Identification of biochemically neutral positions in liver pyruvate kinase.

Understanding how each residue position contributes to protein function has been a long-standing goal in protein science. Substitution studies have historically focused on conserved protein positions. However, substitutions of nonconserved positions can also modify function. Indeed, we recently identified nonconserved positions that have large substitution effects in human liver pyruvate kinase (hLPYK), including altered allosteric coupling. To facilitate a comparison of which characteristics determine when a nonconserved position does vs does not contribute to function, the goal of the current work was to identify neutral positions in hLPYK. However, existing hLPYK data showed that three features commonly associated with neutral positions-high sequence entropy, high surface exposure, and alanine scanning-lacked the sensitivity needed to guide experimental studies. We used multiple evolutionary patterns identified in a sequence alignment of the PYK family to identify which positions were least patterned, reasoning that these were most likely to be neutral. Nine positions were tested with a total of 117 amino acid substitutions. Although exploring all potential functions is not feasible for any protein, five parameters associated with substrate/effector affinities and allosteric coupling were measured for hLPYK variants. For each position, the aggregate functional outcomes of all variants were used to quantify a "neutrality" score. Three positions showed perfect neutral scores for all five parameters. Furthermore, the nine positions showed larger neutral scores than 17 positions located near allosteric binding sites. Thus, our strategy successfully enriched the dataset for positions with neutral and modest substitutions.

Whole-protein alanine-scanning mutagenesis of allostery: A large percentage of a protein can contribute to mechanism.

Many studies of allosteric mechanisms use limited numbers of mutations to test whether residues play "key" roles. However, if a large percentage of the protein contributes to allosteric function, mutating any residue would have a high probability of modifying allostery. Thus, a predicted mechanism that is dependent on only a few residues could erroneously appear to be supported. We used whole-protein alanine-scanning mutagenesis to determine which amino acid sidechains of human liver pyruvate kinase (hL-PYK; approved symbol PKLR) contribute to regulation by fructose-1,6-bisphosphate (Fru-1,6-BP; activator) and alanine (inhibitor). Each nonalanine/nonglycine residue of hL-PYK was mutated to alanine to generate 431 mutant proteins. Allosteric functions in active proteins were quantified by following substrate affinity over a concentration range of effectors. Results show that different residues contribute to the two allosteric functions. Only a small fraction of mutated residues perturbed inhibition by alanine. In contrast, a large percentage of mutated residues influenced activation by Fru-1,6-BP; inhibition by alanine is not simply the reverse of activation by Fru-1,6-BP. Moreover, the results show that Fru-1,6-BP activation would be extremely difficult to elucidate using a limited number of mutations. Additionally, this large mutational data set will be useful to train and test computational algorithms aiming to predict allosteric mechanisms.

Exploring the limits of the usefulness of mutagenesis in studies of allosteric mechanisms.

The outcome of structure-guided mutational analyses is often used in support of postulated mechanisms of protein allostery. However, the limits of how informative mutations can be in understanding allosteric mechanisms are not completely clear. Here, we report an exercise to evaluate whether mutational data can support a simplistic mechanistic model, developed with minimal data inputs. Due to the lack of a mechanism to explain how alanine allosterically modifies the affinity of human liver pyruvate kinase (approved symbol PKLR) for its substrate, phosphoenolpyruvate, we proposed a speculative allosteric mechanism for this system. Within the allosteric amino-acid-binding site (something in the effector site must, of necessity, contribute to the allosteric mechanism), we implemented multiple mutational strategies: (1) site-directed random mutagenesis at positions that contact bound alanine and (2) mutations to probe specific questions. Despite acknowledged inadequacies used to formulate the speculative mechanism, many mutations modified the allosteric coupling constant (Qax ) consistent with that mechanism. The observed support for this speculative mechanism leaves us to ponder the best use of mutational data in structure-function studies of allosteric mechanisms. The mutational databank derived from this exercise has an independent value for training and testing algorithms specific to allostery.

Benchmarking predictions of allostery in liver pyruvate kinase in CAGI4.

