A thermally induced reversible conformational transition of the tryptophan synthase beta2 subunit probed by the spectroscopic properties of pyridoxal phosphate and by enzymatic activity

Neutron diffraction from a microgravity-grown crystal reveals the active site hydrogens of the internal aldimine form of tryptophan synthase

Pyridoxal 5'-phosphate (PLP), the biologically active form of vitamin B6, is an essential cofactor in many biosynthetic pathways. The emergence of PLP-dependent enzymes as drug targets and biocatalysts, such as tryptophan synthase (TS), has underlined the demand to understand PLP-dependent catalysis and reaction specificity. The ability of neutron diffraction to resolve the positions of hydrogen atoms makes it an ideal technique to understand how the electrostatic environment and selective protonation of PLP regulates PLP-dependent activities. Facilitated by microgravity crystallization of TS with the Toledo Crystallization Box, we report the 2.1 Å joint X-ray/neutron (XN) structure of TS with PLP in the internal aldimine form. Positions of hydrogens were directly determined in both the α- and β-active sites, including PLP cofactor. The joint XN structure thus provides insight into the selective protonation of the internal aldimine and the electrostatic environment of TS necessary to understand the overall catalytic mechanism.

Dual species sphingosine-1-phosphate lyase inhibitors to combine antifungal and anti-inflammatory activities in cystic fibrosis: a feasibility study

Cystic fibrosis (CF) is an autosomal recessive disorder characterized by respiratory failure due to a vicious cycle of defective Cystic Fibrosis Transmembrane conductance Regulator (CFTR) function, chronic inflammation and recurrent bacterial and fungal infections. Although the recent introduction of CFTR correctors/potentiators has revolutionized the clinical management of CF patients, resurgence of inflammation and persistence of pathogens still posit a major concern and should be targeted contextually. On the background of a network-based selectivity that allows to target the same enzyme in the host and microbes with different outcomes, we focused on sphingosine-1-phosphate (S1P) lyase (SPL) of the sphingolipid metabolism as a potential candidate to uniquely induce anti-inflammatory and antifungal activities in CF. As a feasibility study, herein we show that interfering with S1P metabolism improved the immune response in a murine model of CF with aspergillosis while preventing germination of Aspergillus fumigatus conidia. In addition, in an early drug discovery process, we purified human and A. fumigatus SPL, characterized their biochemical and structural properties, and performed an in silico screening to identify potential dual species SPL inhibitors. We identified two hits behaving as competitive inhibitors of pathogen and host SPL, thus paving the way for hit-to-lead and translational studies for the development of drug candidates capable of restraining fungal growth and increasing antifungal resistance.

Structure of the Oxygen, Pyridoxal Phosphate-Dependent Capuramycin Biosynthetic Protein Cap15

Pyridoxal phosphate-dependent enzymes able to use oxygen as a co-substrate have emerged in multiple protein families. Here, we use crystallography to solve the 2.40 Å resolution crystal structure of Cap15, a nucleoside biosynthetic enzyme that catalyzes the oxidative decarboxylation of glycyl uridine. Our structural study captures the internal aldimine, pinpointing the active site lysine as K230 and showing the site of phosphate binding. Our docking studies reveal how Cap15 is able to catalyze a stereoselective deprotonation reaction, and bioinformatic analysis reveals active site residues that distinguish Cap15 from the structurally related d-glucosaminate-6-phosphate ammonia lyase and l-seryl-tRNA(Sec) selenium transferase (SelA). Our work provides the structural basis for further mechanistic investigation of a unique biosynthetic enzyme and provides a blueprint for understanding how oxygen reactivity emerged in the SelA-like protein family.

Two-Photon Excited Fluorescence of NADH-Alcohol Dehydrogenase Complex in a Mixture with Bacterial Enzymes

