Biosynthetic consequences of multiple sequential post-transition-state bifurcations

Discovering Monoterpene Catalysis Inside Nanocapsules with Multiscale Modeling and Experiments

Large-scale production of natural products, such as terpenes, presents a significant scientific and technological challenge. One promising approach to tackle this problem is chemical synthesis inside nanocapsules, although enzyme-like control of such chemistry has not yet been achieved. In order to better understand the complex chemistry inside nanocapsules, we design a multiscale nanoreactor simulation approach. The nanoreactor simulation protocol consists of hybrid quantum mechanics-molecular mechanics-based high temperature Langevin molecular dynamics simulations. Using this approach we model the tail-to-head formation of monoterpenes inside a resorcin[4]arene-based capsule (capsule I). We provide a rationale for the experimentally observed kinetics of monoterpene product formation and product distribution using capsule I, and we explain why additional stable monoterpenes, like camphene, are not observed. On the basis of the in-capsule I simulations, and mechanistic insights, we propose that feeding the capsule with pinene can yield camphene, and this proposal is verified experimentally. This suggests that the capsule may direct the dynamic reaction cascades by virtue of π-cation interactions.

Enzyme-catalysed [6+4] cycloadditions in the biosynthesis of natural products

Pericyclic reactions are powerful transformations for the construction of carbon-carbon and carbon-heteroatom bonds in organic synthesis. Their role in biosynthesis is increasingly apparent, and mechanisms by which pericyclases can catalyse reactions are of major interest¹. [4+2] cycloadditions (Diels-Alder reactions) have been widely used in organic synthesis² for the formation of six-membered rings and are now well-established in biosynthesis^3-6. [6+4] and other 'higher-order' cycloadditions were predicted⁷ in 1965, and are now increasingly common in the laboratory despite challenges arising from the generation of a highly strained ten-membered ring system^8,9. However, although enzyme-catalysed [6+4] cycloadditions have been proposed^10-12, they have not been proven to occur. Here we demonstrate a group of enzymes that catalyse a pericyclic [6+4] cycloaddition, which is a crucial step in the biosynthesis of streptoseomycin-type natural products. This type of pericyclase catalyses [6+4] and [4+2] cycloadditions through a single ambimodal transition state, which is consistent with previous proposals^11,12. The [6+4] product is transformed to a less stable [4+2] adduct via a facile Cope rearrangement, and the [4+2] adduct is converted into the natural product enzymatically. Crystal structures of three pericyclases, computational simulations of potential energies and molecular dynamics, and site-directed mutagenesis establish the mechanism of this transformation. This work shows how enzymes are able to catalyse concerted pericyclic reactions involving ambimodal transition states.

Vinyl Triflimides-A Case of Assisted Vinyl Cation Formation

A new concept for selectivity control in carbocation-driven reactions has been identified which allows for the chemo-, regio-, and stereoselective addition of nucleophiles to alkynes-assisted vinyl cation formation-enabled by a Li⁺ -based supramolecular framework. Mechanistic analysis of a model complex (Li₂ NTf₂ ⁺ ⋅3 H₂ O) confirms that solely the formation of a complex between the incoming nucleophile and the transition state of the alkyne protonation is responsible for the resulting selective N addition to the vinyl cation. Into the bargain, a general, operationally simple synthetic procedure to previously inaccessible vinyl triflimides is provided.

Pushing the limits of concertedness. A waltz of wandering carbocations

Among the array of complex terpene-forming carbocation cyclization/rearrangement reactions, the so-called "triple shift" reactions are among the most unexpected. Such reactions involve the asynchronous combination of three 1,n-shifts into a concerted process, e.g., a 1,2-alkyl shift followed by a 1,3-hydride shift followed by a second 1,2-alkyl shift. This type of reaction so far has been proposed to occur during the biosynthesis of diterpenes and the sidechains of sterols. Here we describe efforts to push the limits of concertedness in this type of carbocation reaction by designing, and characterizing with quantum chemical computations, systems that could couple additional 1,n-shift events to a triple shift leading, in principle to quadruple, pentuple, etc. shifts. While our designs did not lead to clear-cut examples of quadruple, etc. shifts, they did lead to reactions with surprisingly flat energy surfaces where more than five chemical events connect reactants and plausible products. Ab initio molecular dynamics simulations demonstrate that the formal minima on these surfaces interchange on short timescales, both with each other and with additional unexpected structures, allowing us a glimpse into a very complex manifold that allows ready access to great structural diversity.

