuricemia and gout, and also in a diagnostic kit. In this study, error-prone polymerase chain reaction and staggered extension process was used to generate a mutant urate oxidase with improved enzyme activity from Bacillus subtilis. After several rounds of mutagenesis and screening, two mutants 6E9 and 8E279 were obtained which exhibited 2.99 and 3.43 times higher catalytic efficiency, respectively. They also exhibited lower optimal reaction temperature and higher thermo-stability. D44V, Q268R and K285Q were identified as the three most beneficial amino acid substitutions introduced by site-directed mutagenesis. D44V/Q268R, which was obtained through random combination of the three mutants, displayed the highest catalytic activity. The K_m,k_cat/K_m and enzyme activity of D44V/Q268R increased by 68%, 83% and 129% respectively, compared with that of wild-type urate oxidase. Structural modeling indicated that mutations far from the active site can have significant effects on activity. For many of them, the underlying mechanisms are still difficult to explain from the static structural model. We also compared the effects of the same set of single point mutations on the wild type and on the final mutant. The results indicate strong effects of epistasis, which may imply that the mutations affect catalysis through influences on protein dynamics besides equilibrium structures.

Structural and functional innovations in the real-time evolution of new (βα)8 barrel enzymes

New genes can arise by duplication and divergence, but there is a fundamental gap in our understanding of the relationship between these genes, the evolving proteins they encode, and the fitness of the organism. Here we used crystallography, NMR dynamics, kinetics, and mass spectrometry to explain the molecular innovations that arose during a previous real-time evolution experiment. In that experiment, the (βα)₈ barrel enzyme HisA was under selection for two functions (HisA and TrpF), resulting in duplication and divergence of the hisA gene to encode TrpF specialists, HisA specialists, and bifunctional generalists. We found that selection affects enzyme structure and dynamics, and thus substrate preference, simultaneously and sequentially. Bifunctionality is associated with two distinct sets of loop conformations, each essential for one function. We observed two mechanisms for functional specialization: structural stabilization of each loop conformation and substrate-specific adaptation of the active site. Intracellular enzyme performance, calculated as the product of catalytic efficiency and relative expression level, was not linearly related to fitness. Instead, we observed thresholds for each activity above which further improvements in catalytic efficiency had little if any effect on growth rate. Overall, we have shown how beneficial substitutions selected during real-time evolution can lead to manifold changes in enzyme function and bacterial fitness. This work emphasizes the speed at which adaptive evolution can yield enzymes with sufficiently high activities such that they no longer limit the growth of their host organism, and confirms the (βα)₈ barrel as an inherently evolvable protein scaffold.

Flexibility and Design: Conformational Heterogeneity along the Evolutionary Trajectory of a Redesigned Ubiquitin

Although protein design has been used to introduce new functions, designed variants generally only function as well as natural proteins after rounds of laboratory evolution. One possibility for this pattern is that designed mutants frequently sample nonfunctional conformations. To test this idea, we exploited advances in multiconformer modeling of room-temperature X-ray data collection on redesigned ubiquitin variants selected for increasing binding affinity to the deubiquitinase USP7. Initial core mutations disrupt natural packing and lead to increased flexibility. Additional, experimentally selected mutations quenched conformational heterogeneity through new stabilizing interactions. Stabilizing interactions, such as cation-pi stacking and ordered waters, which are not included in standard protein design energy functions, can create specific interactions that have long-range effects on flexibility across the protein. Our results suggest that increasing flexibility may be a useful strategy to escape local minima during initial directed evolution and protein design steps when creating new functions.

Structural and energetic study of cation–π–cation interactions in proteins

Statistical and energetic analysis of cation–π–cation motifs in protein structures suggests a potential stabilizing role in the protein fold.

Constrained evolution of a bispecific enzyme: lessons for biocatalyst design

Analysis of the natural evolution of bispecificity in triazine hydrolase highlights the importance of epistasis in protein engineering and evolution.

