Excited-state processes in protochlorophyllide a – a femtosecond time-resolved absorption study

The Role of Membranes and Lipid-Protein Interactions in the Mg-Branch of Tetrapyrrole Biosynthesis

Chlorophyll (Chl) is essential for photosynthesis and needs to be produced throughout the whole plant life, especially under changing light intensity and stress conditions which may result in the destruction and elimination of these pigments. All steps of the Mg-branch of tetrapyrrole biosynthesis leading to Chl formation are carried out by enzymes associated with plastid membranes. Still the significance of these protein-membrane and protein-lipid interactions in Chl synthesis and chloroplast differentiation are not very well-understood. In this review, we provide an overview on Chl biosynthesis in angiosperms with emphasis on its association with membranes and lipids. Moreover, the last steps of the pathway including the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), the biosynthesis of the isoprenoid phytyl moiety and the esterification of Chlide are also summarized. The unique biochemical and photophysical properties of the light-dependent NADPH:protochlorophyllide oxidoreductase (LPOR) enzyme catalyzing Pchlide photoreduction and located to peculiar tubuloreticular prolamellar body (PLB) membranes of light-deprived tissues of angiosperms and to envelope membranes, as well as to thylakoids (especially grana margins) are also reviewed. Data about the factors influencing tubuloreticular membrane formation within cells, the spectroscopic properties and the in vitro reconstitution of the native LPOR enzyme complexes are also critically discussed.

Photocatalysis as the 'master switch' of photomorphogenesis in early plant development

Enzymatic photocatalysis is seldom used in biology. Photocatalysis by light-dependent protochlorophyllide oxidoreductase (LPOR)-one of only a few natural light-dependent enzymes-is an exception, and is responsible for the conversion of protochlorophyllide to chlorophyllide in chlorophyll biosynthesis. Photocatalysis by LPOR not only regulates the biosynthesis of the most abundant pigment on Earth but it is also a 'master switch' in photomorphogenesis in early plant development. Following illumination, LPOR promotes chlorophyll production, plastid membranes are transformed and the photosynthetic apparatus is established. Given these remarkable, light-induced pigment and morphological changes, the LPOR-catalysed reaction has been extensively studied from catalytic, physiological and plant development perspectives, highlighting vital, and multiple, cellular roles of this intriguing enzyme. Here, we offer a perspective in which the link between LPOR photocatalysis and plant photomorphogenesis is explored. Notable breakthroughs in LPOR structural biology have uncovered the structural-mechanistic basis of photocatalysis. These studies have clarified how photon absorption by the pigment protochlorophyllide-bound in a ternary LPOR-protochlorophyllide-NADPH complex-triggers photocatalysis and a cascade of complex molecular and cellular events that lead to plant morphological changes. Photocatalysis is therefore the master switch responsible for early-stage plant development and ultimately life on Earth.

Structural basis for enzymatic photocatalysis in chlorophyll biosynthesis

The enzyme protochlorophyllide oxidoreductase (POR) catalyses a light-dependent step in chlorophyll biosynthesis that is essential to photosynthesis and, ultimately, all life on Earth^1-3. POR, which is one of three known light-dependent enzymes^4,5, catalyses reduction of the photosensitizer and substrate protochlorophyllide to form the pigment chlorophyllide. Despite its biological importance, the structural basis for POR photocatalysis has remained unknown. Here we report crystal structures of cyanobacterial PORs from Thermosynechococcus elongatus and Synechocystis sp. in their free forms, and in complex with the nicotinamide coenzyme. Our structural models and simulations of the ternary protochlorophyllide-NADPH-POR complex identify multiple interactions in the POR active site that are important for protochlorophyllide binding, photosensitization and photochemical conversion to chlorophyllide. We demonstrate the importance of active-site architecture and protochlorophyllide structure in driving POR photochemistry in experiments using POR variants and protochlorophyllide analogues. These studies reveal how the POR active site facilitates light-driven reduction of protochlorophyllide by localized hydride transfer from NADPH and long-range proton transfer along structurally defined proton-transfer pathways.