Post-transition state bifurcations induce dynamical detours in Pummerer-like reactions

A post-transition state bifurcation (PTSB) involved in a Pummerer-type rearrangement is characterized using density functional theory (DFT) calculations on potential energy stationary points and direct dynamics simulations. A sensitivity of the ratio of products produced via this PTSB to solvent dielectric constant is revealed and implications of such a dependence for selectivity control of organic reactions are discussed.

Liberation of H2 from (o-C6H4Me)3P-H(+) + (-)H-B(p-C6F4H)3 ion-pair: A transition-state in the minimum energy path versus the transient species in Born-Oppenheimer molecular dynamics

Using Born-Oppenheimer molecular dynamics (BOMD) with density functional theory, transition-state (TS) calculations, and the quantitative energy decomposition analysis (EDA), we examined the mechanism of H₂-liberation from LB-H⁽⁺⁾ + ^(-)H-LA ion-pair, 1, in which the Lewis base (LB) is (o-C₆H₄Me)₃P and the Lewis acid (LA) is B(p-C₆F₄H)₃. BOMD simulations indicate that the path of H₂ liberation from the ion-pair 1 goes via the short-lived transient species, LB⋯H₂⋯LA, which are structurally reminiscent of the TS-structure in the minimum-energy-path describing the reversible reaction between H₂ and (o-C₆H₄Me)₃P/B(p-C₆F₄H)₃ frustrated Lewis pair (FLP). With electronic structure calculations performed on graphics processing units, our BOMD data-set covers more than 1 ns of evolution of the ion-pair 1 at temperature T ≈ 400 K. BOMD simulations produced H₂-recombination events with various durations of H₂ remaining fully recombined as a molecule within a LB/LA attractive "pocket"-from very short vibrational-time scale to time scales in the range of a few hundred femtoseconds. With the help of perturbational approach to trajectory-propagation over a saddle-area, we directly examined dynamics of H₂-liberation. Using EDA, we elucidated interactions between the cationic and anionic fragments in the ion-pair 1 and between the molecular fragments in the TS-structure. We have also considered a model that qualitatively takes into account the potential energy characteristics of H-H recombination and H₂-release plus inertia of molecular motion of the (o-C₆H₄Me)₃P/B(p-C₆F₄H)₃ FLP.

Predicting Productive Binding Modes for Substrates and Carbocation Intermediates in Terpene Synthases-Bornyl Diphosphate Synthase as a Representative Case

Terpene synthases comprise a family of enzymes that convert acyclic oligo-isoprenyl diphosphates to terpene natural products with complex, polycyclic carbon backbones via the generation and protection of carbocation intermediates. To accommodate this chemistry, terpene synthase active sites generally are lined with alkyl and aromatic, i.e., nonpolar, sidechains. Predicting the correct, mechanistically relevant binding modes for entire terpene synthase reaction pathways remains an unsolved challenge. Here we describe a method for identifying such modes: TerDockin, a series of protocols to predict the orientation of carbon skeletons of substrates and derived carbocations relative to the bound diphosphate group in terpene synthase active sites. Using this recipe for bornyl diphosphate synthase, we have predicted binding modes that are consistent with all current experimental observations, including the results of isotope labeling experiments and known stereoselectivity. In addition, the predicted binding modes recapitulate key findings of a seminal study involving more computationally demanding QM/MM molecular dynamics methods on part of this pathway. This work illustrates the value of the TerDockin approach as a starting point for more involved calculations and sets the stage for the rational engineering of this family of enzymes.

A Maze of Dyotropic Rearrangements and Triple Shifts: Carbocation Rearrangements Connecting Stemarene, Stemodene, Betaerdene, Aphidicolene, and Scopadulanol

Results of quantum chemical investigations shed new light on the mechanisms of formation of the stemarene, stemodene, betaerdene, aphidicolene, and scopadulanol diterpenes from syn-copalyl diphosphate ( syn-CPP). These terpenes are shown to be connected by a complex network of reaction pathways involving concerted but asynchronous dyotropic rearrangements and triple shift rearrangements. The interconnection of these pathways leads to multiple routes for formation of each diterpene, which could lead to different origins for some carbon atoms in a given diterpenes under different conditions.