Thorough study of composition and fluorescence properties of a commercial reagent of active equine NAD-dependent alcohol dehydrogenase expressed and purified from E. coli has been carried out. Several experimental methods: spectral- and time-resolved two-photon excited fluorescence, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, fast protein liquid chromatography, and mass spectrometry were used for analysis. The reagent under study was found to contain also a number of natural fluorophores: free NAD(P)H, NADH-alcohol dehydrogenase, NADPH-isocitrate dehydrogenase, and pyridoxal 5-phosphate-serine hydroxymethyltransferase complexes. The results obtained demonstrated the potential and limitations of popular optical methods as FLIM for separation of fluorescence signals from free and protein-bound forms of NADH, NADPH, and FAD that are essential coenzymes in redox reactions in all living cells. In particular, NADH-alcohol dehydrogenase and NADPH-isocitrate dehydrogenase complexes could not be optically separated in our experimental conditions although fast protein liquid chromatography and mass spectrometry analysis undoubtedly indicated the presence of both enzymes in the molecular sample used. Also, the results of fluorescence, fast protein liquid chromatography, and mass spectrometry analysis revealed a significant contribution of the enzyme-bound coenzyme pyridoxal 5-phosphate to the fluorescence signal that could be separated from enzyme-bound NADH by using bandpass filters, but could effectively mask contribution from enzyme-bound FAD because the fluorescence spectra of the species practically overlapped. It was shown that enzyme-bound pyridoxal 5-phosphate fluorescence can be separated from enzyme-bound NAD(P)H and FAD through analysis of short fluorescence decay times of about tens of picoseconds. However, this analysis was found to be effective only at relatively high number of peak photon counts in recorded fluorescence signals. The results obtained in this study can be used for interpretation of fluorescence signals from a mixture of enzyme-bound fluorophores and should be taken into consideration when determining the intracellular NADH/FAD ratio using FLIM.

Preparation and characterization of an imogolite/chitosan hybrid with pyridoxal-5′-phosphate as an interfacial modifier

Imogolite/chitosan hybrid films were prepared using pyridoxal-5′-phosphate (PLP) as an interfacial modifier. Thermogravimetric analysis and spectroscopic measurements revealed that the phosphate group of PLP was adsorbed on the imogolite. Furthermore, rheological measurements suggested that the PLP-modified imogolites (PLP–imogolite) had strong interactions with chitosan in solution. Moreover, UV absorption of the hybrid film showed that PLP and chitosan formed Schiff base linkages. Therefore, the hybrid films exhibited a significant improvement in their mechanical properties compared to those of pristine chitosan/imogolite hybrid films.

An eco-friendly hybrid film of chitosan and imogolite was prepared using pyridoxal-5′-phosphate as a surface modifier.

Mass Spectrometry-Based Protein Footprinting for Higher-Order Structure Analysis: Fundamentals and Applications

Proteins adopt different higher-order structures (HOS) to enable their unique biological functions. Understanding the complexities of protein higher-order structures and dynamics requires integrated approaches, where mass spectrometry (MS) is now positioned to play a key role. One of those approaches is protein footprinting. Although the initial demonstration of footprinting was for the HOS determination of protein/nucleic acid binding, the concept was later adapted to MS-based protein HOS analysis, through which different covalent labeling approaches "mark" the solvent accessible surface area (SASA) of proteins to reflect protein HOS. Hydrogen-deuterium exchange (HDX), where deuterium in D2O replaces hydrogen of the backbone amides, is the most common example of footprinting. Its advantage is that the footprint reflects SASA and hydrogen bonding, whereas one drawback is the labeling is reversible. Another example of footprinting is slow irreversible labeling of functional groups on amino acid side chains by targeted reagents with high specificity, probing structural changes at selected sites. A third footprinting approach is by reactions with fast, irreversible labeling species that are highly reactive and footprint broadly several amino acid residue side chains on the time scale of submilliseconds. All of these covalent labeling approaches combine to constitute a problem-solving toolbox that enables mass spectrometry as a valuable tool for HOS elucidation. As there has been a growing need for MS-based protein footprinting in both academia and industry owing to its high throughput capability, prompt availability, and high spatial resolution, we present a summary of the history, descriptions, principles, mechanisms, and applications of these covalent labeling approaches. Moreover, their applications are highlighted according to the biological questions they can answer. This review is intended as a tutorial for MS-based protein HOS elucidation and as a reference for investigators seeking a MS-based tool to address structural questions in protein science.