Theoretical Model of the Protochlorophyllide Oxidoreductase from a Hierarchy of Protocols

The enzyme protochlorophyllide oxidoreductase (LPOR) catalyzes the light-driven reduction of protochlorophyllide (Pchlide), a crucial step in chlorophyll biosynthesis. Molecular understanding of the photocatalytic mechanism of LPOR is essential for harnessing light energy to mediate enzymatic reactions. The absence of X-ray crystal structure has promoted the development of LPOR homology models that lack a catalytically competent active site and could not explain the variously reported spectroscopic evidence, including time-resolved optical spectroscopy data. We have refined previous structural models to account for the catalytic active site and the characteristic experimental spectral features of Pchlide binding, including the 26 cm^-1 red shift of the C₁₃₍₁₎ carbonyl stretch vibration in the mid-infrared (IR) and the 12 nm red shift of the Q _x electronic band. A hierarchy of theoretical methods, including homology modeling, molecular dynamics simulations, hybrid quantum mechanics [(TD-)DFT]/molecular mechanics [AMBER] calculations, and computational vibrational and electronic spectroscopies, have been combined in an iterative protocol to reproduce experimental evidence and to predict ultrafast transient IR spectroscopic fingerprints associated with the catalytic process. The successful application to the LPOR enzyme indicates that the presented hierarchical protocol provides a general workflow to protein structure refinement.

Stepwise Hydride Transfer in a Biological System: Insights into the Reaction Mechanism of the Light-Dependent Protochlorophyllide Oxidoreductase

Hydride transfer plays a crucial role in a wide range of biological systems. However, its mode of action (concerted or stepwise) is still under debate. Light‐dependent NADPH: protochlorophyllide oxidoreductase (POR) catalyzes the stereospecific trans addition of a hydride anion and a proton across the C₁₇−C₁₈ double bond of protochlorophyllide. Time‐resolved absorption and emission spectroscopy were used to investigate the hydride transfer mechanism in POR. Apart from excited states of protochlorophyllide, three discrete intermediates were resolved, consistent with a stepwise mechanism that involves an initial electron transfer from NADPH. A subsequent proton‐coupled electron transfer followed by a proton transfer yield distinct different intermediates for wild type and the C226S variant, that is, initial hydride attaches to either C₁₇ or C₁₈, but ends in the same chlorophyllide stereoisomer. This work provides the first evidence of a stepwise hydride transfer in a biological system.

Direct Evidence of an Excited-State Triplet Species upon Photoactivation of the Chlorophyll Precursor Protochlorophyllide

The chlorophyll precursor protochlorophyllide (Pchlide), which is the substrate for the light-driven enzyme protochlorophyllide oxidoreductase, has unique excited-state properties that facilitate photocatalysis. Previous time-resolved spectroscopy measurements have implied that a long-lived triplet state is formed during the excited-state relaxation of Pchlide, although direct evidence of its existence is still lacking. Here we use time-resolved electron paramagnetic resonance (EPR) in combination with time-resolved absorption measurements at a range of temperatures (10-290 K), solvents, and oxygen concentrations to provide a detailed characterization of the triplet state of Pchlide. The triplet decays in a biphasic, oxygen-dependent manner, while the first reported EPR signature of a Pchlide triplet displays both emissive and absorptive features and an antisymmetric spectrum similar to other porphyrin triplet states. This work demonstrates that the Pchlide triplet is accessible to various cryogenic spectroscopic probes over a range of time scales and paves the way for understanding its potential role in catalysis.

Excited-State Properties of Protochlorophyllide Analogues and Implications for Light-Driven Synthesis of Chlorophyll

Protochlorophyllide (Pchlide), an intermediate in the biosynthesis of chlorophyll, is the substrate for the light-driven enzyme protochlorophyllide oxidoreductase. Pchlide has excited-state properties that allow it to initiate photochemistry in the enzyme active site, which involves reduction of Pchlide by sequential hydride and proton transfer. The basis of this photochemical behavior has been investigated here using a combination of time-resolved spectroscopies and density functional theory calculations of a number of Pchlide analogues with modifications to various substituent groups. A keto group on ring E is essential for excited-state charge separation in the molecule, which is the driving force for the photoreactivity of the pigment. Vibrational "fingerprints" of specific regions of the Pchlide chromophore have been assigned, allowing identification of the modes that are crucial for excited-state chemistry in the enzyme. This work provides an understanding of the structural determinants of Pchlide that are important for harnessing light energy.