Diverged Plant Terpene Synthases Reroute the Carbocation Cyclization Path towards the Formation of Unprecedented 6/11/5 and 6/6/7/5 Sesterterpene Scaffolds

Sesterterpenoids are a relatively rare class of plant terpenes. Sesterterpene synthase (STS)-mediated cyclization of the linear C25 isoprenoid precursor geranylfarnesyl diphosphate (GFPP) defines sesterterpene scaffolds. So far only a very limited number of STSs have been characterized. The discovery of three new plant STSs is reported that produce a suite of sesterterpenes with unprecedented 6/11/5 and 6/6/7/5 fused ring systems when transiently co-expressed with a GFPP synthase in Nicotiana benthamiana. Structural elucidation, feeding experiments, and quantum chemical calculations suggest that these STSs catalyze an unusual cyclization path involving reprotonation, intramolecular 1,6 proton transfer, and concerted but asynchronous bicyclization events. The cyclization is diverted from those catalyzed by the characterized plant STSs by forming unified 15/5 bicyclic sesterterpene intermediates. Mutagenesis further revealed a conserved amino acid residue implicated in reprotonation.

Computational evidence for a reaction pathway bifurcation in Sasaki-type (4 + 3)-cycloadditions

Evidence for a post-transition state bifurcation in a (4 + 3)-cycloaddition is presented.

SAM-dependent enzyme-catalysed pericyclic reactions in natural product biosynthesis

Pericyclic reactions are among the most powerful synthetic transformations to make multiple regioselective and stereoselective carbon-carbon bonds¹. These reactions have been widely applied for the synthesis of biologically active complex natural products containing contiguous stereogenic carbon centers^2–6. Despite the prominence of pericyclic reactions in total synthesis, only three naturally existing enzymatic examples, intramolecular Diels-Alder (IMDA) reaction⁷, Cope⁸ and Claisen rearrangements⁹, have been characterized. Here, we report the discovery of a S-adenosyl-L-methionine (SAM) dependent enzyme LepI that can catalyse stereoselective dehydration, bifurcating IMDA/hetero-DA (HDA) reactions via an ambimodal transition state, and a [3,3]-sigmatropic retro-Claisen rearrangement leading to the formation of dihydopyran core in the fungal natural product leporin¹⁰. Combined in vitro enzymatic characterization and computational studies provide evidence and mechanistic insight about how the O-methyltransferase-like protein LepI regulates the bifurcating biosynthetic reaction pathways (“direct” HDA and “byproduct recycle” IMDA/retro-Claisen reaction pathways) by utilizing SAM as the cofactor in order to converge to the desired biosynthetic end product. This work highlights that LepI is the first example of an enzyme catalysing a (SAM-dependent) retro-Claisen rearrangement. We suggest that more pericyclic biosynthetic enzymatic transformations are yet to be discovered in the intriguing enzyme toolboxes in Nature¹¹, and propose an ever expanding role of the versatile cofactor SAM in enzyme catalysis.

Structural and Chemical Biology of Terpenoid Cyclases

The year 2017 marks the twentieth anniversary of terpenoid cyclase structural biology: a trio of terpenoid cyclase structures reported together in 1997 were the first to set the foundation for understanding the enzymes largely responsible for the exquisite chemodiversity of more than 80000 terpenoid natural products. Terpenoid cyclases catalyze the most complex chemical reactions in biology, in that more than half of the substrate carbon atoms undergo changes in bonding and hybridization during a single enzyme-catalyzed cyclization reaction. The past two decades have witnessed structural, functional, and computational studies illuminating the modes of substrate activation that initiate the cyclization cascade, the management and manipulation of high-energy carbocation intermediates that propagate the cyclization cascade, and the chemical strategies that terminate the cyclization cascade. The role of the terpenoid cyclase as a template for catalysis is paramount to its function, and protein engineering can be used to reprogram the cyclization cascade to generate alternative and commercially important products. Here, I review key advances in terpenoid cyclase structural and chemical biology, focusing mainly on terpenoid cyclases and related prenyltransferases for which X-ray crystal structures have informed and advanced our understanding of enzyme structure and function.

Post-transition state bifurcations gain momentum – current state of the field

The existence of post-transition state bifurcations on potential energy surfaces for organic and biological reaction mechanisms has been known for decades, but recently, new reports of bifurcations have been occurring at a much higher rate. Beyond simply discovering bifurcations, computational chemists are developing techniques to understand what aspects of molecular structure and vibrations control the product selectivity in systems containing bifurcations. For example, the distribution of products seen in simulations has been found to be extremely sensitive to the local environment of the reacting system (i.e. the presence of a catalyst, enzyme, or explicit solvent molecules). The outlook for the future of this field is discussed, with an eye towards the application of the principles discussed here by experimental chemists to design a reaction setup to efficiently generate desired products.