Semi-rational approach to expand the Acyl-CoA Chain length tolerance of Sphingomonas paucimobilis serine palmitoyltransferase

Serine palmitoyltransferase (SPTase), the first enzyme of the sphingolipid biosynthesis pathway, produces 3-ketodihydrosphingosine by a Claisen-like condensation/decarboxylation reaction of l-Ser and palmitoyl-CoA (n-C₁₆-CoA). Previous structural analysis of Sphingomonas paucimobilis SPTase (SpSPTase) revealed a dynamic active site loop (RPPATP; amino acids 378-383) in which R378 (underlined) forms a salt bridge with the carboxylic acid group of the PLP : l-Ser external aldimine. We hypothesized that this interaction might play a key role in acyl group substrate selectivity and therefore performed site-saturation mutagenesis at position 378 based on semi-rational design to expand tolerance for shorter acyl-CoA's. The resulting library was initially screened for the reaction between l-Ser and dodecanoyl-CoA (n-C₁₂-CoA). The most interesting mutant (R378 K) was then purified and compared to wild-type SpSPTase against a panel of acyl-CoA's. These data showed that the R378 K substitution shifted the acyl group preference to shorter chain lengths, opening the possibility of using this and other engineered variants for biocatalytic C-C bond-forming reactions.

Biochemical and Proteomic Studies of Human Pyridoxal 5'-Phosphate-Binding Protein (PLPBP)

The pyridoxal 5'-phosphate-binding protein (PLPBP) is an evolutionarily conserved protein linked to pyridoxal 5'-phosphate-binding. Although mutations in PLPBP were shown to cause vitamin B6-dependent epilepsy, its cellular role and function remain elusive. We here report a detailed biochemical investigation of human PLPBP and its epilepsy-causing mutants by evaluating stability, cofactor binding, and oligomerization. In this context, chemical cross-linking combined with mass spectrometry unraveled an unexpected dimeric assembly of PLPBP. Furthermore, the interaction network of PLPBP was elucidated by chemical cross-linking paired with co-immunoprecipitation. A mass spectrometric analysis in a PLPBP knockout cell line resulted in distinct proteomic changes compared to wild type cells, including upregulation of several cytoskeleton- and cell division-associated proteins. Finally, transfection experiments with vitamin B6-dependent epilepsy-causing PLPBP variants indicate a potential role of PLPBP in cell division as well as proper muscle function. Taken together, our studies on the structure and cellular role of human PLPBP enable a better understanding of the physiological and pathological mechanism of this important protein.

Saccharomyces cerevisiae Differential Functionalization of Presumed ScALT1 and ScALT2 Alanine Transaminases Has Been Driven by Diversification of Pyridoxal Phosphate Interactions

Saccharomyces cerevisiae arose from an interspecies hybridization (allopolyploidiza-tion), followed by Whole Genome Duplication. Diversification analysis of ScAlt1/ScAlt2 indicated that while ScAlt1 is an alanine transaminase, ScAlt2 lost this activity, constituting an example in which one of the members of the gene pair lacks the apparent ancestral physiological role. This paper analyzes structural organization and pyridoxal phosphate (PLP) binding properties of ScAlt1 and ScAlt2 indicating functional diversification could have determined loss of ScAlt2 alanine transaminase activity and thus its role in alanine metabolism. It was found that ScAlt1 and ScAlt2 are dimeric enzymes harboring 67% identity and intact conservation of the catalytic residues, with very similar structures. However, tertiary structure analysis indicated that ScAlt2 has a more open conformation than that of ScAlt1 so that under physiological conditions, while PLP interaction with ScAlt1 allows the formation of two tautomeric PLP isomers (enolimine and ketoenamine) ScAlt2 preferentially forms the ketoenamine PLP tautomer, indicating a modified polarity of the active sites which affect the interaction of PLP with these proteins, that could result in lack of alanine transaminase activity in ScAlt2. The fact that ScAlt2 forms a catalytically active Schiff base with PLP and its position in an independent clade in "sensu strictu" yeasts suggests this protein has a yet undiscovered physiological function.

Human liver peroxisomal alanine:glyoxylate aminotransferase: characterization of the two allelic forms and their pathogenic variants

The hepatic peroxisomal alanine:glyoxylate aminotransferase (AGT) is a pyridoxal 5'-phosphate (PLP)-enzyme whose deficiency is responsible for Primary Hyperoxaluria Type 1 (PH1), an autosomal recessive disorder. In the last few years the knowledge of the characteristics of AGT and the transfer of this information into some pathogenic variants have significantly contributed to the improvement of the understanding at the molecular level of the PH1 pathogenesis. In this review, the spectroscopic features, the coenzyme's binding affinity, the steady-state kinetic parameters as well as the sensitivity to thermal and chemical stress of the two allelic forms of AGT, the major (AGT-Ma) and the minor (AGT-Mi) allele, have been described. Moreover, we summarize the characterization obtained by means of biochemical and bioinformatic analyses of the following PH1-causing variants in the recombinant purified forms: G82E associated with the major allele, F152I encoded on the background of the minor allele, and the G41 mutants which co-segregate either with the major allele (G41R-Ma and G41V-Ma) or with the minor allele (G41R-Mi). The data have been correlated with previous clinical and cell biology results, which allow us to (i) highlight the functional differences between AGT-Ma and AGT-Mi, (ii) identify the structural and functional molecular defects of the pathogenic variants, (iii) improve the correlation between the genotype and the enzymatic phenotype, (iv) foresee or understand the molecular basis of the responsiveness to pyridoxine treatment of patients bearing these mutations, and (v) pave the way for new treatment strategies. This article is part of a Special Issue entitled: Pyridoxal Phospate Enzymology.