Excited-state charge separation in the photochemical mechanism of the light-driven enzyme protochlorophyllide oxidoreductase

The unique light-driven enzyme protochlorophyllide oxidoreductase (POR) is an important model system for understanding how light energy can be harnessed to power enzyme reactions. The ultrafast photochemical processes, essential for capturing the excitation energy to drive the subsequent hydride- and proton-transfer chemistry, have so far proven difficult to detect. We have used a combination of time-resolved visible and IR spectroscopy, providing complete temporal resolution over the picosecond-microsecond time range, to propose a new mechanism for the photochemistry. Excited-state interactions between active site residues and a carboxyl group on the Pchlide molecule result in a polarized and highly reactive double bond. This so-called "reactive" intramolecular charge-transfer state creates an electron-deficient site across the double bond to trigger the subsequent nucleophilic attack of NADPH, by the negatively charged hydride from nicotinamide adenine dinucleotide phosphate. This work provides the crucial, missing link between excited-state processes and chemistry in POR. Moreover, it provides important insight into how light energy can be harnessed to drive enzyme catalysis with implications for the design of light-activated chemical and biological catalysts.

Protochlorophyllide in model systems--an approach to in vivo conditions

Absorption and fluorescence properties of protochlorophyllide (Pchlide) monomers and aggregates in various model systems are presented in this study. The absorption and fluorescence maxima, and fluorescence lifetimes of Pchlide monomers were not dependent on liposome composition. Fluorescence quenching experiments using KI and SASLs as fluorescence quenchers, revealed that Pchlide molecules entered a lipid bilayer and were localized close to the polar lipid headgroup area. The process of Pchlide aggregation was evident for high (i.e. at least 9 mol%) Pchlide content in liposomes prepared from galactolipids. To our knowledge, this is the first study of Pchlide aggregation in membrane-mimicking model systems. The aggregates showed absorption maxima at 480 and 650 nm. Fluorescence of the aggregates measured for excitation at 480 nm had a maximum at 656 nm and was characterized with two fluorescence lifetime components, i.e. 0.1 and 1-2 ns. Pchlide aggregates observed in the buffer had similar position of absorption and fluorescence bands to those observed in liposomes, although the overall fluorescence intensity was considerably lower. Some differences in the relative intensity of Soret absorption bands were observed. These results showed that the presence of liposomes decreased the efficiency of the process of Pchlide aggregation. Water bound at the interface region of AOT/isooctane/water reversed micelles induced disaggregation of the Pchlide aggregates indicating that Pchlide aggregates are buried into hydrophilic core of micelles. The results are discussed with respect to the role of lipids in Pchlide aggregation found in plant etioplasts and their significance for light-induced Pchlide photoreduction.

Protein-induced excited-state dynamics of protochlorophyllide

The light-driven NADPH:protochlorophyllide oxidoreductase (POR) is a key enzyme of chlorophyll biosynthesis in angiosperms. POR's unique requirement for light to become catalytically active makes the enzyme an attractive model to study the dynamics of enzymatic reactions in real time. Here, we use picosecond time-resolved fluorescence and femtosecond pump-probe spectroscopy to examine the influence of the protein environment on the excited-state dynamics of the substrate, protochlorophyllide (PChlide), in the enzyme/substrate (PChlide/POR) and pseudoternary complex including the nucleotide cofactor NADP(+) (PChlide/NADP(+)/ POR). In comparison with the excited-state processes of unbound PChlide, the lifetime of the thermally equilibrated S(1) excited state is lengthened from 3.4 to 4.4 and 5.4 ns in the PChlide/POR and PChlide/NADP(+)/POR complex, whereas the nonradiative rates are decreased by ∼30 and 40%, respectively. This effect is most likely due to the reduced probability of nonradiative decay into the triplet excited state, thus keeping the risk of photosensitized side reactions in the enzyme low. Further, the initial reaction path involves the formation of an intramolecular charge-transfer state (S(ICT)) as an intermediate product. From a strong blue shift in the excited-state absorption, it is concluded that the S(ICT) state is stabilized by local interactions with specific protein sites in the catalytic pocket. The possible relevance of this result for the catalytic reaction in the enzyme POR is discussed.

