Structural characterization of the apo form and NADH binary complex of human lactate dehydrogenase

Exploring the Key Amino Acid Residues Surrounding the Active Center of Lactate Dehydrogenase A for the Development of Ideal Inhibitors

Lactate dehydrogenase A (LDHA) primarily catalyzes the conversion between lactic acid and pyruvate, serving as a key enzyme in the aerobic glycolysis pathway of sugar in tumor cells. LDHA plays a crucial role in the occurrence, development, progression, invasion, metastasis, angiogenesis, and immune escape of tumors. Consequently, LDHA not only serves as a biomarker for tumor diagnosis and prognosis but also represents an ideal target for tumor therapy. Although LDHA inhibitors show great therapeutic potential, their development has proven to be challenging. In the development of LDHA inhibitors, the key active sites of LDHA are emphasized. Nevertheless, there is a relative lack of research on the amino acid residues around the active center of LDHA. Therefore, in this study, we investigated the amino acid residues around the active center of LDHA. Through structure comparison analysis, five key amino acid residues (Ala30, Met41, Lys131, Gln233, and Ala259) were identified. Subsequently, the effects of these five residues on the enzymatic properties of LDHA were investigated using site-directed mutagenesis. The results revealed that the catalytic activities of the five mutants varied to different degrees in both the reaction from lactic acid to pyruvate and pyruvate to lactic acid. Notably, the catalytic activities of LDHAM41G and LDHAK131I were improved, particularly in the case of LDHAK131I. The results of the molecular dynamics analysis of LDHAK131I explained the reasons for this phenomenon. Additionally, the optimum temperature of LDHAM41G and LDHAQ233M increased from 35 °C to 40 °C, whereas in the reverse reaction, the optimum temperature of LDHAM41G and LDHAK131I decreased from 70 °C to 60 °C. These findings indicate that Ala30, Met41, Lys131, Gln233, and Ala259 exert diverse effects on the catalytic activity and optimum temperature of LHDA. Therefore, these amino acid residues, in addition to the key catalytic site of the active center, play a crucial role. Considering these residues in the design and screening of LDHA inhibitors may lead to the development of more effective inhibitors.

Deciphering Evolutionary Trajectories of Lactate Dehydrogenases Provides New Insights into Allostery

Lactate dehydrogenase (LDH, EC.1.1.127) is an important enzyme engaged in the anaerobic metabolism of cells, catalyzing the conversion of pyruvate to lactate and NADH to NAD+. LDH is a relevant enzyme to investigate structure-function relationships. The present work provides the missing link in our understanding of the evolution of LDHs. This allows to explain (i) the various evolutionary origins of LDHs in eukaryotic cells and their further diversification and (ii) subtle phenotypic modifications with respect to their regulation capacity. We identified a group of cyanobacterial LDHs displaying eukaryotic-like LDH sequence features. The biochemical and structural characterization of Cyanobacterium aponinum LDH, taken as representative, unexpectedly revealed that it displays homotropic and heterotropic activation, typical of an allosteric enzyme, whereas it harbors a long N-terminal extension, a structural feature considered responsible for the lack of allosteric capacity in eukaryotic LDHs. Its crystallographic structure was solved in 2 different configurations typical of the R-active and T-inactive states encountered in allosteric LDHs. Structural comparisons coupled with our evolutionary analyses helped to identify 2 amino acid positions that could have had a major role in the attenuation and extinction of the allosteric activation in eukaryotic LDHs rather than the presence of the N-terminal extension. We tested this hypothesis by site-directed mutagenesis. The resulting C. aponinum LDH mutants displayed reduced allosteric capacity mimicking those encountered in plants and human LDHs. This study provides a new evolutionary scenario of LDHs that unifies descriptions of regulatory properties with structural and mutational patterns of these important enzymes.

