Interaction of human 3-phosphoglycerate kinase with L-ADP, the mirror image of D-ADP

l-Nucleoside-analogues, mirror images of the natural d-nucleosides, are a new class of antiviral and anticancer agents. In the cell they have to be phosphorylated to pharmacologically active triphosphate forms, the last step seems to involve human 3-phosphoglycerate kinase (hPGK). Here we present a steady state kinetic and biophysical study of the interaction of the model compound l-MgADP with hPGK. l-MgADP is a good substrate with k_cat and K_m values of 685 s⁻¹ and 0.27 mM, respectively. Double inhibition studies suggest that l-MgADP binds to the specific adenosine-binding site and protects the conformation of hPGK molecule against heat denaturation, as detected by microcalorimetry. Structural details of the interaction in the enzyme active site are different for the d- and l-enantiomers (e.g. the effect of Mg²⁺), but these differences do not prevent the occurrence of the catalytic cycle, which is accompanied by the hinge-bending domain closure, as indicated by SAXS measurements.     

The contribution of surface residues to membrane binding and ligand transfer by the α-tocopherol transfer protein (α-TTP)

Previous work has shown that the α-tocopherol transfer protein (α-TTP) can bind to vesicular or immobilized phospholipid membranes. Revealing the molecular mechanisms by which α-TTP associates with membranes is thought to be critical to understanding its function and role in the secretion of tocopherol from hepatocytes into the circulation. Calculations presented in the Orientations of Proteins in Membranes database have provided a testable model for the spatial arrangement of α-TTP and other CRAL-TRIO family proteins with respect to the lipid bilayer. These calculations predicted that a hydrophobic surface mediates the interaction of α-TTP with lipid membranes. To test the validity of these predictions, we used site-directed mutagenesis and examined the substituted mutants with regard to intermembrane ligand transfer, association with lipid layers and biological activity in cultured hepatocytes. Substitution of residues in helices A8 (F165A and F169A) and A10 (I202A, V206A and M209A) decreased the rate of intermembrane ligand transfer as well as protein adsorption to phospholipid bilayers. The largest impairment was observed upon mutation of residues that are predicted to be fully immersed in the lipid bilayer in both apo (open) and holo (closed) conformations such as Phe165 and Phe169. Mutation F169A, and especially F169D, significantly impaired α-TTP-assisted secretion of α-tocopherol outside cultured hepatocytes. Mutation of selected basic residues (R192H, K211A, and K217A) had little effect on transfer rates, indicating no significant involvement of nonspecific electrostatic interactions with membranes.     

Amino-terminal domain stability mediates apolipoprotein E aggregation into neurotoxic fibrils

The three isoforms of apolipoprotein (apo) E are strongly associated with different risks for Alzheimer's disease: apoE4>apoE3>apoE2. Here, we show at physiological salt concentrations and pH that native tetramers of apoE form soluble aggregates in vitro that bind the amyloid dyes thioflavin T and Congo red. However, unlike classic amyloid fibrils, the aggregates adopt an irregular protofilament-like morphology and are seemingly highly alpha-helical. The aggregates formed at substantially different rates (apoE4>apoE3>apoE2) and were significantly more toxic to cultured neuronal cells than the tetramer. Since the three isoforms have large differences in conformational stability that can influence aggregation and amyloid pathways, we tested the effects of mutations that increased or decreased stability. Decreasing the conformational stability of the amino-terminal domain of apoE increased aggregation rates and vice versa. Our findings provide a new perspective for an isoform-specific pathogenic role for apoE aggregation in which differences in the conformational stability of the amino-terminal domain mediate neurodegeneration.     