The Critical Assessment of Genome Interpretation (CAGI) is a global community experiment to objectively assess computational methods for predicting phenotypic impacts of genomic variation. One of the 2015-2016 competitions focused on predicting the influence of mutations on the allosteric regulation of human liver pyruvate kinase. More than 30 different researchers accessed the challenge data. However, only four groups accepted the challenge. Features used for predictions ranged from evolutionary constraints, mutant site locations relative to active and effector binding sites, and computational docking outputs. Despite the range of expertise and strategies used by predictors, the best predictions were marginally greater than random for modified allostery resulting from mutations. In contrast, several groups successfully predicted which mutations severely reduced enzymatic activity. Nonetheless, poor predictions of allostery stands in stark contrast to the impression left by more than 700 PubMed entries identified using the identifiers "computational + allosteric." This contrast highlights a specialized need for new computational tools and utilization of benchmarks that focus on allosteric regulation.

Distinguishing the interactions in the fructose 1,6-bisphosphate binding site of human liver pyruvate kinase that contribute to allostery.

In the study of allosteric proteins, understanding which effector-protein interactions contribute to allosteric activation is important both for designing allosteric drugs and for understanding allosteric mechanisms. The antihyperglycemic target, human liver pyruvate kinase (hL-PYK), binds its allosteric activator, fructose 1,6-bisphosphate (Fru-1,6-BP), such that the 1'-phosphate interacts with side chains of Arg501 and Trp494 and the 6'-phosphate interacts with Thr444, Thr446, Ser449 (i.e., the 444-449 loop), and Ser531. Additionally, backbone atoms from the 527-533 loop interact with a sugar ring hydroxyl and the two effector phosphate moieties. An effector analogue series indicates that only one phosphate on the sugar is required for activation. However, singly phosphorylated sugars, including Fru-1-P and Fru-6-P, bind with a Kix in the range of 0.07-1 mM. The second phosphate of Fru-1,6-BP causes tight effector binding, because this native effector has a Kix of 0.061 µM. Glucose 1,6-bisphosphate and ribulose 1,5-bisphosphate bind in the 0.07-1 mM range. The contrast with a higher Fru-1,6-BP binding indicates specificity for the fructose sugar conformation. Site-directed random mutagenesis at each residue that contacts bound Fru-1,6-BP showed that a negative charge introduced at position 531 mimics allosteric activation, even in the absence of Fru-1,6-BP. Collectively, analogue and mutagenesis studies are consistent with the 527-533 loop playing a key role in allosteric function. Deletion mutations that shortened the 527-533 loop were expected to prevent formation of hydrogen bonds between backbone atoms on the loop and Fru-1,6-BP. Indeed, Fru-1,6-BP did not activate these loop-shortened mutant proteins. Previous structural comparisons of M1-PYK and M2-PYK indicate that the 527-533 loop makes interactions across a subunit interface when an activator is not present. Mutating the hL-PYK subunit interface interactions among Trp527, Arg528, and Asp499 mimics allosteric activation. Considered with published structures, these results are consistent with (1) the two phosphates of Fru-1,6-BP docking to Arg501/Trp494 and the 444-449 loop, respectively, and (2) the formation of hydrogen bonds among Fru-1,6-BP and backbone atoms of the 527-533 loop pulling this loop away from the subunit interface, which results in breaking of the Trp527-Arg528-Asp499 interactions to elicit an allosteric response.

Optimizing Pichia pastoris protein secretion: Role of N-linked glycosylation on the α-mating factor secretion signal leader.