Molecular dynamics simulations elucidate conformational selection and induced fit mechanisms in the binding of PD-1 and PD-L1

PD-L1 binds PD-1 through a complex mechanism including both the conformational selection and induced fit pathways.

Dynamic Conformational States Dictate Selectivity toward the Native Substrate in a Substrate-Permissive Acyltransferase

Hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyltransferase (HCT) is an essential acyltransferase that mediates flux through plant phenylpropanoid metabolism by catalyzing a reaction between p-coumaroyl-CoA and shikimate, yet it also exhibits broad substrate permissiveness in vitro. How do enzymes like HCT avoid functional derailment by cellular metabolites that qualify as non-native substrates? Here, we combine X-ray crystallography and molecular dynamics to reveal distinct dynamic modes of HCT under native and non-native catalysis. We find that essential electrostatic and hydrogen-bonding interactions between the ligand and active site residues, permitted by active site plasticity, are elicited more effectively by shikimate than by other non-native substrates. This work provides a structural basis for how dynamic conformational states of HCT favor native over non-native catalysis by reducing the number of futile encounters between the enzyme and shikimate.

Protein dynamics: Conformational footprints

The mechanisms by which proteins evolve new functions can be slow and mysterious. Comprehensive structural analysis of enzyme variants reveal how gradual enrichments of pre-existing populations with the right productive dynamics for new functions can accomplish this aim.

Functional Trade-Offs in Promiscuous Enzymes Cannot Be Explained by Intrinsic Mutational Robustness of the Native Activity

The extent to which an emerging new function trades off with the original function is a key characteristic of the dynamics of enzyme evolution. Various cases of laboratory evolution have unveiled a characteristic trend; a large increase in a new, promiscuous activity is often accompanied by only a mild reduction of the native, original activity. A model that associates weak trade-offs with “evolvability” was put forward, which proposed that enzymes possess mutational robustness in the native activity and plasticity in promiscuous activities. This would enable the acquisition of a new function without compromising the original one, reducing the benefit of early gene duplication and therefore the selection pressure thereon. Yet, to date, no experimental study has examined this hypothesis directly. Here, we investigate the causes of weak trade-offs by systematically characterizing adaptive mutations that occurred in two cases of evolutionary transitions in enzyme function: (1) from phosphotriesterase to arylesterase, and (2) from atrazine chlorohydrolase to melamine deaminase. Mutational analyses in various genetic backgrounds revealed that, in contrast to the prevailing model, the native activity is less robust to mutations than the promiscuous activity. For example, in phosphotriesterase, the deleterious effect of individual mutations on the native phosphotriesterase activity is much larger than their positive effect on the promiscuous arylesterase activity. Our observations suggest a revision of the established model: weak trade-offs are not caused by an intrinsic robustness of the native activity and plasticity of the promiscuous activity. We propose that upon strong adaptive pressure for the new activity without selection against the original one, selected mutations will lead to the largest possible increases in the new function, but whether and to what extent they decrease the old function is irrelevant, creating a bias towards initially weak trade-offs and the emergence of generalist enzymes.

Understanding how enzymes evolve is a fundamental question that can help us decipher not only the mechanisms of evolution on a higher level, i.e., whole organisms, but also advances our knowledge of sequence-structure-function relationships as a guide to artificial evolution in the test tube. An important yet unexplained phenomenon occurs during the evolution of a new enzymatic function; it has been observed that new and ancestral functions often trade-off only weakly, meaning the original native activity is initially maintained at a high level despite drastic improvement of the new promiscuous activity. It has previously been proposed that weak trade-offs occur because the native activity is robust to mutations while the promiscuous activity is not. However, the present work contradicts this hypothesis, based on the detailed characterization of mutational effects on both activities in two examples of enzyme evolution. We propose an alternative explanation: the weak activity trade-off is consistent with being a by-product of strong selection for the new activity rather than an intrinsic property of the native activity.