Structural and functional determinants of conserved lipid interaction domains of inward rectifying Kir6.2 channels

All members of the inward rectifiier K(+) (Kir) channel family are activated by phosphoinositides and other amphiphilic lipids. To further elucidate the mechanistic basis, we examined the membrane association of Kir6.2 fragments of K(ATP) channels, and the effects of site-directed mutations of these fragments and full-length Kir6.2 on membrane association and K(ATP) channel activity, respectively. GFP-tagged Kir6.2 COOH terminus and GFP-tagged pleckstrin homology domain from phospholipase C delta1 both associate with isolated membranes, and association of each is specifically reduced by muscarinic m1 receptor-mediated phospholipid depletion. Kir COOH termini are predicted to contain multiple beta-strands and a conserved alpha-helix (residues approximately 306-311 in Kir6.2). Systematic mutagenesis of D307-F315 reveals a critical role of E308, I309, W311 and F315, consistent with residues lying on one side of a alpha-helix. Together with systematic mutation of conserved charges, the results define critical determinants of a conserved domain that underlies phospholipid interaction in Kir channels.     

Insilico Structural 3D Modelling of Novel Cry1Ib9 and Cry3A Toxins from Local Isolates of Bacillus thuringiensis

Three-dimensional (3D) models for the 79.2 kDa activated Cry1Ib9 and 77.4 kDa activated Cry3A δ-endotoxins from Bacillus thuringiensis (Bt) native isolates that are specifically toxic to Coleopteran insect pests were constructed by utilizing homology modeling online tool. Evidences presented here, based on the identification of structural equivalent residues of Cry1Ib9 and Cry3A toxin through homology modelling indicate that, they share a common Bt toxin tridimensional structure. The main differences observed in Cry1I9 domain I at positions α2b (S56-I60), α4 (F78-l93) and additionally β0 (Q10-L12), α8a (T280-V282) were observed, in domain II at positions α9b (P333-L339), β6(T390-Q393), β7(V398-W404), β8 (V418-W425), β9 (E453-N454), β10 (S470-I479) where as in domain III the changes were observed at positions β19 (R601-F607), β20 (609-L613), β21 (S618-F627) and α11a (K655-F664), α13, α14 components present at downstream sites, where as in Cry3A main differences observed in domain I is at the position of α4 (P105-I152), α5 (Q163-A185), β1A(E190-L192), α6 (F193-Y217), Domain II is not consevered and main variations were observed at β2 (E292-L295), β3(V299-L308), β4(I340-F347), β5(D356-P368), β6(I375-T377), β7(V389-F394), β8(K398-N405), β9(Y416-Y427), β10 (T436-Y439), β12(G476-H495), β12A (M503-I504) where as in domain III main variations observed at positions of β18 (P583-I593), β19(F604-S610), β20(P611-L615), β21(N619-G626). Cry1Ib9 and Cry3A contain the most variable regions in the loops of domain II, which determine the specificity of these toxins. These are the first models of Coleopteran-active protein from native isolates of Bt and its importance can be perceived since members of this group of toxins are potentially important candidates for coleoptera insect pest control programs.     

Examination of the long-range effects of aminofluorene-induced conformational heterogeneity and its relevance to the mechanism of translesional DNA synthesis

Adduct-induced conformational heterogeneity complicates the understanding of how DNA adducts exert mutation. A case in point is the N-deacetylated AF lesion [N-(2'-deoxyguanosin-8-yl)-2-aminofluorene], the major adduct derived from the strong liver carcinogen N-acetyl-2-aminofluorene. Three conformational families have been previously characterized and are dependent on the positioning of the aminofluorene rings: B is in the "B-DNA" major groove, S is "stacked" into the helix with base-displacement, and W is "wedged" into the minor groove. Here, we conducted (19)F NMR, CD, T(m), and modeling experiments at various primer positions with respect to a template modified by a fluorine tagged AF-adduct (FAF). In the first set, the FAF-G was paired with C and in the second set it was paired with A. The FAF-G:C oligonucleotides were found to preferentially adopt the B or S-conformers while the FAF-G:A mismatch ones preferred the B and W-conformers. The conformational preferences of both series were dependent on temperature and complementary strand length; the largest differences in conformation were displayed at lower temperatures. The CD and T(m) results are in general agreement with the NMR data. Molecular modeling indicated that the aminofluorene moiety in the minor groove of the W-conformer would impose a steric clash with the tight-packing amino acid residues on the DNA binding area of the Bacillus fragment (BF), a replicative DNA polymerase. In the case of the B-type conformer, the carcinogenic moiety resides in the solvent-exposed major groove throughout the replication/translocation process. The present dynamic NMR results, combined with previous primer extension kinetic data by Miller & Grollman, support a model in which adduct-induced conformational heterogeneities at positions remote from the replication fork affect polymerase function through a long-range DNA-protein interaction.     