The methylotrophic yeast, Pichia pastoris (P. pastoris; syn. Komagataella spp.), known for its ability to grow to high cell densities, its strong and tightly regulated promoters, and mammalian liked secretion pathway, has been widely used as a robust system to secrete heterologous proteins. The α-mating factor (MF) secretion signal leader from Saccharomyces cerevisiae (S. cerevisiae) is currently the most successfully used secretion signal sequence in the P. pastoris system. In this study, the secretion efficiency mediated by the α-MF secretion signal leaders from Komagataella pastoris (K. pastoris) and Komagataella phaffii (K. phaffii) was assessed using Enhanced Green Fluorescent Protein (EGFP) as a reporter. The results indicated that the secretion efficiency associated with the α-MF secretion signal leaders from K. pastoris and K. phaffii was notably lower in comparison to the α-MF secretion signal leader from S. cerevisiae. Further research indicated that N-linked glycosylation of the α-MF secretion signal leader enhanced the secretion of EGFP. Disruption of calnexin impaired the secretion of EGFP mediated by the N-linked glycosylated α-MF secretion signal leader, without affecting EGFP secretion mediated by the non-N-linked glycosylation α-MF secretion signal leader. The N-linked glycosylated of the α-MF secretion signal leader reduced the unfolded protein response (UPR) in the endoplasmic reticulum (ER). The enhancement of EGFP secretion by the N-linked glycosylated α-MF secretion signal leader might be achieved through the acceleration of proper folding of glycoproteins by the molecular chaperone calnexin. This study enhances the understanding of protein secretion in P. pastoris, specifically highlighting the influence of N-linked glycosylation on secretion efficiency, and could have implications for the production of recombinant proteins in bioengineering and biotechnological applications in P. pastoris.

Energetic coupling between an oxidizable cysteine and the phosphorylatable N-terminus of human liver pyruvate kinase.

During our efforts to characterize the regulatory properties of human liver pyruvate kinase (L-PYK), we have noted that the affinity of the protein for phosphoenolpyruvate (PEP) becomes reduced several days after cell lysis. A 1.8 Å crystallographic structure of L-PYK with the S12D mimic of phosphorylation indicates that Cys436 is oxidized, the first potential insight into explaining the effect of "aging". Interestingly, the oxidation is only to sulfenic acid despite the crystal growth time period of 2 weeks. Mutagenesis confirms that the side chain of residue 436 is energetically coupled to PEP binding. Mass spectrometry confirms that the oxidation is present in solution and is not an artifact caused by X-ray exposure. Exposure of the L-PYK mutations to H2O2 also confirms that PEP affinity is sensitive to the nature of the side chain at position 436. A 1.95 Å structure of the C436M mutant of L-PYK, the only mutation at position 436 that has been shown to strengthen PEP affinity, revealed that the methionine substitution results in the ordering of several N-terminal residues that have not been ordered in previous structures. This result allowed speculation that oxidation of Cys436 and phosphorylation of the N-terminus at Ser12 may function through a similar mechanism, namely the interruption of an activating interaction between the nonphosphorylated N-terminus with the nonoxidized main body of the protein. Mutant cycles were used to provide evidence that mutations of Cys436 are energetically synergistic with N-terminal modifications, a result that is consistent with phosphorylation of the N-terminus and oxidation of Cys436 functioning through mechanisms with common features. Alanine-scanning mutagenesis was used to confirm that the newly ordered N-terminal residues were important to the regulation of enzyme function by the N-terminus of the enzyme (i.e., not an artifact caused by the introduced methionine substitution) and to further define which residues in the N-terminus are energetically coupled to PEP affinity. Collectively, these studies indicate energetic coupling (and potentially mechanistic similarities) between the oxidation of Cys436 and phosphorylation of Ser12 in the N-terminus of L-PYK.

Comprehensively analyzing the genetic alterations, and identifying key genes in ovarian cancer.