Excited-state dynamics of protochlorophyllide revealed by subpicosecond infrared spectroscopy

To gain a better understanding of the light-induced reduction of protochlorophyllide (PChlide) to chlorophyllide as a key regulatory step in chlorophyll synthesis, we performed transient infrared absorption measurements on PChlide in d4-methanol. Excitation in the Q-band at 630 nm initiates dynamics characterized by three time constants: τ₁ = 3.6 ± 0.2, τ₂ = 38 ± 2, and τ₃ = 215 ± 8 ps. As indicated by the C13'=O carbonyl stretching mode in the electronic ground state at 1686 cm⁻¹, showing partial ground-state recovery, and in the excited electronic state at 1625 cm⁻¹, showing excited-state decay, τ₂ describes the formation of a state with a strong change in electronic structure, and τ₃ represents the partial recovery of the PChlide electronic ground state. Furthermore, τ₁ corresponds with vibrational energy relaxation. The observed kinetics strongly suggest a branched reaction scheme with a branching ratio of 0.5 for the path leading to the PChlide ground state on the 200 ps timescale and the path leading to a long-lived state (>>700 ps). The results clearly support a branched reaction scheme, as proposed previously, featuring the formation of an intramolecular charge transfer state with ∼25 ps, its decay into the PChlide ground state with 200 ps, and a parallel reaction path to the long-lived PChlide triplet state.

Single and multi-exciton dynamics in aqueous protochlorophyllide aggregates

In plants, the oxidoreductase enzyme POR reduces protochlorophyllide (Pchlide) into chlorophyllide (Chlide), using NADPH as a cofactor. The reduction involves the transfer of two electrons and two protons to the C17═C18 double bond of Pchlide, and the reaction is initiated by the absorption of light by Pchlide itself. In this work we have studied the excited state dynamics of Pchlide dissolved in water, where it forms excitonically coupled aggregates, by ultrafast time-resolved transient absorption and fluorescence experiments performed in the 480-720 nm visible region and in the 1780-1590 cm(-1) mid-IR region. The ground state visible absorption spectrum of aqueous Pchlide red shifts and broadens in comparison to the spectrum of monomeric Pchlide in organic solvents. The population of the one-exciton state occurs at low excitation densities, of <1 photon per aggregate. We characterized the multiexciton manifolds spectra by measuring the absorption difference spectra at increasingly higher photon densities. The multiexciton states are characterized by blue-shifted stimulated emission and red-shifted excited state absorption in comparison to those of the one-exciton manifold. The relaxation dynamics of the multiexciton manifolds into the one-exciton manifold is found to occur in ∼10 ps. This surprisingly slow rate we suggest is due to the intrinsic charge transfer character of the PChlide excited state that leads to solvation, stabilizing the CT state, and subsequent charge recombination, which limits the exciton relaxation.

Protochlorophyllide excited-state dynamics in organic solvents studied by time-resolved visible and mid-infrared spectroscopy