Site-directed mutagenesis of glutamate 317 of bovine α-1,3Galactosyltransferase and its effect on enzyme activity: Implications for reaction mechanism

Bovine α1,3galactosyltransferase (α1,3GalT) transfers galactose from UDP-α-galactose to terminal β-linked galactosyl residues with retention of configuration of the incoming galactose residue. The epitope synthesized has been shown to be critical for xenotransplantation. According to a proposed double-displacement reaction mechanism, glutamate-317 (E317) is thought to be the catalytic nucleophile. The proposed catalytic role of E317 involves an initial nucleophilic attack with inversion of configuration and formation of a covalent sugar–enzyme intermediate between E317 and galactose from the donor substrate, followed by a second nucleophilic attack performed by the acceptor substrate with a second inversion of configuration. To determine whether E317 of α1,3GalT is critical for enzyme activity, site-directed mutagenesis was used to substitute alanine, aspartic acid, cysteine and histidine for E317. If the proposed reaction mechanism for the α1,3GalT enzyme is correct, E317D and E317H would produce active enzymes since they can act as nucleophiles. The non-conservative mutation E317A and conservative mutation E317C are predicted to produce inactive or very low activity enzymes since the E317A mutant cannot engage in a nucleophilic attack, and the E317C mutant would trap the galactose residue. The results obtained demonstrate that E317D and E317H mutants retained activity, albeit significantly less than the wild-type enzyme. Additionally, both E317A and E317C mutant also retained enzyme activity, suggesting that E317 is not the catalytic nucleophile proposed in the double-displacement mechanism. Therefore, either a different amino acid may act as the catalytic nucleophile or the reaction must proceed by a different mechanism.     

Highly organized but pliant active site of DNA polymerase beta: compensatory mechanisms in mutant enzymes revealed by dynamics simulations and energy analyses

To link conformational transitions noted for DNA polymerases with kinetic results describing catalytic efficiency and fidelity, we investigate the role of key DNA polymerase beta residues on subdomain motion through simulations of five single-residue mutants: Arg-283-Ala, Tyr-271-Ala, Asp-276-Val, Arg-258-Lys, and Arg-258-Ala. Since a movement toward a closed state was only observed for R258A, we suggest that Arg(258) is crucial in modulating motion preceding chemistry. Analyses of protein/DNA interactions in the mutant active site indicate distinctive hydrogen bonding and van der Waals patterns arising from compensatory structural adjustments. By comparing closed mutant complexes with the wild-type enzyme, we interpret experimentally derived nucleotide binding affinities in molecular terms: R283A (decreased), Y271A (increased), D276V (increased), and R258A (decreased). Thus, compensatory interactions (e.g., in Y271A with adjacent residues Phe(272), Asn(279), and Arg(283)) increase the overall binding affinity for the incoming nucleotide although direct interactions may decrease. Together with energetic analyses, we predict that R258G might increase the rate of nucleotide insertion and maintain enzyme fidelity as R258A; D276L might increase the nucleotide binding affinity more than D276V; and R283A/K280A might decrease the nucleotide binding affinity and increase misinsertion more than R283A. The combined observations regarding key roles of specific residues (e.g., Arg(258)) and compensatory interactions echo the dual nature of polymerase active site, namely versatility (to accommodate various basepairs) and specificity (for preserving fidelity) and underscore an organized but pliant active site essential to enzyme function.     