Though significant improvements have been made in the treatment methods for ovarian cancer (OC), the prognosis for OC patients is still poor. Exploring hub genes associated with the development of OC and utilizing them as appropriate potential biomarkers or therapeutic targets is highly valuable. In this study, the differentially expressed genes (DEGs) were identified from an independent GSE69428 Gene Expression Omnibus (GEO) dataset between OC and control samples. The DEGs were processed to construct the protein-protein interaction (PPI) network using STRING. Later, hub genes were identified through Cytohubba analysis of the Cytoscape. Expression and survival profiling of the hub genes were validated using GEPIA, OncoDB, and GENT2. For exploring promoter methylation levels and genetic alterations in hub genes, MEXPRESS and cBioPortal were utilized, respectively. Moreover, DAVID, HPA, TIMER, CancerSEA, ENCORI, DrugBank, and GSCAlite were used for gene enrichment analysis, subcellular localization analysis, immune cell infiltration analysis, exploring correlations between hub genes and different diverse states, lncRNA-miRNA-mRNA co-regulatory network analysis, predicting hub gene-associated drugs, and conducting drug sensitivity analysis, respectively. In total, 8947 DEGs were found between OC and normal samples in GSE69428. After STRING and Cytohubba analysis, 4 hub genes including TTK (TTK Protein Kinase), (BUB1 mitotic checkpoint serine/threonine kinase B) BUB1B, (Nucleolar and spindle-associated protein 1) NUSAP1, and (ZW10 interacting kinetochore protein) ZWINT were selected as the hub genes. Further, it was validated that these 4 hub genes were significantly up-regulated in OC samples compared to normal controls, but overexpression of these genes was not associated with overall survival (OS). However, genetic alterations in those genes were found to be linked with OS and disease-free (DFS) survival. Moreover, this study also revealed some novel links between TTK, BUB1B, NUSAP1, and ZWINT overexpression and promoter methylation status, immune cell infiltration, miRNAs, gene enrichment terms, and various chemotherapeutic drugs. Four hub genes, including TTK, BUB1B, NUSAP1, and ZWINT, were revealed as tumor-promotive factors in OC, having the potential to be utilized as novel biomarkers and therapeutic targets for OC management.

PYK-SubstitutionOME: an integrated database containing allosteric coupling, ligand affinity and mutational, structural, pathological, bioinformatic and computational information about pyruvate kinase isozymes.

Interpreting changes in patient genomes, understanding how viruses evolve and engineering novel protein function all depend on accurately predicting the functional outcomes that arise from amino acid substitutions. To that end, the development of first-generation prediction algorithms was guided by historic experimental datasets. However, these datasets were heavily biased toward substitutions at positions that have not changed much throughout evolution (i.e. conserved). Although newer datasets include substitutions at positions that span a range of evolutionary conservation scores, these data are largely derived from assays that agglomerate multiple aspects of function. To facilitate predictions from the foundational chemical properties of proteins, large substitution databases with biochemical characterizations of function are needed. We report here a database derived from mutational, biochemical, bioinformatic, structural, pathological and computational studies of a highly studied protein family-pyruvate kinase (PYK). A centerpiece of this database is the biochemical characterization-including quantitative evaluation of allosteric regulation-of the changes that accompany substitutions at positions that sample the full conservation range observed in the PYK family. We have used these data to facilitate critical advances in the foundational studies of allosteric regulation and protein evolution and as rigorous benchmarks for testing protein predictions. We trust that the collected dataset will be useful for the broader scientific community in the further development of prediction algorithms. Database URL https://github.com/djparente/PYK-DB.

Odd one out? Functional tuning of Zymomonas mobilis pyruvate kinase is narrower than its allosteric, human counterpart.

Various protein properties are often illuminated using sequence comparisons of protein homologs. For example, in analyses of the pyruvate kinase multiple sequence alignment, the set of positions that changed during speciation ("phylogenetic" positions) were enriched for "rheostat" positions in human liver pyruvate kinase (hLPYK). (Rheostat positions are those which, when substituted with various amino acids, yield a range of functional outcomes). However, the correlation was moderate, which could result from multiple biophysical constraints acting on the same position during evolution and/or various sources of noise. To further examine this correlation, we here tested Zymomonas mobilis PYK (ZmPYK), which has <65% sequence identity to any other PYK sequence. Twenty-six ZmPYK positions were selected based on their phylogenetic scores, substituted with multiple amino acids, and assessed for changes in Kapp-PEP . Although we expected to identify multiple, strong rheostat positions, only one moderate rheostat position was detected. Instead, nearly half of the 271 ZmPYK variants were inactive and most others showed near wild-type function. Indeed, for the active ZmPYK variants, the total range of Kapp,PEP values ("tunability") was 40-fold less than that observed for hLPYK variants. The combined functional studies and sequence comparisons suggest that ZmPYK has evolved functional and/or structural attributes that differ from the rest of the family. We hypothesize that including such "orphan" sequences in MSA analyses obscures the correlations used to predict rheostat positions. Finally, results raise the intriguing biophysical question as to how the same protein fold can support rheostat positions in one homolog but not another.