From Hydrindane to Decalin: A Mild Transformation through a Dyotropic Ring Expansion

An unexpected ring expansion converting hydrindane cores into decalins has been observed. The process occurs under very mild conditions and with exquisite transfer of chiral information. The ring expansion provides access to decorated decalins with complete stereocontrol. The reaction mechanism is studied in order to gain insight into the underlying causes for the low thermal requirements in this reaction and the nature of the chirality transfer process. Interestingly, both result from an unprecedented dyotropic reaction involving a mesylate group.

Mechanism of Formation of the Nonstandard Product in the Prenyltransferase Reaction of the G115T Mutant of FtmPT1: A Case of Reaction Dynamics Calling the Shots?

FtmPT1 is a fungal indole prenyltransferase that affords Tryprostatin B from Brevianamide F and dimethylallyl pyrophosphate; however, when a single residue in the active site is mutated (Gly115Thr), a novel five-membered ring compound is obtained as the major product with Tryprostatin B as the minor product. Herein, we describe detailed studies of the catalysis of the Gly115Thr mutant of FtmPT1 with a focus on the observed regioselectivity of the reaction. We employ one- and two-dimensional potential of mean force simulations to explore the catalytic mechanism, along with molecular dynamics simulations exploring the reaction dynamics of the prenyl transfer reaction. Single-point electronic structure calculations were also used to explore the performance of the self-consistent charge density functional tight-binding method to model specific reaction steps. Importantly, we observe that the two reaction pathways have comparable activation parameters and propose that the origin of the novel product is predicated, at least in part, on the topology of the potential energy surface as revealed by reaction dynamics studies.

Navigating Past a Fork in the Road: Carbocation-π Interactions Can Manipulate Dynamic Behavior of Reactions Facing Post-Transition-State Bifurcations

Dynamics calculations are described for carbocation rearrangements involving product-forming pathways with post-transition-state bifurcations. We show that noncovalent interactions with associated benzene rings (a simple model of aromatic amino acid side chains) can switch inherent dynamical tendencies for competing modes of disrotation, establishing that meaningful changes in dynamically controlled product selectivity can be achieved with few weak noncovalent interactions.

Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces

In this Introduction, we show the basic problems of non-statistical and non-equilibrium phenomena related to the papers collected in this themed issue. Over the past few years, significant advances in both computing power and development of theories have allowed the study of larger systems, increasing the time length of simulations and improving the quality of potential energy surfaces. In particular, the possibility of using quantum chemistry to calculate energies and forces 'on the fly' has paved the way to directly study chemical reactions. This has provided a valuable tool to explore molecular mechanisms at given temperatures and energies and to see whether these reactive trajectories follow statistical laws and/or minimum energy pathways. This themed issue collects different aspects of the problem and gives an overview of recent works and developments in different contexts, from the gas phase to the condensed phase to excited states.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Gas-phase reactivity of [Ca(formamide)]2+ complex: an example of different dynamical behaviours

In the present contribution, we have summarized our recent work on the comprehension of [Ca(formamide)]²⁺ complex gas-phase unimolecular dissociation. By using different theoretical approaches, we were able to revise the original (and typical for such kind of problems) understanding given in terms of stationary points on the potential energy surface, which did not provide a satisfactory explanation of the experimentally observed reactivity. In particular, we point out how non-statistical and non-intrinsic reaction coordinate mechanisms are of fundamental importance.This article is part of the themed issue 'Theoretical and computational studies of non-equilibrium and non-statistical dynamics in the gas phase, in the condensed phase and at interfaces'.

Biosynthesis of the oxygenated diterpene nezukol in the medicinal plant Isodon rubescens is catalyzed by a pair of diterpene synthases