LDHA: The Obstacle to T cell responses against tumor

Immunotherapy has become a successful therapeutic strategy in certain solid tumors and hematological malignancies. However, this efficacy of immunotherapy is impeded by limited success rates. Cellular metabolic reprogramming determines the functionality and viability in both cancer cells and immune cells. Extensive research has unraveled that the limited success of immunotherapy is related to immune evasive metabolic reprogramming in tumor cells and immune cells. As an enzyme that catalyzes the final step of glycolysis, lactate dehydrogenase A (LDHA) has become a major focus of research. Here, we have addressed the structure, localization, and biological features of LDHA. Furthermore, we have discussed the various aspects of epigenetic regulation of LDHA expression, such as histone modification, DNA methylation, N6-methyladenosine (m6A) RNA methylation, and transcriptional control by noncoding RNA. With a focus on the extrinsic (tumor cells) and intrinsic (T cells) functions of LDHA in T-cell responses against tumors, in this article, we have reviewed the current status of LDHA inhibitors and their combination with T cell-mediated immunotherapies and postulated different strategies for future therapeutic regimens.

Identification of human LDHC4 as a potential target for anticancer drug discovery

One of the distinct hallmarks of cancer cells is aerobic glycolysis (Warburg effect). Lactate dehydrogenase A (LDHA) is thought to play a key role in aerobic glycolysis and has been extensively studied, while lactate dehydrogenase C (LDHC), an isoform of LDHA, has received much less attention. Here we showed that human LDHC was significantly expressed in lung cancer tissues, overexpression of Ldhc in mice could promote tumor growth, and knock-down of LDHC could inhibit the proliferation of lung cancer A549 cells. We solved the first crystal structure of human LDHC4 and found that the active-site loop of LDHC4 adopted a distinct conformation compared to LDHA4 and lactate dehydrogenase B4 (LDHB4). Moreover, we found that (ethylamino) (oxo)acetic acid shows about 10 times selective inhibition against LDHC4 over LDHA4 and LDHB4. Our studies suggest that LDHC4 is a potential target for anticancer drug discovery and (ethylamino) (oxo)acetic acid provides a good start to develop lead compounds for selective drugs targeting LDHC4.

Reversal of insulin resistance by Ficus benghalensis bark in fructose-induced insulin-resistant rats

ETHNOPHARMACOLOGICAL RELEVANCE

Bark of Ficus benghalensis L. (family: Moraceae), commonly known as Banyan is recorded as Nyagrodha in Ayurvedic Pharmacopeia of India to manage burning sensation, obesity, diabetes, bleeding disorders, thirst, skin diseases, wounds, and dysmenorrhoea. However, the effect of F. benghalensis bark over glycolysis, gluconeogenesis, and appetite regulation in insulin-resistant pathogenesis has not been reported yet.

AIM OF THE STUDY

The present study aimed to investigate the effect of hydroalcoholic extract of F. benghalensis bark in gluconeogenesis, glycolysis, and appetite regulation in fructose-induced insulin resistance in experimental rats.

MATERIALS AND METHODS

Male Wister rats were supplemented with fructose in drinking water (10% w/v for 42 days and 20% w/v for next 12 days; a total of 54 days); insulin resistance was confirmed via the elevated area under the curve of the glucose during oral glucose tolerance test after 54 days and was subjected with extract treatment for next 30 days. After 30 days of treatment, animals were fasted to perform oral glucose and insulin tolerance test to estimate glucose and insulin levels. The blood sample was collected for biochemical estimation and the liver homogenate was prepared to estimate hepatic enzymes and enzymatic and non-enzymatic anti-oxidant biomarkers followed by histopathological evaluation. Also, glycogen content was quantified in gastrocnemius muscle and liver homogenates. Further, reported bioactives from the F. benghalensis were retrieved from the ChEBI database and docked against hexokinase, phosphofructokinase, glucose-6-phosphatase, lactate dehydrogenase, and fructose-1,6-biphosphatase to identify the probable lead hits against the enzymes involved in gluconeogenesis.

RESULTS

Treatment with the F. benghalensis bark extract significantly increased the body weight and food intake and significantly decreased fructose supplemented water intake. Further, treatment with extract significantly increased the exogenous glucose clearance and well responded to the exogenous insulin. Further, extract treatment improved lipid metabolism, ameliorated plasma leptin, and multiple enzymatic and non-enzymatic antioxidant biomarkers. Likewise, it also improved gluconeogenesis mediated pathogenesis of non-alcoholic fatty liver injury. Additionally, molecular docking also identified mucusisoflavone A and B as lead hits in downregulating gluconeogenesis.

CONCLUSION

Hydroalcoholic extract of F. benghalensis bark may prevent insulin resistance by downregulating gluconeogenesis and improving the appetite in fructose-induced insulin-resistant rats.