Impact of intra-subunit interactions on the dimeric arginine kinase activity and structural stability

Arginine kinase (AK) catalyzes the reversible phosphorylation of arginine by ATP, yielding the phosphoarginine. In this research, six conserved residues located on the intra-subunit domain-domain interfaces were mutated to explore their roles in the activity and structural stability of dimer AK. The mutations D69A, E70A, E71A and F80A led to pronounced loss of AK activity and structural stability. Although the mutations V75A and F76A had little effect on AK activity and structure, they caused gradually decreased the stability and reactivation of dimer AK. Our results suggested that the mutations might affect the correct positioning of the N-loop and C-loop thus disrupted the efficient recognition and interactions between the N-terminal domain and C-terminal domain which may influence the compact dimer structure, and result in decreased activity and structural stability.     

Structural conservation of residues in BH1 and BH2 domains of Bcl-2 family proteins

The sequence of Bcl-2 homology domains, BH1 and BH2, is known to be conserved among anti- and pro-apoptotic members of Bcl-2 family proteins. But structural conservation of these domains with respect to functionally active residues playing role in heterodimerization-mediated regulation of apoptosis has never been elucidated. Here, we have suggested the formation of an active site by structurally conserved residues in BH1 (glycine, arginine) and BH2 (tryptophan) domains of Bcl-2 family members, which also accounts for the functional effect of known mutations in BH1 (G145A, G145E) and BH2 (W188A) domains of Bcl-2.     

Stabilization of the activity of ATP-sensitive potassium channels by ion pairs formed between adjacent Kir6.2 subunits

ATP-sensitive potassium (KATP) channels are formed by the coassembly of four Kir6.2 subunits and four sulfonylurea receptor subunits (SUR). The cytoplasmic domains of Kir6.2 mediate channel gating by ATP, which closes the channel, and membrane phosphoinositides, which stabilize the open channel. Little is known, however, about the tertiary or quaternary structures of the domains that are responsible for these interactions. Here, we report that an ion pair between glutamate 229 and arginine 314 in the intracellular COOH terminus of Kir6.2 is critical for maintaining channel activity. Mutation of either residue to alanine induces inactivation, whereas charge reversal at positions 229 and 314 (E229R/R314E) abolishes inactivation and restores the wild-type channel phenotype. The close proximity of these two residues is demonstrated by disulfide bond formation between cysteine residues introduced at the two positions (E229C/R314C); disulfide bond formation abolishes inactivation and stabilizes the current. Using Kir6.2 tandem dimer constructs, we provide evidence that the ion pair likely forms by residues from two adjacent Kir6.2 subunits. We propose that the E229/R314 intersubunit ion pairs may contribute to a structural framework that facilitates the ability of other positively charged residues to interact with membrane phosphoinositides. Glutamate and arginine residues are found at homologous positions in many inward rectifier subunits, including the G-protein-activated inwardly rectifying potassium channel (GIRK), whose cytoplasmic domain structure has recently been solved. In the GIRK structure, the E229- and R314-corresponding residues are oriented in opposite directions in a single subunit such that in the tetramer model, the E229 equivalent residue from one subunit is in close proximity of the R314 equivalent residue from the adjacent subunit. The structure lends support to our findings in Kir6.2, and raises the possibility that a homologous ion pair may be involved in the gating of GIRKs.     

Properties of the inulinase gene levH1 of Lactobacillus casei IAM 1045; cloning, mutational and biochemical characterization