Protochlorophyllide (PChlide) is a precursor in the biosynthesis of chlorophyll. Complexed with NADPH to the enzyme protochlorophyllide oxidoreductase (POR), it is reduced to chlorophyllide, a process that occurs via a set of spectroscopically distinct intermediate states and is initiated from the excited state of PChlide. To obtain a better understanding of these catalytic events, we characterized the excited state dynamics of PChlide in the solvents tetrahydrofuran (THF), methanol, and Tris/Triton buffer using ultrafast transient absorption in the visible and mid-infrared spectral regions and time-resolved fluorescence emission experiments. For comparison, we present time-resolved transient absorption measurements of chlorophyll a in THF. From the combined analysis of these experiments, we derive that during the 2-3 ns excited state lifetime an extensive multiphasic quenching of the emission occurs due to solvation of the excited state, which is in agreement with the previously proposed internal charge transfer (ICT) character of the S1 state ( Zhao , G. J. ; Han , K. L. Biophys. J. 2008 , 94 , 38 ). The solvation process in methanol occurs in conjunction with a strengthening of a hydrogen bond to the Pchlide keto carbonyl group. We demonstrate that the internal conversion from the S2 to S1 excited states is remarkably slow and stretches out on to the 700 fs time scale, causing a rise of blue-shifted signals in the transient absorption and a gain of emission in the time-resolved fluorescence. A triplet state is populated on the nanosecond time scale with a maximal yield of approximately 23%. The consequences of these observations for the catalytic pathway and the role of the triplet and ICT state in activation of the enzyme are discussed.

Ultrafast catalytic processes and conformational changes in the light-driven enzyme protochlorophyllide oxidoreductase (POR)

The enzyme POR (protochlorophyllide oxidoreductase), from the family of alcohol dehydrogenases, reduces protochlorophyllide into chlorophyllide on the absorption of light. The reduction involves the transfer of two protons and two electrons and is an important regulatory step in the biosynthesis of chlorophyll. In recent years, due to the availability of large quantities of the pure enzyme, much of the catalytic reaction has been unravelled by using a variety of spectroscopic methods, including ultrafast initial events in catalysis. In addition, it has been demonstrated that a light-activated conformational change of the protein is necessary to activate catalysis. This makes POR a very important model system to study the relationship between structural changes of enzymes and functionality.

Protochlorophyllide a: A Comprehensive Photophysical Picture

The photochemistry of protochlorophyllide a, a precursor in the biosynthesis of chlorophyll and substrate of the light regulated enzyme protochlorophyllide oxidoreductase, is investigated by pump-probe spectroscopy. Upon excitation into the lowest lying Q-band the light induced changes are recorded over a wide range of probe wavelengths in the visible and near-IR region between 500 and 1000 nm. Following excitation, an initial ultrafast 450 fs process is observed related to the motion out of the Franck-Condon region on the excited state surface; thus directly unraveling previous suggestions based on time-resolved fluorescence measurements (ChemPhysChem 2006, 7, 1727-1733). Furthermore, the data reveals a previously concealed photointermediate, whose formation on a nanosecond timescale matches the overall fluorescence decay and is assigned to a triplet state. The implications of this finding with respect to the photochemistry of NADPH:protochlorophyllide oxidoreductase (POR) are discussed.

Site-specific solvation of the photoexcited protochlorophyllide a in methanol: formation of the hydrogen-bonded intermediate state induced by hydrogen-bond strengthening

The site-specific solvation of the photoexcited protochlorophyllide a (Pchlide a) in methanol solvent was investigated using the time-dependent density functional theory method for the first time to our knowledge. The intermolecular site-specific coordination and hydrogen-bonding interactions between Pchlide a and methanol molecules play a very important role in the steady-state and time-resolved spectra. All the calculated absorption and fluorescence spectra of the isolated Pchlide a and its coordinated and hydrogen-bonded complexes with methanol demonstrate that the novel fluorescence shoulder at approximately 690 nm of Pchlide a in methanol should be ascribed to the coordinated and hydrogen-bonded Pchlide a-(MeOH)(4) complex. This coordinated and hydrogen-bonded complex can also account for the intermediate state found in the time-resolved spectroscopic studies. Herein, we have theoretically confirmed that the intermolecular coordination and hydrogen bonds between Pchlide a and methanol molecules can be strengthened in the electronically excited state of Pchlide a. Furthermore, the site-specific solvation of the photoexcited Pchlide a can be induced by the intermolecular coordination and hydrogen-bond strengthening upon photoexcitation. Then the hydrogen-bonded intermediate state is formed in 22-27 ps timescales after the site-specific solvation. All the steady-state and time-resolved spectral features of Pchlide a in different solvents can be explained by the formation of this hydrogen-bonded intermediate state after the site-specific solvation, which is induced by the coordination and hydrogen-bond strengthening.