Heme regulation of human cystathionine beta-synthase activity: insights from fluorescence and Raman spectroscopy

Cystathionine beta-synthase (CBS) plays a central role in homocysteine metabolism, and malfunction of the enzyme leads to homocystinuria, a devastating metabolic disease. CBS contains a pyridoxal 5'-phosphate (PLP) cofactor which catalyzes the synthesis of cystathionine from homocysteine and serine. Mammalian forms of the enzyme also contain a heme group, which is not involved in catalysis. It may, however, play a regulatory role, since the enzyme is inhibited when CO or NO are bound to the heme. We have investigated the mechanism of this inhibition using fluorescence and resonance Raman spectroscopies. CO binding is found to induce a tautomeric shift of the PLP from the ketoenamine to the enolimine form. The ketoenamine is key to PLP reactivity because its imine C horizontal lineN bond is protonated, facilitating attack by the nucleophilic substrate, serine. The same tautomer shift is also induced by heat inactivation of Fe(II)CBS, or by an Arg266Met replacement in Fe(II)CBS, which likewise inactivates the enzyme; in both cases the endogenous Cys52 ligand to the heme is replaced by another, unidentified ligand. CO binding also displaces Cys52 from the heme. We propose that the tautomer shift results from loss of a stabilizing H-bond from Asn149 to the PLP ring O3' atom, which is negatively charged in the ketoenamine tautomer. This loss would be induced by displacement of the PLP as a result of breaking the salt bridge between Cys52 and Arg266, which resides on a short helix that is also anchored to the PLP via H-bonds to its phosphate group. The salt bridge would be broken when Cys52 is displaced from the heme. Cys52 protonation is inferred to be the rate-limiting step in breaking the salt bridge, since the rate of the tautomer shift, following CO binding, increases with decreasing pH. In addition, elevation of the concentration of phosphate buffer was found to diminish the rate and extent of the tautomer shift, suggesting a ketoenamine-stabilizing phosphate binding site, possibly at the protonated imine bond of the PLP. Implications of these findings for CBS regulation are discussed.

Liver Tumor Gross Margin Identification and Ablation Monitoring during Liver Radiofrequency Treatment

PURPOSE

To determine whether tissue visible light spectroscopy (VLS) used during radiofrequency (RF) ablation of liver tumors could aid in detecting when tissue becomes adequately ablated, locate grossly ablated regions long after temperature and hydration measures would no longer be reliable, and differentiate tumor from normal hepatic tissue based on VLS spectral characteristics.

MATERIALS AND METHODS

Studies were performed on human liver in vivo and animal liver ex vivo. In three ex vivo cow livers, RF-induced lesions were created at 80 degrees C. A 28-gauge needle embedded with VLS optical fibers was inserted alongside an RF ablation array, and tissue spectral characteristics were recorded throughout ablation. In one anesthetized sheep in vivo, a VLS needle probe was passed through freshly ablated liver lesions, and ablated region spectral characteristics were recorded during probe transit. In two human subjects, a VLS needle probe was passed through liver tumors in patients undergoing hepatic tumor resection without ablation, and tumor spectral characteristics were recorded during probe transit.

RESULTS

In bovine studies, there was significant change in baseline absorbance (P < .0001) as a result of increased light scattering as liver was ablated. Liver exhibited native differential absorbance peaks at 550 nm that disappeared during ablation, suggesting that optical spectroscopy detects markers of tissue altered during ablation. In sheep, liver gross ablation margins were clearly defined with millimeter resolution during needle transit through the region, suggesting that VLS is sensitive to gross margins of ablation, even after the temperature has normalized. In humans, absorbance decreased as the needle passed from normal tissue into tumor and normalized after emerging from the tumor, suggesting that absence of native liver pigment may serve as a marker for the gross margins and presence of tumors of extrahepatic origin.