Though some genetic features of lactobacillar fructan hydrolases were elucidated, information about their enzymology or mutational analyses were scarce. Lactobacillus casei IAM1045 exhibits extracellular activity degrading inulin. After partial purification of the inulin-degrading protein from the spent culture medium, several fragments were obtained by protease digestion. Based on their partial amino-acid sequences, oligonucleotide primers were designed, and its structural gene (levH1) was determined using the gene library constructed in the E. coli system. The levH1 gene encoded a protein (designated as LevH1), of which calculated molecular mass and pI were 138.8-kDa and 4.66, respectively. LevH1 (1296 amino-acids long) was predicted to have a four-domain structure, containing (i) an N-terminal secretion signal of 40 amino-acids, (ii) variable domain of about 140 residues whose function is unclear, (iii) a catalytic domain of about 630 residues with glycoside-hydrolase activity consisting of two modules, a five-blade β-propeller module linked to a β-sandwich module, (iv) a C-terminal domain of about 490 residues comprising five nearly perfect repeat sequences of 80 residues homologous to equivalents of other hypothetical cell surface proteins, followed by 37-residues rich in Ser/Thr/Pro/Gly, a pentad LPQAG (the LPXTG homologue). When overproduced in E. coli, the putative variable-catalytic domain region of about 770 residues exhibited exo-inulinase activity. Deletion analyses demonstrated that the variable-catalytic domain region containing two modules is important for enzymatic activity. Presence of eight conserved motifs (I-VIII) was suggested in the catalytic domain by comparative analysis, among which motif VIII was newly identified in the β-sandwich module in this study. Site-directed mutagenesis of conserved amino-acids in these motifs revealed that D198, R388, D389 and E440, were crucial for inulinase activity. Moreover, mutations of D502A and D683A in motif VI and VIII respectively caused significant decrease in the activity. These results suggested that the variable domain and β-sandwich module, besides the β-propeller module, are important for inulin-degrading activity of LevH1.     

Functional Analysis of the Transmembrane Domain in Paramyxovirus F Protein-Mediated Membrane Fusion

To enter cells, enveloped viruses use fusion-mediating glycoproteins to facilitate the merger of the viral and host cell membranes. These glycoproteins undergo large-scale irreversible refolding during membrane fusion. The paramyxovirus parainfluenza virus 5 mediates membrane merger through its fusion protein (F). The transmembrane (TM) domains of viral fusion proteins are typically required for fusion. The TM domain of F is particularly interesting in that it is potentially unusually long; multiple calculations suggest a TM helix length between 25 and 48 residues. Oxidative cross-linking of single-cysteine substitutions indicates the F TM trimer forms a helical bundle within the membrane. To assess the functional role of the paramyxovirus parainfluenza virus 5 F protein TM domain, alanine scanning mutagenesis was performed. Two residues located in the outer leaflet of the bilayer are critical for fusion. Multiple amino acid substitutions at these positions indicate the physical properties of the side chain play a critical role in supporting or blocking fusion. Analysis of intermediate steps in F protein refolding indicated that the mutants were not trapped at the open stalk intermediate or the prehairpin intermediate. Incorporation of a known F protein destabilizing mutation that causes a hyperfusogenic phenotype restored fusion activity to the mutants. Further, altering the curvature of the lipid bilayer by addition of oleic acid promoted fusion of the F protein mutants. In aggregate, these data indicate that the TM domain plays a functional role in fusion beyond merely anchoring the protein in the viral envelope and that it can affect the structures and steady-state concentrations of the various conformational intermediates en route to the final postfusion state. We suggest that the unusual length of this TM helix might allow it to serve as a template for formation of or specifically stabilize the lipid stalk intermediate in fusion.     

Mutation of the highly conserved Arg165 and Glu168 residues of human Gsalpha disrupts the alphaD-alphaE loop and enhances basal GDP/GTP exchange rate