Identification of the first highly selective inhibitor of human lactate dehydrogenase B

Lactate dehydrogenase (LDH) catalyses the conversion of pyruvate to lactate and NADH to NAD⁺; it has two isoforms, LDHA and LDHB. LDHA is a promising target for cancer therapy, whereas LDHB is necessary for basal autophagy and cancer cell proliferation in oxidative and glycolytic cancer cells. To the best of our knowledge, selective inhibitors for LDHB have not yet been reported. Here, we developed a high-throughput mass spectrometry screening system using an LDHB enzyme assay by detecting NADH and NAD⁺. As a result, we identified a small-molecule LDHB selective inhibitor AXKO-0046, an indole derivative. This compound exhibited uncompetitive LDHB inhibition (EC₅₀ = 42 nM). X-ray crystallography revealed that AXKO-0046 bound to the potential allosteric site away from the LDHB catalytic active site, suggesting that targeting the tetramerisation interface of the two dimers is critical for the enzymatic activity. AXKO-0046 and its derivatives can be used to validate LDHB-associated pathways in cancer metabolism.

Preparation, stability and commutability of candidate reference materials for lactate dehydrogenase (LDH)

BACKGROUND

Lactate dehydrogenase (LDH) is a key enzyme that functions as a marker of cell damage. Its activity can be measured by a variety of laboratory methods. To eliminate inter-method bias and enhance equivalence among different measurement procedures, LDH detection needs to be standardized.

METHODS

Optimized sequences coding for lactate dehydrogenase subunit A (LDH-A) and subunit B (LDH-B) were synthesized and cloned into the pRSFDuet vector, and then the constructed 6His-LDHA-pRSFDuet, 6His-LDHB-pRSFDuet, and 6His-LDHA-LDHB-pRSFDuet plasmids were transformed into Escherichia coli BL21 (DE3) for expression. The enzyme activity and specific activity of recombinant LDHs were detected. Electrophoresis of LDH isoenzymes was performed. The stability of recombinant LDHs and serum LDH was evaluated. Commutability of recombinant LDH-B was studied by the IFCC reference method and six routine methods.

RESULTS

Three plasmids were constructed and three highly concentrated recombinant LDH isoenzymes were obtained. The specific activities of LDH-A, LDH-AB, and LDH-B were 18.08 U/mg, 21.74 U/mg, and 14.18 U/mg, respectively. There was a desirable linear correlation between the activities of recombinant LDH isoenzymes and their protein concentrations. Electrophoresis of LDH isoenzymes showed that the recombinant LDH-B corresponded to LDH1 and it demonstrated good stability at 4 °C and 25 °C for 5 weeks. LDH-B formulations in saline-bovine serum albumin solution and human serum matrix were commutable for six routine methods.

CONCLUSION

Human recombinant LDH-B has great potential to become an improved and less expensive standard or reference material in external quality assessment for clinical LDH measurement.

Small Molecule-Based Enzyme Inhibitors in the Treatment of Primary Hyperoxalurias

Primary hyperoxalurias (PHs) are a group of inherited alterations of the hepatic glyoxylate metabolism. PHs classification based on gene mutations parallel a variety of enzymatic defects, and all involve the harmful accumulation of calcium oxalate crystals that produce systemic damage. These geographically widespread rare diseases have a deep impact in the life quality of the patients. Until recently, treatments were limited to palliative measures and kidney/liver transplants in the most severe forms. Efforts made to develop pharmacological treatments succeeded with the biotechnological agent lumasiran, a siRNA product against glycolate oxidase, which has become the first effective therapy to treat PH1. However, small molecule drugs have classically been preferred since they benefit from experience and have better pharmacological properties. The development of small molecule inhibitors designed against key enzymes of glyoxylate metabolism is on the focus of research. Enzyme inhibitors are successful and widely used in several diseases and their pharmacokinetic advantages are well known. In PHs, effective enzymatic targets have been determined and characterized for drug design and interesting inhibitory activities have been achieved both in vitro and in vivo. This review describes the most recent advances towards the development of small molecule enzyme inhibitors in the treatment of PHs, introducing the multi-target approach as a more effective and safe therapeutic option.