Plants produce an immense diversity of natural products (i.e. secondary or specialized metabolites) that offer a rich source of known and potentially new pharmaceuticals and other desirable bioproducts. The Traditional Chinese Medicinal plant Isodon rubescens (Lamiaceae) contains an array of bioactive labdane-related diterpenoid natural products. Of these, the ent-kauranoid oridonin is the most prominent specialized metabolite that has been extensively studied for its potent antimicrobial and anticancer efficacy. Mining of a previously established transcriptome of I. rubescens leaf tissue identified seven diterpene synthase (diTPSs) candidates. Here we report the functional characterization of four I. rubescens diTPSs. IrTPS5 and IrTPS3 were identified as an ent-copalyl diphosphate (CPP) synthase and a (+)-CPP synthase, respectively. Distinct transcript abundance of IrTPS5 and the predicted ent-CPP synthase IrTPS1 suggested a role of IrTPS5 in specialized ent-kaurene metabolism possibly en route to oridonin. Nicotiana benthamiana co-expression assays demonstrated that IrTPS4 functions sequentially with IrTPS3 to form miltiradiene. In addition, IrTPS2 converted the IrTPS3 product (+)-CPP into the hydroxylated tricyclic diterpene nezukol not previously identified in I. rubescens. Metabolite profiling verified the presence of nezukol in I. rubescens leaf tissue. The proposed IrTPS2-catalyzed reaction mechanism proceeds via the common ionization of the diphosphate group of (+)-CPP, followed by formation of an intermediary pimar-15-en-8-yl+ carbocation and neutralization of the carbocation by water capture at C-8 to yield nezukol, as confirmed by nuclear magnetic resonance (NMR) analysis. Oxygenation activity is rare for the family of class I diTPSs and offers new catalysts for developing metabolic engineering platforms to produce a broader spectrum of bioactive diterpenoid natural products.

REVIEW: Epistasis and dominance in the emergence of catalytic function as exemplified by the evolution of plant terpene synthases

Epistasis, the interaction between mutations and the genetic background, is a pervasive force in evolution that is difficult to predict yet derives from a simple principle - biological systems are interconnected. Therefore, one effect may be intimately linked to another, hence interdependent. Untangling epistatic interactions between and within genes is a vibrant area of research. Deriving a mechanistic understanding of epistasis is a major challenge. Particularly, elucidating how epistasis can attenuate the effects of otherwise dominant mutations that control phenotypes. Using the emergence of terpene cyclization in specialized metabolism as an excellent example, this review describes the process of discovery and interpretation of dominance and epistasis in relation to current efforts. Specifically, we outline experimental approaches to isolating epistatic networks of mutations in protein structure, formally quantifying epistatic interactions, then building biochemical models with chemical mechanisms in efforts to achieve an understanding of the physical basis for epistasis. From these models we describe informed conjectures about past evolutionary events that underlie the emergence, divergence and specialization of terpene synthases to illustrate key principles of the constraining forces of epistasis in enzyme function.

External electric field control: driving the reactivity of metal-free azide–alkyne click reactions

An external electric field is demonstrated as an efficient catalyst to accelerate click reactions.

Natural [4 + 2]-Cyclases

[4 + 2]-Cycloadditions are increasingly being recognized in the biosynthetic pathways of many structurally complex natural products. A relatively small collection of enzymes from these pathways have been demonstrated to increase rates of cyclization and impose stereochemical constraints on the reactions. While mechanistic investigation of these enzymes is just beginning, recent studies have provided new insights with implications for understanding their biosynthetic roles, mechanisms of catalysis, and evolutionary origin.

Dynamics and the Regiochemistry of Nitration of Toluene

The regiochemistry of the nitration of toluene by NO₂⁺BF₄^- in dichloromethane is accurately predicted from trajectories in explicit solvent. Simpler models and approaches based on transition state theory fail to account for the selectivity. Potential of mean force calculations find no free-energy barrier for reaction of the toluene/NO₂⁺BF₄^- encounter complex, yet the trajectories require an extraordinary 3 ps to descend an exergonic slope. The selectivity is decided late in long trajectories because their completion requires solvent and counterion reorganization. The normal descriptive understanding of the regiochemistry based on transition-state energies is unsupported.

Decarboxylation Facilitated by Carbocation Formation and Rearrangement during Steam Distillation of Vetiver Oil

Density functional theory (DFT) calculations are used to probe the validity of mechanistic proposals for the conversion of isozizanoic acid to 12-norisoziza-5-ene, a reaction that occurs during steam distillation of vetiver oil. While this conversion corresponds overall to a simple decarboxylation, a multistep mechanism involving carbocation intermediates is supported by the computational results.

Cryptic post-transition state bifurcations that reduce the efficiency of lactone-forming Rh-carbenoid C-H insertions

Post-transition state bifurcations are described that lead to unexpected byproducts in Rh-promoted C–H insertion reactions.