G protein signalling regulates a wide range of cellular processes such as motility, differentiation, secretion, neurotransmission, and cell division. G proteins consist of three subunits organized as a Galpha monomer associated with a Gbetagamma heterodimer. Structural studies have shown that Galpha subunits are constituted by two domains: a Ras-like domain, also called the GTPase domain (GTPaseD), and an helical domain (HD), which is unique to heterotrimeric G-proteins. The HD display significantly higher primary structure diversity than the GTPaseD. Regardless of this diversity, there are small regions of the HD which show high degree of identity with residues that are 100% conserved. One of such regions is the alpha helixD-alpha helixE loop (alphaD-alphaE) in the HD, which contains the consensus aminoacid sequence R*-[RSA]-[RSAN]-E*-[YF]-[QH]-L in all mammalian Galpha subunits. Interestingly, the highly conserved arginine (R*) and glutamic acid (E*) residues form a salt bridge that stabilizes the alphaD-alphaE loop, that is localized in the top of the cleft formed between the GTPaseD and HD. Because the guanine nucleotide binding site is deeply buried in this cleft and those interdomain interactions are playing an important role in regulating the basal GDP/GTP nucleotide exchange rate of Galpha subunits, we studied the role of these highly conserved R and E residues in Galpha function. In the present study, we mutated the human Gsalpha R165 and E168 residues to alanine (A), thus generating the R165--> A, E168--> A, and R165/E168--> A mutants. We expressed these human Gsalpha (hGsalpha) mutants in bacteria as histidine tagged proteins, purified them by niquel-agarose chromatography and studied their nucleotide exchange properties. We show that the double R165/E168--> A mutant exhibited a fivefold increased GTP binding kinetics, a higher GDP dissociation rate, and an augmented capacity to activate adenylyl cyclase. Structure analysis showed that disruption of the salt bridge between R165 and E168 by the introduced mutations, caused important structural changes in the HD at the alphaD-alphaE loop (residues 160-175) and in the GTPaseD at a region required for Gsalpha activation by the receptor (residues 308-315). In addition, other two GTPaseD regions that surround the GTP binding site were also affected.     

Agonist-induced conformational changes in bovine rhodopsin: insight into activation of G-protein-coupled receptors

Activation of G-protein-coupled receptors (GPCRs) is initiated by conformational changes in the transmembrane (TM) helices and the intra- and extracellular loops induced by ligand binding. Understanding the conformational changes in GPCRs leading to activation is imperative in deciphering the role of these receptors in the pathology of diseases. Since the crystal structures of activated GPCRs are not yet available, computational methods and biophysical techniques have been used to predict the structures of GPCR active states. We have recently applied the computational method LITiCon to understand the ligand-induced conformational changes in β₂-adrenergic receptor by ligands of varied efficacies. Here we report a study of the conformational changes associated with the activation of bovine rhodopsin for which the crystal structure of the inactive state is known. Starting from the inactive (dark) state, we have predicted the TM conformational changes that are induced by the isomerization of 11-cis retinal to all-trans retinal leading to the fully activated state, metarhodopsin II. The predicted active state of rhodopsin satisfies all of the 30 known experimental distance constraints. The predicted model also correlates well with the experimentally observed conformational switches in rhodopsin and other class A GPCRs, namely, the breaking of the ionic lock between R135^3.50 at the intracellular end of TM3 (part of the DRY motif) and E247^6.30 on TM6, and the rotamer toggle switch on W265^6.48 on TM6. We observe that the toggling of the W265^6.48 rotamer modulates the bend angle of TM6 around the conserved proline. The rotamer toggling is facilitated by the formation of a water wire connecting S298^7.45, W265^6.48 and H211^5.46. As a result, the intracellular ends of TMs 5 and 6 move outward from the protein core, causing large conformational changes at the cytoplasmic interface. The predicted outward movements of TM5 and TM6 are in agreement with the recently published crystal structure of opsin, which is proposed to be close to the active-state structure. In the predicted active state, several residues in the intracellular loops, such as R69, V139^3.54, T229, Q237, Q239, S240, T243 and V250^6.33, become more water exposed compared to the inactive state. These residues may be involved in mediating the conformational signal from the receptor to the G protein. From mutagenesis studies, some of these residues, such as V139^3.54, T229 and V250^6.33, are already implicated in G-protein activation. The predicted active state also leads to the formation of new stabilizing interhelical hydrogen-bond contacts, such as those between W265^6.48 and H211^5.46 and E122^3.37 and C167^4.56. These hydrogen-bond contacts serve as potential conformational switches offering new opportunities for future experimental investigations. The calculated retinal binding energy surface shows that binding of an agonist makes the receptor dynamic and flexible and accessible to many conformations, while binding of an inverse agonist traps the receptor in the inactive state and makes the other conformations inaccessible.     