Lactate dehydrogenase isoenzymes in patients with acute exacerbation of chronic obstructive pulmonary disease: An exploratory cross-sectional study

We aimed to evaluate differences in serum lactate dehydrogenase (LDH) isoenzymes between patients hospitalized for acute exacerbation of chronic obstructive pulmonary disease (AECOPD) and other lower respiratory tract infections (LRTIs). Based on self-reported COPD diagnosis, 71 participants were divided into AECOPD (n = 38, 29 males, mean age 70.5 years) and LRTI (n = 33, 12 males, mean age 70.4 years) groups. Information on demographics, comorbidities, and COPD severity markers, as well as arterial blood gases and laboratory data were collected, while serum LDH electrophoresis was performed to examine the LDH isoenzymes. Adjusting for sex, age, comorbidities, degree of hypoxemia, inflammation markers, muscle and myocardial enzymes, and total serum LDH, the mean differences (95 % confidence intervals) in the ratios of serum LDH isoenzymes to total serum LDH between groups (LDHx_AECOPD - LDHx_LRTI) were statistically significant for LDH1 [4.9 (1.4 to 8.3)], LDH2 [3.0 (0.1 to 5.8)], LDH3 [-4.3 (-6.3 to -2.3)], and LDH4 [-3.2 (-4.9 to -1.5)]. A sum of LDH3 and LDH4 ratios below 29 % had the highest discriminative ability to classify a subject in the AECOPD group (AUC 0.841, sensitivity 76 %, specificity 87 %). Aerobic metabolic adaptive mechanisms in respiratory muscles during AECOPD could explain the above differences.

The usefulness of lactate dehydrogenase measurements in current oncological practice

One of the hallmarks of cancer cells is increased energy requirements associated with the higher rate of cellular proliferative activity. Metabolic changes in rapidly dividing cancer cells are closely associated with increased uptake of glucose and abnormal activity of lactate dehydrogenase (LDH), which regulates the processing of glucose to lactic acid. As serum LDH levels were found to be commonly increased in cancer patients and correlated with poor clinical outcome and resistance to therapy, the determination of LDH has become a standard supportive tool in diagnosing cancers or monitoring the effects of cancer treatment. The aim of this review is to summarize the current knowledge about methods and the practical utility for measuring both the total LDH and LDH isoenzymatic activities in the diagnosis, prognosis and prediction of cancer diseases.

Structural Evidence for Isoform-Selective Allosteric Inhibition of Lactate Dehydrogenase A

Lactate dehydrogenase A (LDHA) is frequently overexpressed in tumors, thereby sustaining high glycolysis rates, tumor growth, and chemoresistance. High-throughput screening resulted in the identification of phthalimide and dibenzofuran derivatives as novel lactate dehydrogenase inhibitors, selectively inhibiting the activity of the LDHA isoenzyme. Cocrystallization experiments confirmed target engagement in addition to demonstrating binding to a novel allosteric binding site present in all four LDHA subunits of the LDH5 homotetramer.

Structural and functional regulation of lactate dehydrogenase-A in cancer

Dysregulated metabolism is one of the hallmarks of cancer. Under normal physiological conditions, ATP is primarily generated by oxidative phosphorylation. Cancers commonly undergo a dramatic shift toward glycolysis, despite the presence of oxygen. This phenomenon is known as the Warburg effect, and requires the activity of LDHA. LDHA converts pyruvate to lactate in the final step of glycolysis and is often upregulated in cancer. LDHA inhibitors present a promising therapeutic option, as LDHA blockade leads to apoptosis in cancer cells. Despite this, existing LDHA inhibitors have shown limited clinical efficacy. Here, we review recent progress in LDHA structure, function and regulation as well as strategies to target this critical enzyme.

Machilin A Inhibits Tumor Growth and Macrophage M2 Polarization Through the Reduction of Lactic Acid