Inhibition of Pyruvate Kinase From Thermoanaerobacterium saccharolyticum by IMP Is Independent of the Extra-C Domain.

The pyruvate kinase (PYK) isozyme from Thermoanaerobacterium saccharolyticum (TsPYK) has previously been used in metabolic engineering for improved ethanol production. This isozyme belongs to a subclass of PYK isozymes that include an extra C-domain. Like other isozymes that include this extra C-domain, we found that TsPYK is activated by AMP and ribose-5-phosphate (R5P). Our use of sugar-phosphate analogs generated a surprising result in that IMP and GMP are allosteric inhibitors (rather than activators) of TsPYK. We believe this to be the first report of any PYK isozyme being inhibited by IMP and GMP. A truncated protein that lacks the extra C-domain is also inhibited by IMP. A screen of several other bacterial PYK enzymes (include several that have the extra-C domain) indicates that the inhibition by IMP is specific to only a subset of those isozymes.

Rheostat functional outcomes occur when substitutions are introduced at nonconserved positions that diverge with speciation.

When amino acids vary during evolution, the outcome can be functionally neutral or biologically-important. We previously found that substituting a subset of nonconserved positions, "rheostat" positions, can have surprising effects on protein function. Since changes at rheostat positions can facilitate functional evolution or cause disease, more examples are needed to understand their unique biophysical characteristics. Here, we explored whether "phylogenetic" patterns of change in multiple sequence alignments (such as positions with subfamily specific conservation) predict the locations of functional rheostat positions. To that end, we experimentally tested eight phylogenetic positions in human liver pyruvate kinase (hLPYK), using 10-15 substitutions per position and biochemical assays that yielded five functional parameters. Five positions were strongly rheostatic and three were non-neutral. To test the corollary that positions with low phylogenetic scores were not rheostat positions, we combined these phylogenetic positions with previously-identified hLPYK rheostat, "toggle" (most substitution abolished function), and "neutral" (all substitutions were like wild-type) positions. Despite representing 428 variants, this set of 33 positions was poorly statistically powered. Thus, we turned to the in vivo phenotypic dataset for E. coli lactose repressor protein (LacI), which comprised 12-13 substitutions at 329 positions and could be used to identify rheostat, toggle, and neutral positions. Combined hLPYK and LacI results show that positions with strong phylogenetic patterns of change are more likely to exhibit rheostat substitution outcomes than neutral or toggle outcomes. Furthermore, phylogenetic patterns were more successful at identifying rheostat positions than were co-evolutionary or eigenvector centrality measures of evolutionary change.

An activating interaction between the unphosphorylated n-terminus of human liver pyruvate kinase and the main body of the protein is interrupted by phosphorylation.

The initial 26 amino acids of human liver pyruvate kinase (L-PYK) are not present/observed in the crystal structure. This region includes Ser12, the site of hormone-dependent phosphorylation. Truncating the N-terminus of L-PYK mimics the effects of phosphorylation by causing a decrease in apparent phosphoenolpyruvate (PEP) affinity. An N-terminus truncation series was used to map the minimum number of residues that could be removed to result in the decrease in apparent PEP affinity. Results are consistent with a mechanism by which phosphorylation at Ser12 interrupts an activating interaction of N-terminal residues (including those at positions 7-10) with the main body of the protein, as a means of inhibiting substrate affinity.

[Relationship of adiponectin and visfatin with fetus intrauterine growth].