Studies of the melatonin binding site location onto quinone reductase 2 by directed mutagenesis

Melatonin is a neurohormone implicated in both biorhythm synchronization and neuroprotection from oxidative stress. Its functions are mediated by two G-protein-coupled-receptors (MT1 and MT2) and MT3, which corresponds to quinone oxidoreductase 2 (QR2). To determine the binding site of QR2 for melatonin, point mutations of residues crucial for the enzymatic activity of hQR2 were performed. The substitution of the hydrophobic residues Phe126, Ile128 and Phe178 by tyrosines at the active site significantly increased enzymatic activity and decreased the affinity of a structural analog of melatonin, the 2[¹²⁵I]iodo-MCANAT. The mutation of residues implicated in zinc chelating (His¹⁷³; His¹⁷⁷) had no effect on radioligand binding. Destabilisation of the cofactor FAD by mutation N18E showed that 2[¹²⁵I]iodo-MCANAT binding was closely linked to the conformational integrity of human QR2. Surprisingly, the mutations C222F and N161A, which are distant from the determined binding site of the ligand, increased the affinity of 2[¹²⁵I]iodo-MCANAT for hQR2. What seems to better explain the binding variations among the mutants are the activity recorded with BNAH and coenzyme Q1. Various hypotheses are discussed based on the various parameters used in the study: nature of the substrates and co-substrates and nature of the amino acid changes. This study, which constitutes the first structural analysis of hQR2, should enable to better understand the biological role of melatonin on this enzyme and particularly, the discrepancies between the pharmacologies of the melatonin binding site (MT3) and the QR2 catalytic activity.     

The interplay between protein stability and dynamics in conformational diseases: The case of hPGK1 deficiency

Conformational diseases often show defective protein folding efficiency in vivo upon mutation, affecting protein properties such as thermodynamic stability and folding/unfolding/misfolding kinetics as well as the interactions of the protein with the protein homeostasis network. Human phosphoglycerate kinase 1 (hPGK1) deficiency is a rare inherited disease caused by mutations in hPGK1 that lead to loss-of-function. This disease offers an excellent opportunity to explore the complex relationships between protein stability and dynamics because of the different unfolding mechanisms displayed towards chemical and thermal denaturation. This work explores these relationships using two thermostable mutants (p.E252A and p.T378P) causing hPGK1 deficiency and WT hPGK1 using proteolysis and chemical denaturation. p.T378P is degraded ~ 30-fold faster at low protease concentrations (here, the proteolysis step is rate-limiting) and ~ 3-fold faster at high protease concentrations (where unfolding kinetics is rate-limiting) than WT and p.E252A, indicating that p.T378P is thermodynamically and kinetically destabilized. Urea denaturation studies support the decrease in thermodynamic stability and folding cooperativity for p.T378P, as well as changes in folding/unfolding kinetics. The present study reveals changes in the folding landscape of hPGK1 upon mutation that may affect protein folding efficiency and stability in vivo, also suggesting that native state stabilizers and protein homeostasis modulators may help to correct folding defects in hPGK1 deficiency. Moreover, detailed kinetic proteolysis studies are shown to be powerful and simple tools to provide deep insight into mutational effects on protein folding and stability in conformational diseases.     

The origin of the high sensitivity of muscle fructose 1,6-bisphosphatase towards AMP

Adenosine 5'-monophosphate (AMP) inhibits muscle fructose 1,6-bisphosphatase (FBPase) about 44 times stronger than the liver isozyme. The key role in strong AMP binding to muscle isozyme play K20, T177 and Q179. Muscle FBPase which has been mutated towards the liver enzyme (K20E/T177M/Q179C) is inhibited by AMP about 26 times weaker than the wild-type muscle enzyme, but it binds the fluorescent AMP analogue, 2',3'-O-(2,4,6-trinitrophenyl)adenosine 5'-monophosphate (TNP-AMP), similarly to the wild-type liver enzyme. The reverse mutation of liver FBPase towards the muscle isozyme significantly increases the affinity of the mutant to TNP-AMP. High affinity to the inhibitor but low sensitivity to AMP of the liver triple mutant suggest differences between the isozymes in the mechanism of allosteric signal transmission.