Lactate dehydrogenase A (LDHA) is an important enzyme responsible for cancer growth and energy metabolism in various cancers via the aerobic glycolytic pathway. Here, we report that machilin A (MA), which acts as a competitive inhibitor by blocking the nicotinamide adenine dinucleotide (NAD) binding site of LDHA, suppresses growth of cancer cells and lactate production in various cancer cell types, including colon, breast, lung, and liver cancers. Furthermore, MA markedly decreased LDHA activity, lactate production, and intracellular adenosine triphosphate (ATP) levels induced by hypoxia-induced LDHA expression in cancer cells, and significantly inhibited colony formation, leading to reduced cancer cell survival. In mouse models inoculated with murine Lewis lung carcinoma, MA significantly suppressed tumor growth as observed by a reduction of tumor volume and weight; resulting from the inhibition of LDHA activity. Subsequently, the suppression of tumor-derived lactic acid in MA-treated cancer cells resulted in decrease of neovascularization through the regulation of alternatively activated macrophages (M2) polarization in macrophages. Taken together, we suggest that the reduction of lactate by MA in cancer cells directly results in a suppression of cancer cell growth. Furthermore, macrophage polarization and activation of endothelial cells for angiogenesis were indirectly regulated preventing lactate production in MA-treated cancer cells.

Predicting protein-ligand binding affinity and correcting crystal structures with quantum mechanical calculations: lactate dehydrogenase A

Accurately computing the geometry and energy of host-guest and protein-ligand interactions requires a physically accurate description of the forces in action. Quantum mechanics can provide this accuracy but the calculations can require a prohibitive quantity of computational resources. The size of the calculations can be reduced by including only the atoms of the receptor that are in close proximity to the ligand. We show that when combined with log P values for the ligand (which can be computed easily) this approach can significantly improve the agreement between computed and measured binding energies. When the approach is applied to lactate dehydrogenase A, it can make quantitative predictions about conformational, tautomeric and protonation state preferences as well as stereoselectivity and even identifies potential errors in structures deposited in the Protein Data Bank for this enzyme. By broadening the evidence base for these structures from only the diffraction data, more chemically realistic structures can be proposed.

Exploration of ligand-induced protein conformational alteration, aggregate formation, and its inhibition: A biophysical insight

The association of protein aggregates with plentiful human diseases has fascinated studies regarding the biophysical characterization of protein misfolding and ultimately their aggregate formation mechanism. Protein-ligand interaction, their mechanism, conformational changes by ligands, and protein aggregate formation have been studied upon exploiting experimental techniques and computational methodologies. Such studies for the exploration of ligand-induced conformational changes in protein, misfolding and aggregation, has confirmed drastic progresses in the study of aggregate formation pathways. This review comprises of an inclusive description of contemporary experimental techniques as well as theoretical improvements in the interpretation of the conformational properties of protein. We have also discussed various factors responsible for the microenvironment change around protein that sequentially causes amyloidoses. Biophysical techniques and cell-based assays to gain comprehensive understandings of protein-ligand interaction, protein folding, and aggregation pathways have also been described. The promising therapeutic methods used to inhibit the protein fibrillogenesis have also been discussed.

Three-Dimensional Analysis of the Interactions between hLDH5 and Its Inhibitors

Inhibitors of human lactate dehydrogenase (hLDH5)—the enzyme responsible for the conversion of pyruvate to lactate coupled with oxidation of NADH to NAD⁺—are promising therapeutic agents against cancer because this enzyme is generally found to be overexpressed in most invasive cancer cells and is linked to their vitality especially under hypoxic conditions. Consequently, significant efforts have been made for the identification of small-molecule hLDH5 inhibitors displaying high inhibitory potencies. X-ray structure of hLDH5 complexes as well as molecular modeling studies contribute to identify and explain the main binding modes of hLDH5 inhibitors reported in literature. The purpose of this review is to analyze the main three-dimensional interactions between some of the most potent inhibitors and hLDH5, in order to provide useful suggestions for the design of new derivatives.

Liver glucose metabolism in humans

Information about normal hepatic glucose metabolism may help to understand pathogenic mechanisms underlying obesity and diabetes mellitus. In addition, liver glucose metabolism is involved in glycosylation reactions and connected with fatty acid metabolism. The liver receives dietary carbohydrates directly from the intestine via the portal vein. Glucokinase phosphorylates glucose to glucose 6-phosphate inside the hepatocyte, ensuring that an adequate flow of glucose enters the cell to be metabolized. Glucose 6-phosphate may proceed to several metabolic pathways. During the post-prandial period, most glucose 6-phosphate is used to synthesize glycogen via the formation of glucose 1-phosphate and UDP–glucose. Minor amounts of UDP–glucose are used to form UDP–glucuronate and UDP–galactose, which are donors of monosaccharide units used in glycosylation. A second pathway of glucose 6-phosphate metabolism is the formation of fructose 6-phosphate, which may either start the hexosamine pathway to produce UDP-N-acetylglucosamine or follow the glycolytic pathway to generate pyruvate and then acetyl-CoA. Acetyl-CoA may enter the tricarboxylic acid (TCA) cycle to be oxidized or may be exported to the cytosol to synthesize fatty acids, when excess glucose is present within the hepatocyte. Finally, glucose 6-phosphate may produce NADPH and ribose 5-phosphate through the pentose phosphate pathway. Glucose metabolism supplies intermediates for glycosylation, a post-translational modification of proteins and lipids that modulates their activity. Congenital deficiency of phosphoglucomutase (PGM)-1 and PGM-3 is associated with impaired glycosylation. In addition to metabolize carbohydrates, the liver produces glucose to be used by other tissues, from glycogen breakdown or from de novo synthesis using primarily lactate and alanine (gluconeogenesis).