CONCLUSIONS

In human subjects, VLS during RF liver tumor ablation depicted gross hepatic tumor margins in real time; in animal subjects, VLS achieved monitoring of when and where RF ablation endpoints were achieved, even long after the tissue cooled. Real-time in vivo monitoring and treatment feedback may be possible with the use of real-time VLS sensors placed along side of, or embedded into, the RF probe, which can then be used as an adjunct to standard imaging during tumor localization and RF ablation treatment.

Conversion of 5-aminolevulinate synthase into a more active enzyme by linking the two subunits: spectroscopic and kinetic properties

The two active sites of dimeric 5-aminolevulinate synthase (ALAS), a pyridoxal 5'-phosphate (PLP)-dependent enzyme, are located on the subunit interface with contribution of essential amino acids from each subunit. Linking the two subunits into a single polypeptide chain dimer (2XALAS) yielded an enzyme with an approximate sevenfold greater turnover number than that of wild-type ALAS. Spectroscopic and kinetic properties of 2XALAS were investigated to explore the differences in the coenzyme structure and kinetic mechanism relative to those of wild-type ALAS that confer a more active enzyme. The absorption spectra of both ALAS and 2XALAS had maxima at 410 and 330 nm, with a greater A(410)/A(330) ratio at pH approximately 7.5 for 2XALAS. The 330 nm absorption band showed an intense fluorescence at 385 nm but not at 510 nm, indicating that the 330 nm absorption species is the substituted aldamine rather than the enolimine form of the Schiff base. The 385 nm emission intensity increased with increasing pH with a single pK of approximately 8.5 for both enzymes, and thus the 410 and 330 nm absorption species were attributed to the ketoenamine and substituted aldamine, respectively. Transient kinetic analysis of the formation and decay of the quinonoid intermediate EQ(2) indicated that, although their rates were similar in ALAS and 2XALAS, accumulation of this intermediate was greater in the 2XALAS-catalyzed reaction. Collectively, these results suggest that ketoenamine is the active form of the coenzyme and forms a more prominent coenzyme structure in 2XALAS than in ALAS at pH approximately 7.5.

Regulation of tryptophan synthase by temperature, monovalent cations, and an allosteric ligand. Evidence from Arrhenius plots, absorption spectra, and primary kinetic isotope effects

To investigate the linkage between enzyme conformation and catalysis, we have determined the effects of temperature on catalytic properties of the tryptophan synthase alpha(2)beta(2) complex and beta(2) subunit in the absence or presence of different monovalent cations (Cs(+), Na(+), and GuH(+)) and of an allosteric ligand, alpha-glycerol 3-phosphate. Arrhenius plots of the activity data between 5 and 50 degrees C are nonlinear in the presence of certain ligands but not others. The conditions that yield nonlinear Arrhenius plots also yield temperature-dependent changes in the equilibrium distribution of enzyme-substrate intermediates and in primary kinetic isotope effects. The results provide evidence that the nonlinear Arrhenius plots are caused by a temperature-dependent conformational change that precedes the rate-limiting step in catalysis. Thermodynamic analysis of the data associated with the conformational change shows that the activation energies are much higher at low temperatures than at high temperatures. We correlate the results with a model in which the enzyme is converted by increased temperature under certain conditions from a low-activity "open" conformation to a high-activity "closed" conformation. The allosteric ligand and different monovalent cations, including GuH(+), which also acts as a chaotropic agent, affect the equilibrium between the open and closed forms. The large positive entropy changes in the conformational conversion suggest that the closed conformation results from tightened hydrophobic interactions that exclude water from the active site of the beta subunit.

Tryptophan synthase mutations that alter cofactor chemistry lead to mechanism-based inactivation