The excited-state chemistry of protochlorophyllide a: a time-resolved fluorescence study

The excited-state processes of protochlorophyllide a, the precursor of chlorophyll a in chlorophyll biosynthesis, are studied using picosecond time-resolved fluorescence spectroscopy. Following excitation into the Soret band, two distinct fluorescence components, with emission maxima at 640 and 647 nm, are observed. The 640 nm emitting component appears within the time resolution of the experiment and then decays with a time constant of 27 ps. In contrast, the 647 nm emitting component is built up with a 3.5 ps rise time and undergoes a subsequent decay with a time constant of 3.5 ns. The 3.5 ps rise kinetics are attributed to relaxations in the electronically excited state preceding the nanosecond fluorescence, which is ascribed to emission out of the thermally equilibrated S(1) state. The 27 ps fluorescence, which appears within the experimental response of the streak camera, is suggested to originate from a second minimum on the excited-state potential-energy surface. The population of the secondary excited state is suggested to reflect a very fast motion out of the Franck-Condon region along a reaction coordinate different from the one connecting the Franck-Condon region with the S(1) potential-energy minimum. The 27 ps-component is an emissive intermediate on the reactive excited-state pathway, as its decay yields the intermediate photoproduct, which has been identified previously (J. Phys. Chem. B 2006, 110, 4399-4406). No emission of the photoproduct is observed. The results of the time-resolved fluorescence study allow a detailed spectral characterization of the emission of the excited states in protochlorophyllide a, and the refinement of the kinetic model deduced from ultrafast absorption measurements.

Solvent Effects on the Excited-State Processes of Protochlorophyllide: A Femtosecond Time-Resolved Absorption Study

The excited-state dynamics of protochlorophyllide a, a porphyrin-like compound and, as substrate of the NADPH/protochlorophyllide oxidoreductase, a precursor of chlorophyll biosynthesis, is studied by femtosecond absorption spectroscopy in a variety of solvents, which were chosen to mimic different environmental conditions in the oxidoreductase complex. In the polar solvents methanol and acetonitrile, the excited-state dynamics differs significantly from that in the nonpolar solvent cyclohexane. In methanol and acetonitrile, the relaxation dynamics is multiexponential with three distinguishable time scales of 4.0-4.5 ps for vibrational relaxation and vibrational energy redistribution of the initially excited S1 state, 22-27 ps for the formation of an intermediate state, most likely with a charge transfer character, and 200 ps for the decay of this intermediate state back to the ground state. In the nonpolar solvent cyclohexane, only the 4.5 ps relaxational process can be observed, whereas the intermediate intramolecular charge transfer state is not populated any longer. In addition to polarity, solvent viscosity also affects the excited-state processes. Upon increasing the viscosity by adding up to 60% glycerol to a methanolic solution, a deceleration of the 4 and 22 ps decay rates from the values in pure methanol is found. Apparently not only vibrational cooling of the S1 excited state is slowed in the more viscous surrounding, but the formation rate of the intramolecular charge transfer state is also reduced, suggesting that nuclear motions along a reaction coordinate are involved in the charge transfer. The results of the present study further specify the model of the excited-state dynamics in protochlorophyllide a as recently suggested (Chem. Phys. Lett. 2004, 397, 110).

Making light work of enzyme catalysis: protochlorophyllide oxidoreductase

In the chlorophyll biosynthetic pathway, the enzyme protochlorophyllide oxidoreductase (POR) catalyses a key light-driven reaction that triggers a profound transformation in plant development. Because POR is activated by light, it can provide information on the way in which light energy can be harnessed to power enzyme reactions and it presents us with a unique opportunity to study catalysis at low temperatures and on ultrafast timescales that are not accessible for most analyses of enzyme function. Recent advances in our understanding of the catalytic mechanism of POR illustrate why it is an important generic model for studying enzyme catalysis and reaction dynamics.