Identification of lactate dehydrogenase as a mammalian pyrroloquinoline quinone (PQQ)-binding protein

Pyrroloquinoline quinone (PQQ), a redox-active o-quinone, is an important nutrient involved in numerous physiological and biochemical processes in mammals. Despite such beneficial functions, the underlying molecular mechanisms remain to be established. In the present study, using PQQ-immobilized Sepharose beads as a probe, we examined the presence of protein(s) that are capable of binding PQQ in mouse NIH/3T3 fibroblasts and identified five cellular proteins, including l-lactate dehydrogenase (LDH) A chain, as potential mammalian PQQ-binding proteins. In vitro studies using a purified rabbit muscle LDH show that PQQ inhibits the formation of lactate from pyruvate in the presence of NADH (forward reaction), whereas it enhances the conversion of lactate to pyruvate in the presence of NAD⁺ (reverse reaction). The molecular mechanism underlying PQQ-mediated regulation of LDH activity is attributed to the oxidation of NADH to NAD⁺ by PQQ. Indeed, the PQQ-bound LDH oxidizes NADH, generating NAD⁺, and significantly catalyzes the conversion of lactate to pyruvate. Furthermore, PQQ attenuates cellular lactate release and increases intracellular ATP levels in the NIH/3T3 fibroblasts. Our results suggest that PQQ, modulating LDH activity to facilitate pyruvate formation through its redox-cycling activity, may be involved in the enhanced energy production via mitochondrial TCA cycle and oxidative phosphorylation.

Structures of lactate dehydrogenase A (LDHA) in apo, ternary and inhibitor-bound forms

Lactate dehydrogenase (LDH) is an essential metabolic enzyme that catalyzes the interconversion of pyruvate and lactate using NADH/NAD(+) as a co-substrate. Many cancer cells exhibit a glycolytic phenotype known as the Warburg effect, in which elevated LDH levels enhance the conversion of glucose to lactate, making LDH an attractive therapeutic target for oncology. Two known inhibitors of the human muscle LDH isoform, LDHA, designated 1 and 2, were selected, and their IC50 values were determined to be 14.4 ± 3.77 and 2.20 ± 0.15 µM, respectively. The X-ray crystal structures of LDHA in complex with each inhibitor were determined; both inhibitors bind to a site overlapping with the NADH-binding site. Further, an apo LDHA crystal structure solved in a new space group is reported, as well as a complex with both NADH and the substrate analogue oxalate bound in seven of the eight molecules and an oxalate only bound in the eighth molecule in the asymmetric unit. In this latter structure, a kanamycin molecule is located in the inhibitor-binding site, thereby blocking NADH binding. These structures provide insights into LDHA enzyme mechanism and inhibition and a framework for structure-assisted drug design that may contribute to new cancer therapies.

Search for Human Lactate Dehydrogenase A Inhibitors Using Structure-Based Modeling

The human lactate dehydrogenase isoform A plays an important role in the anaerobic metabolism of tumour cells and therefore constitutes an attractive target in the oncology field. Full-atom models of lactate dehydrogenase A (in complex with NADH and in the apo form) have been generated to enable structure-based design of novel inhibitors competing with pyruvate and NADH. The structural criteria for the selection of potential inhibitors were established, and virtual screening of a library of low-molecular-weight compounds was performed. A potential inhibitor, STK381370, was identified whose docking pose was stabilized through additional interactions with the loop 96-111 providing for the transition from the open to the closed conformation.