OBJECTIVE: To explore the correlation between adipocyte factors (adiponectin and visfatin) and fetus intrauterine growth. METHODS: Enzyme immunoassay was used to measure the adiponectin and visfatin levels in maternal and umbilical serum from 14 women with fetal growth restriction (FGR group), 14 women with macrosomia (macrosomia group) and 14 normal pregnant women (control group). The correlations of cord serum adiponectin and visfatin with maternal serum adiponectin and visfatin were analyzed. RESULTS: (1) Serum visfatin levels in FGR mothers [(41.4 +/- 5.5)] microg/L were significantly higher than that in control women [(34.7 +/- 4.9) microg/L] and macrosomia mothers [(37.3 +/- 4.4) microg/L; P < 0.01, P < 0.05]. Serum adiponectin levels in macrosomia mothers [(4.1 +/- 1.3) mg/L] were significantly lower than that in control women [(6.6 +/- 1.5) mg/L] and FGR mothers [(6.4 +/- 1.3) mg/L; P < 0.01]. (2) Serum visfatin levels in FGR babies [(58.1 +/- 7.6) microg/L] were significantly increased than that in control newborns [(42.6 +/- 7.8) microg/L] and macrosomia babies [(48.5 +/- 9.1) microg/L; P < 0.01, P < 0.05]. Serum adiponectin levels in macrosomia babies [(6.5 +/- 1.3) mg/L] were significantly decreased than that in control newborns [(7.7 +/- 1.5) mg/L] and FGR babies [(7.7 +/- 1.0) mg/L; P < 0.05, P < 0.05]. (3) Maternal serum visfatin levels were positively correlated with umbilical serum visfatin levels (r = 0.720, P < 0.01). Umbilical serum adiponectin levels were higher than that in maternal serum, but there were no relationship between them (r = 0.301, P > 0.05). CONCLUSION: The changes of visfatin and adiponectin levels may be related to the occurrence of FGR and fetal macrosomia.

Mutational mimics of allosteric effectors: a genome editing design to validate allosteric drug targets.

Development of drugs that allosterically regulate enzyme functions to treat disease is a costly venture. Amino acid susbstitutions that mimic allosteric effectors in vitro will identify therapeutic regulatory targets enhancing the likelihood of developing a disease treatment at a reasonable cost. We demonstrate the potential of this approach utilizing human liver pyruvate kinase (hLPYK) as a model. Inhibition of hLPYK was the first desired outcome of this study. We identified individual point mutations that: 1) mimicked allosteric inhibition by alanine, 2) mimicked inhibition by protein phosphorylation, and 3) prevented binding of fructose-1,6-bisphosphate (Fru-1,6-BP). Our second desired outcome was activation of hLPYK. We identified individual point mutations that: 1) prevented hLPYK from binding alanine, the allosteric inhibitor, 2) prevented inhibitory protein phosphorylation, or 3) mimicked allosteric activation by Fru-1,6-BP. Combining the three activating point mutations produced a constitutively activated enzyme that was unresponsive to regulators. Expression of a mutant hLPYK transgene containing these three mutations in a mouse model was not lethal. Thus, mutational mimics of allosteric effectors will be useful to confirm whether allosteric activation of hLPYK will control glycolytic flux in the diabetic liver to reduce hepatic glucose production and, in turn, reduce or prevent hyperglycemia.

Allosteric regulation of human liver pyruvate kinase by peptides that mimic the phosphorylated/dephosphorylated N-terminus.

An advantage of studying allosteric regulation over covalent modification is that allostery allows the experimentalist to vary the concentration of effector, thereby allowing independent quantification of effector binding and allosteric coupling. In turn, this capacity allows the use of effector analogues to determine which regions of the effector contribute to effector binding and which contribute to allosteric regulation. Like many other proteins, human liver pyruvate kinase (hL-PYK) is regulated by phosphorylation. The phosphorylation of hL-PYK occurs on Ser12 of the N-terminus. Phosphorylation appears to interrupt an interaction (distant from the active site) between the N-terminus and the main body of the protein. Since this interaction increases the affinity of hL-PYK for the substrate (phosphoenolpyruvate, PEP), phosphorylation-dependent interruption of the N-terminus/main-body interaction results in an antagonism of PEP binding. Due to the advantages of studying an allosteric system, we detail a protocol to express and purify N-terminal peptides of hL-PYK using a SUMO-fusion system. We further demonstrate that these peptides act as allosteric regulators that modulate the affinity of hL-PYK for PEP.