Mutations in the pyridoxal phosphate binding site of the tryptophan synthase beta subunit (S377D and S377E) alter cofactor chemistry [Jhee, K.-H., et al. (1998) J. Biol. Chem. 273, 11417-11422]. We now report that the S377D, S377E, and S377A beta2 subunits form alpha2 beta2 complexes with the alpha subunit and activate the alpha subunit-catalyzed cleavage of indole 3-glycerol phosphate. The apparent Kd for dissociation of the alpha and beta subunits is unaffected by the S377A mutation but is increased up to 500-fold by the S377D and S377E mutations. Although the three mutant alpha2 beta2 complexes exhibit very low activities in beta elimination and beta replacement reactions catalyzed at the beta site in the presence of Na+, the activities and spectroscopic properties of the S377A alpha2 beta2 complex are partially repaired by addition of Cs+. The S377D and S377E alpha2 beta2 complexes, unlike the wild-type and S377A alpha2 beta2 complexes and the mutant beta2 subunits, undergo irreversible substrate-induced inactivation by L-serine or by beta-chloro-L-alanine. The rates of inactivation (kinact) are similar to the rates of catalysis (kcat). The partition ratios are very low (kcat/kinact = 0.25-3) and are affected by alpha subunit ligands and monovalent cations. The inactivation product released by alkali was shown by HPLC and by fluorescence, absorption, and mass spectroscopy to be identical to a compound previously synthesized from pyridoxal phosphate and pyruvate. We suggest that alterations in the cofactor chemistry that result from the engineered Asp377 in the active site of the beta subunit may promote the mechanism-based inactivation.

Mutation of an active site residue of tryptophan synthase (beta-serine 377) alters cofactor chemistry

To better understand how an enzyme controls cofactor chemistry, we have changed a tryptophan synthase residue that interacts with the pyridine nitrogen of the pyridoxal phosphate cofactor from a neutral Ser (beta-Ser377) to a negatively charged Asp or Glu. The spectroscopic properties of the mutant enzymes are altered and become similar to those of tryptophanase and aspartate aminotransferase, enzymes in which an Asp residue interacts with the pyridine nitrogen of pyridoxal phosphate. The absorption spectrum of each mutant enzyme undergoes a pH-dependent change (pKa approximately 7.7) from a form with a protonated internal aldimine nitrogen (lambdamax = 416 nm) to a deprotonated form (lambdamax = 336 nm), whereas the absorption spectra of the wild type tryptophan synthase beta2 subunit and alpha2 beta2 complex are pH-independent. The reaction of the S377D alpha2 beta2 complex with L-serine, L-tryptophan, and other substrates results in the accumulation of pronounced absorption bands (lambdamax = 498-510 nm) ascribed to quinonoid intermediates. We propose that the engineered Asp or Glu residue changes the cofactor chemistry by stabilizing the protonated pyridine nitrogen of pyridoxal phosphate, reducing the pKa of the internal aldimine nitrogen and promoting formation of quinonoid intermediates.

Reversible unfolding of sheep liver tetrameric serine hydroxymethyltransferase

Equilibrium unfolding studies of the tetrameric serine hydroxymethyltransferase from sheep liver (SHMT, E.C.2.1.2.1) revealed that the holoenzyme, apoenzyme and the sodium borohydride-reduced holoenzyme had random coil structures in 8 M urea. In the presence of a non-ionic detergent, Brij-35, and polyethylene glycol, the 8 M urea unfolded protein could be completely (> 95%) refolded by a 20-fold dilution. The refolded enzyme was completely active and kinetically similar to the native enzyme. The midpoint of inactivation of the enzyme occurred at a urea concentration that was much below the urea concentration required to bring about a substantial loss of secondary structure. This observation suggested the occurrence of a 'predenaturation transition' in the unfolding pathway. The equilibrium urea-induced denaturation curve of holoSHMT showed two transitions. The midpoint of the first transition was 1.2 M, which was comparable to that required for 50% decrease in enzyme activity. Further, 50% release of the pyridoxal-5'-phosphate (PLP) from the active site, as monitored by decrease in absorbance at 425 nm, also occurred at about 1.2 M urea. Size exclusion chromatography showed that the tetrameric SHMT unfolds via the intermediate formation of dimers. This dissociation occurred at a much lower urea concentration (0.15 M) in the unfolding of the apoenzyme, and at a higher urea concentration (1.2 M) in the unfolding of holoenzyme, thereby demonstrating the involvement of PLP in stabilizing the quaternary structure of the enzyme. Size exclusion chromatography of the refolding intermediates demonstrated that the cofactor shifts the equilibrium towards the formation of the active tetramer. The reduced holoenzyme could also be refolded to its native structure, as observed by fluorescence and CD measurements, indicating that the presence of covalently linked PLP does not affect refolding. The results demonstrate clearly that the dimer is an intermediate in the urea-induced equilibrium unfolding/refolding of sheep liver SHMT; and PLP, in addition to its role in catalysis, is required for the stabilization of the tetrameric structure of the enzyme.