Byproducts of chemical reactions are generally thought to result from the competition between two reaction pathways, each with its own rate-determining transition state structure. We show here, however, that pathways with a single transition state structure followed by a post-transition state bifurcation may also be a source of undesired products, especially those whose appearance is unexpected. The viability of this scenario for intramolecular C–H insertion reactions affording β-lactones via Rh-carbenoid intermediates is assessed through quantum chemical calculations on potential energy surfaces and quasi-classical molecular dynamics simulations. It appears that, in these cases, the rhodium catalyst is to blame for the accessibility of a second, unintended, pathway following the transition state structure for β-lactone formation that leads to fragmentation to a ketene and carbonyl compound. If an unexpected product is formed via a post-transition state bifurcation, conventional strategies for suppressing its formation are unlikely to succeed. Guidelines for recognizing the presence of a post-transition state bifurcation are described here, along with hints at means for controlling product distributions.

Dynamically Complex [6+4] and [4+2] Cycloadditions in the Biosynthesis of Spinosyn A

SpnF, an enzyme involved in the biosynthesis of spinosyn A, catalyzes a transannular Diels-Alder reaction. Quantum mechanical computations and dynamic simulations now show that this cycloaddition is not well described as either a concerted or stepwise process, and dynamical effects influence the identity and timing of bond formation. The transition state for the reaction is ambimodal and leads directly to both the observed Diels-Alder and an unobserved [6+4] cycloadduct. The potential energy surface bifurcates and the cycloadditions occur by dynamically stepwise modes featuring an "entropic intermediate". A rapid Cope rearrangement converts the [6+4] adduct into the observed [4+2] adduct. Control of nonstatistical dynamical effects may serve as another way by which enzymes control reactions.

Dynamic behavior of rearranging carbocations - implications for terpene biosynthesis

This review describes unexpected dynamical behaviors of rearranging carbocations and the modern computational methods used to elucidate these aspects of reaction mechanisms. Unique potential energy surface topologies associated with these rearrangements have been discovered in recent years that are not only of fundamental interest, but also provide insight into the way Nature manipulates chemical space to accomplish specific chemical transformations. Cautions for analyzing both experimental and theoretical data on carbocation rearrangements are included throughout.

"Cation-Stitching Cascade": exquisite control of terpene cyclization in cyclooctatin biosynthesis

Terpene cyclization is orchestrated by terpene cyclases, which are involved in the biosynthesis of various cyclic natural products, but understanding the origin and mechanism of the selectivity of terpene cyclization is challenging. In this work, we describe an in-depth mechanistic study on cyclooctatin biosynthesis by means of theoretical calculations combined with experimental methods. We show that the main framework of cyclooctatin is formed through domino-type carbocation transportation along the terpene chain, which we call a “cation-stitching cascade”, including multiple hydrogen-shifts and a ring rearrangement that elegantly determine the stereoselectivity.

Does the pressure dependence of kinetic isotope effects report usefully on dynamics in enzyme H-transfer reactions?

The temperature dependence of kinetic isotope effects (KIEs) has emerged as the main experimental probe of enzymatic H-transfer by quantum tunnelling. Implicit in the interpretation is a presumed role for dynamic coupling of H-transfer chemistry to the protein environment, the so-called 'promoting motions/vibrations hypothesis'. This idea remains contentious, and others have questioned the importance and/or existence of promoting motions/vibrations. New experimental methods of addressing this problem are emerging, including use of mass-modulated enzymes and time-resolved spectroscopy. The pressure dependence of KIEs has been considered as a potential probe of quantum tunnelling reactions, because semi-classical KIEs, which are defined by differences in zero-point vibrational energy, are relatively insensitive to kbar changes in pressure. Reported combined pressure and temperature (p-T) dependence studies of H-transfer reactions are, however, limited. Here, we extend and review the available p-T studies that have utilized well-defined experimental systems in which quantum mechanical tunnelling is established. These include flavoproteins, quinoproteins, light-activated enzymes and chemical model systems. We show that there is no clear general trend between the p-T dependencies of the KIEs in these systems. Given the complex nature of p-T studies, we conclude that computational simulations using determined (e.g. X-ray) structures are also needed alongside experimental measurements of reaction rates/KIEs to guide the interpretation of p-T effects. In providing new insight into H-transfer/environmental coupling, combined approaches that unite both atomistic understanding with experimental rate measurements will require careful evaluation on a case-by-case basis. Although individually informative, we conclude that p-T studies do not provide the more generalized insight that has come from studies of the temperature dependence of KIEs.